| .. _kernel_hacking_lock: |
| |
| =========================== |
| Unreliable Guide To Locking |
| =========================== |
| |
| :Author: Rusty Russell |
| |
| Introduction |
| ============ |
| |
| Welcome, to Rusty's Remarkably Unreliable Guide to Kernel Locking |
| issues. This document describes the locking systems in the Linux Kernel |
| in 2.6. |
| |
| With the wide availability of HyperThreading, and preemption in the |
| Linux Kernel, everyone hacking on the kernel needs to know the |
| fundamentals of concurrency and locking for SMP. |
| |
| The Problem With Concurrency |
| ============================ |
| |
| (Skip this if you know what a Race Condition is). |
| |
| In a normal program, you can increment a counter like so: |
| |
| :: |
| |
| very_important_count++; |
| |
| |
| This is what they would expect to happen: |
| |
| |
| .. table:: Expected Results |
| |
| +------------------------------------+------------------------------------+ |
| | Instance 1 | Instance 2 | |
| +====================================+====================================+ |
| | read very_important_count (5) | | |
| +------------------------------------+------------------------------------+ |
| | add 1 (6) | | |
| +------------------------------------+------------------------------------+ |
| | write very_important_count (6) | | |
| +------------------------------------+------------------------------------+ |
| | | read very_important_count (6) | |
| +------------------------------------+------------------------------------+ |
| | | add 1 (7) | |
| +------------------------------------+------------------------------------+ |
| | | write very_important_count (7) | |
| +------------------------------------+------------------------------------+ |
| |
| This is what might happen: |
| |
| .. table:: Possible Results |
| |
| +------------------------------------+------------------------------------+ |
| | Instance 1 | Instance 2 | |
| +====================================+====================================+ |
| | read very_important_count (5) | | |
| +------------------------------------+------------------------------------+ |
| | | read very_important_count (5) | |
| +------------------------------------+------------------------------------+ |
| | add 1 (6) | | |
| +------------------------------------+------------------------------------+ |
| | | add 1 (6) | |
| +------------------------------------+------------------------------------+ |
| | write very_important_count (6) | | |
| +------------------------------------+------------------------------------+ |
| | | write very_important_count (6) | |
| +------------------------------------+------------------------------------+ |
| |
| |
| Race Conditions and Critical Regions |
| ------------------------------------ |
| |
| This overlap, where the result depends on the relative timing of |
| multiple tasks, is called a race condition. The piece of code containing |
| the concurrency issue is called a critical region. And especially since |
| Linux starting running on SMP machines, they became one of the major |
| issues in kernel design and implementation. |
| |
| Preemption can have the same effect, even if there is only one CPU: by |
| preempting one task during the critical region, we have exactly the same |
| race condition. In this case the thread which preempts might run the |
| critical region itself. |
| |
| The solution is to recognize when these simultaneous accesses occur, and |
| use locks to make sure that only one instance can enter the critical |
| region at any time. There are many friendly primitives in the Linux |
| kernel to help you do this. And then there are the unfriendly |
| primitives, but I'll pretend they don't exist. |
| |
| Locking in the Linux Kernel |
| =========================== |
| |
| If I could give you one piece of advice on locking: **keep it simple**. |
| |
| Be reluctant to introduce new locks. |
| |
| Two Main Types of Kernel Locks: Spinlocks and Mutexes |
| ----------------------------------------------------- |
| |
| There are two main types of kernel locks. The fundamental type is the |
| spinlock (``include/asm/spinlock.h``), which is a very simple |
| single-holder lock: if you can't get the spinlock, you keep trying |
| (spinning) until you can. Spinlocks are very small and fast, and can be |
| used anywhere. |
| |
| The second type is a mutex (``include/linux/mutex.h``): it is like a |
| spinlock, but you may block holding a mutex. If you can't lock a mutex, |
| your task will suspend itself, and be woken up when the mutex is |
| released. This means the CPU can do something else while you are |
| waiting. There are many cases when you simply can't sleep (see |
| `What Functions Are Safe To Call From Interrupts?`_), |
| and so have to use a spinlock instead. |
| |
| Neither type of lock is recursive: see |
| `Deadlock: Simple and Advanced`_. |
| |
| Locks and Uniprocessor Kernels |
| ------------------------------ |
| |
| For kernels compiled without ``CONFIG_SMP``, and without |
| ``CONFIG_PREEMPT`` spinlocks do not exist at all. This is an excellent |
| design decision: when no-one else can run at the same time, there is no |
| reason to have a lock. |
| |
| If the kernel is compiled without ``CONFIG_SMP``, but ``CONFIG_PREEMPT`` |
| is set, then spinlocks simply disable preemption, which is sufficient to |
| prevent any races. For most purposes, we can think of preemption as |
| equivalent to SMP, and not worry about it separately. |
| |
| You should always test your locking code with ``CONFIG_SMP`` and |
| ``CONFIG_PREEMPT`` enabled, even if you don't have an SMP test box, |
| because it will still catch some kinds of locking bugs. |
| |
| Mutexes still exist, because they are required for synchronization |
| between user contexts, as we will see below. |
| |
| Locking Only In User Context |
| ---------------------------- |
| |
| If you have a data structure which is only ever accessed from user |
| context, then you can use a simple mutex (``include/linux/mutex.h``) to |
| protect it. This is the most trivial case: you initialize the mutex. |
| Then you can call mutex_lock_interruptible() to grab the |
| mutex, and mutex_unlock() to release it. There is also a |
| mutex_lock(), which should be avoided, because it will |
| not return if a signal is received. |
| |
| Example: ``net/netfilter/nf_sockopt.c`` allows registration of new |
| setsockopt() and getsockopt() calls, with |
| nf_register_sockopt(). Registration and de-registration |
| are only done on module load and unload (and boot time, where there is |
| no concurrency), and the list of registrations is only consulted for an |
| unknown setsockopt() or getsockopt() system |
| call. The ``nf_sockopt_mutex`` is perfect to protect this, especially |
| since the setsockopt and getsockopt calls may well sleep. |
| |
| Locking Between User Context and Softirqs |
| ----------------------------------------- |
| |
| If a softirq shares data with user context, you have two problems. |
| Firstly, the current user context can be interrupted by a softirq, and |
| secondly, the critical region could be entered from another CPU. This is |
| where spin_lock_bh() (``include/linux/spinlock.h``) is |
| used. It disables softirqs on that CPU, then grabs the lock. |
| spin_unlock_bh() does the reverse. (The '_bh' suffix is |
| a historical reference to "Bottom Halves", the old name for software |
| interrupts. It should really be called spin_lock_softirq()' in a |
| perfect world). |
| |
| Note that you can also use spin_lock_irq() or |
| spin_lock_irqsave() here, which stop hardware interrupts |
| as well: see `Hard IRQ Context`_. |
| |
| This works perfectly for UP as well: the spin lock vanishes, and this |
| macro simply becomes local_bh_disable() |
| (``include/linux/interrupt.h``), which protects you from the softirq |
| being run. |
| |
| Locking Between User Context and Tasklets |
| ----------------------------------------- |
| |
| This is exactly the same as above, because tasklets are actually run |
| from a softirq. |
| |
| Locking Between User Context and Timers |
| --------------------------------------- |
| |
| This, too, is exactly the same as above, because timers are actually run |
| from a softirq. From a locking point of view, tasklets and timers are |
| identical. |
| |
| Locking Between Tasklets/Timers |
| ------------------------------- |
| |
| Sometimes a tasklet or timer might want to share data with another |
| tasklet or timer. |
| |
| The Same Tasklet/Timer |
| ~~~~~~~~~~~~~~~~~~~~~~ |
| |
| Since a tasklet is never run on two CPUs at once, you don't need to |
| worry about your tasklet being reentrant (running twice at once), even |
| on SMP. |
| |
| Different Tasklets/Timers |
| ~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| If another tasklet/timer wants to share data with your tasklet or timer |
| , you will both need to use spin_lock() and |
| spin_unlock() calls. spin_lock_bh() is |
| unnecessary here, as you are already in a tasklet, and none will be run |
| on the same CPU. |
| |
| Locking Between Softirqs |
| ------------------------ |
| |
| Often a softirq might want to share data with itself or a tasklet/timer. |
| |
| The Same Softirq |
| ~~~~~~~~~~~~~~~~ |
| |
| The same softirq can run on the other CPUs: you can use a per-CPU array |
| (see `Per-CPU Data`_) for better performance. If you're |
| going so far as to use a softirq, you probably care about scalable |
| performance enough to justify the extra complexity. |
| |
| You'll need to use spin_lock() and |
| spin_unlock() for shared data. |
| |
| Different Softirqs |
| ~~~~~~~~~~~~~~~~~~ |
| |
| You'll need to use spin_lock() and |
| spin_unlock() for shared data, whether it be a timer, |
| tasklet, different softirq or the same or another softirq: any of them |
| could be running on a different CPU. |
| |
| Hard IRQ Context |
| ================ |
| |
| Hardware interrupts usually communicate with a tasklet or softirq. |
| Frequently this involves putting work in a queue, which the softirq will |
| take out. |
| |
| Locking Between Hard IRQ and Softirqs/Tasklets |
| ---------------------------------------------- |
| |
| If a hardware irq handler shares data with a softirq, you have two |
| concerns. Firstly, the softirq processing can be interrupted by a |
| hardware interrupt, and secondly, the critical region could be entered |
| by a hardware interrupt on another CPU. This is where |
| spin_lock_irq() is used. It is defined to disable |
| interrupts on that cpu, then grab the lock. |
| spin_unlock_irq() does the reverse. |
| |
| The irq handler does not need to use spin_lock_irq(), because |
| the softirq cannot run while the irq handler is running: it can use |
| spin_lock(), which is slightly faster. The only exception |
| would be if a different hardware irq handler uses the same lock: |
| spin_lock_irq() will stop that from interrupting us. |
| |
| This works perfectly for UP as well: the spin lock vanishes, and this |
| macro simply becomes local_irq_disable() |
| (``include/asm/smp.h``), which protects you from the softirq/tasklet/BH |
| being run. |
| |
| spin_lock_irqsave() (``include/linux/spinlock.h``) is a |
| variant which saves whether interrupts were on or off in a flags word, |
| which is passed to spin_unlock_irqrestore(). This means |
| that the same code can be used inside an hard irq handler (where |
| interrupts are already off) and in softirqs (where the irq disabling is |
| required). |
| |
| Note that softirqs (and hence tasklets and timers) are run on return |
| from hardware interrupts, so spin_lock_irq() also stops |
| these. In that sense, spin_lock_irqsave() is the most |
| general and powerful locking function. |
| |
| Locking Between Two Hard IRQ Handlers |
| ------------------------------------- |
| |
| It is rare to have to share data between two IRQ handlers, but if you |
| do, spin_lock_irqsave() should be used: it is |
| architecture-specific whether all interrupts are disabled inside irq |
| handlers themselves. |
| |
| Cheat Sheet For Locking |
| ======================= |
| |
| Pete Zaitcev gives the following summary: |
| |
| - If you are in a process context (any syscall) and want to lock other |
| process out, use a mutex. You can take a mutex and sleep |
| (``copy_from_user()`` or ``kmalloc(x,GFP_KERNEL)``). |
| |
| - Otherwise (== data can be touched in an interrupt), use |
| spin_lock_irqsave() and |
| spin_unlock_irqrestore(). |
| |
| - Avoid holding spinlock for more than 5 lines of code and across any |
| function call (except accessors like readb()). |
| |
| Table of Minimum Requirements |
| ----------------------------- |
| |
| The following table lists the **minimum** locking requirements between |
| various contexts. In some cases, the same context can only be running on |
| one CPU at a time, so no locking is required for that context (eg. a |
| particular thread can only run on one CPU at a time, but if it needs |
| shares data with another thread, locking is required). |
| |
| Remember the advice above: you can always use |
| spin_lock_irqsave(), which is a superset of all other |
| spinlock primitives. |
| |
| ============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ============== |
| . IRQ Handler A IRQ Handler B Softirq A Softirq B Tasklet A Tasklet B Timer A Timer B User Context A User Context B |
| ============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ============== |
| IRQ Handler A None |
| IRQ Handler B SLIS None |
| Softirq A SLI SLI SL |
| Softirq B SLI SLI SL SL |
| Tasklet A SLI SLI SL SL None |
| Tasklet B SLI SLI SL SL SL None |
| Timer A SLI SLI SL SL SL SL None |
| Timer B SLI SLI SL SL SL SL SL None |
| User Context A SLI SLI SLBH SLBH SLBH SLBH SLBH SLBH None |
| User Context B SLI SLI SLBH SLBH SLBH SLBH SLBH SLBH MLI None |
| ============== ============= ============= ========= ========= ========= ========= ======= ======= ============== ============== |
| |
| Table: Table of Locking Requirements |
| |
| +--------+----------------------------+ |
| | SLIS | spin_lock_irqsave | |
| +--------+----------------------------+ |
| | SLI | spin_lock_irq | |
| +--------+----------------------------+ |
| | SL | spin_lock | |
| +--------+----------------------------+ |
| | SLBH | spin_lock_bh | |
| +--------+----------------------------+ |
| | MLI | mutex_lock_interruptible | |
| +--------+----------------------------+ |
| |
| Table: Legend for Locking Requirements Table |
| |
| The trylock Functions |
| ===================== |
| |
| There are functions that try to acquire a lock only once and immediately |
| return a value telling about success or failure to acquire the lock. |
| They can be used if you need no access to the data protected with the |
| lock when some other thread is holding the lock. You should acquire the |
| lock later if you then need access to the data protected with the lock. |
| |
| spin_trylock() does not spin but returns non-zero if it |
| acquires the spinlock on the first try or 0 if not. This function can be |
| used in all contexts like spin_lock(): you must have |
| disabled the contexts that might interrupt you and acquire the spin |
| lock. |
| |
| mutex_trylock() does not suspend your task but returns |
| non-zero if it could lock the mutex on the first try or 0 if not. This |
| function cannot be safely used in hardware or software interrupt |
| contexts despite not sleeping. |
| |
| Common Examples |
| =============== |
| |
| Let's step through a simple example: a cache of number to name mappings. |
| The cache keeps a count of how often each of the objects is used, and |
| when it gets full, throws out the least used one. |
| |
| All In User Context |
| ------------------- |
| |
| For our first example, we assume that all operations are in user context |
| (ie. from system calls), so we can sleep. This means we can use a mutex |
| to protect the cache and all the objects within it. Here's the code:: |
| |
| #include <linux/list.h> |
| #include <linux/slab.h> |
| #include <linux/string.h> |
| #include <linux/mutex.h> |
| #include <asm/errno.h> |
| |
| struct object |
| { |
| struct list_head list; |
| int id; |
| char name[32]; |
| int popularity; |
| }; |
| |
| /* Protects the cache, cache_num, and the objects within it */ |
| static DEFINE_MUTEX(cache_lock); |
| static LIST_HEAD(cache); |
| static unsigned int cache_num = 0; |
| #define MAX_CACHE_SIZE 10 |
| |
| /* Must be holding cache_lock */ |
| static struct object *__cache_find(int id) |
| { |
| struct object *i; |
| |
| list_for_each_entry(i, &cache, list) |
| if (i->id == id) { |
| i->popularity++; |
| return i; |
| } |
| return NULL; |
| } |
| |
| /* Must be holding cache_lock */ |
| static void __cache_delete(struct object *obj) |
| { |
| BUG_ON(!obj); |
| list_del(&obj->list); |
| kfree(obj); |
| cache_num--; |
| } |
| |
| /* Must be holding cache_lock */ |
| static void __cache_add(struct object *obj) |
| { |
| list_add(&obj->list, &cache); |
| if (++cache_num > MAX_CACHE_SIZE) { |
| struct object *i, *outcast = NULL; |
| list_for_each_entry(i, &cache, list) { |
| if (!outcast || i->popularity < outcast->popularity) |
| outcast = i; |
| } |
| __cache_delete(outcast); |
| } |
| } |
| |
| int cache_add(int id, const char *name) |
| { |
| struct object *obj; |
| |
| if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) |
| return -ENOMEM; |
| |
| strscpy(obj->name, name, sizeof(obj->name)); |
| obj->id = id; |
| obj->popularity = 0; |
| |
| mutex_lock(&cache_lock); |
| __cache_add(obj); |
| mutex_unlock(&cache_lock); |
| return 0; |
| } |
| |
| void cache_delete(int id) |
| { |
| mutex_lock(&cache_lock); |
| __cache_delete(__cache_find(id)); |
| mutex_unlock(&cache_lock); |
| } |
| |
| int cache_find(int id, char *name) |
| { |
| struct object *obj; |
| int ret = -ENOENT; |
| |
| mutex_lock(&cache_lock); |
| obj = __cache_find(id); |
| if (obj) { |
| ret = 0; |
| strcpy(name, obj->name); |
| } |
| mutex_unlock(&cache_lock); |
| return ret; |
| } |
| |
| Note that we always make sure we have the cache_lock when we add, |
| delete, or look up the cache: both the cache infrastructure itself and |
| the contents of the objects are protected by the lock. In this case it's |
| easy, since we copy the data for the user, and never let them access the |
| objects directly. |
| |
| There is a slight (and common) optimization here: in |
| cache_add() we set up the fields of the object before |
| grabbing the lock. This is safe, as no-one else can access it until we |
| put it in cache. |
| |
| Accessing From Interrupt Context |
| -------------------------------- |
| |
| Now consider the case where cache_find() can be called |
| from interrupt context: either a hardware interrupt or a softirq. An |
| example would be a timer which deletes object from the cache. |
| |
| The change is shown below, in standard patch format: the ``-`` are lines |
| which are taken away, and the ``+`` are lines which are added. |
| |
| :: |
| |
| --- cache.c.usercontext 2003-12-09 13:58:54.000000000 +1100 |
| +++ cache.c.interrupt 2003-12-09 14:07:49.000000000 +1100 |
| @@ -12,7 +12,7 @@ |
| int popularity; |
| }; |
| |
| -static DEFINE_MUTEX(cache_lock); |
| +static DEFINE_SPINLOCK(cache_lock); |
| static LIST_HEAD(cache); |
| static unsigned int cache_num = 0; |
| #define MAX_CACHE_SIZE 10 |
| @@ -55,6 +55,7 @@ |
| int cache_add(int id, const char *name) |
| { |
| struct object *obj; |
| + unsigned long flags; |
| |
| if ((obj = kmalloc(sizeof(*obj), GFP_KERNEL)) == NULL) |
| return -ENOMEM; |
| @@ -63,30 +64,33 @@ |
| obj->id = id; |
| obj->popularity = 0; |
| |
| - mutex_lock(&cache_lock); |
| + spin_lock_irqsave(&cache_lock, flags); |
| __cache_add(obj); |
| - mutex_unlock(&cache_lock); |
| + spin_unlock_irqrestore(&cache_lock, flags); |
| return 0; |
| } |
| |
| void cache_delete(int id) |
| { |
| - mutex_lock(&cache_lock); |
| + unsigned long flags; |
| + |
| + spin_lock_irqsave(&cache_lock, flags); |
| __cache_delete(__cache_find(id)); |
| - mutex_unlock(&cache_lock); |
| + spin_unlock_irqrestore(&cache_lock, flags); |
| } |
| |
| int cache_find(int id, char *name) |
| { |
| struct object *obj; |
| int ret = -ENOENT; |
| + unsigned long flags; |
| |
| - mutex_lock(&cache_lock); |
| + spin_lock_irqsave(&cache_lock, flags); |
| obj = __cache_find(id); |
| if (obj) { |
| ret = 0; |
| strcpy(name, obj->name); |
| } |
| - mutex_unlock(&cache_lock); |
| + spin_unlock_irqrestore(&cache_lock, flags); |
| return ret; |
| } |
| |
| Note that the spin_lock_irqsave() will turn off |
| interrupts if they are on, otherwise does nothing (if we are already in |
| an interrupt handler), hence these functions are safe to call from any |
| context. |
| |
| Unfortunately, cache_add() calls kmalloc() |
| with the ``GFP_KERNEL`` flag, which is only legal in user context. I |
| have assumed that cache_add() is still only called in |
| user context, otherwise this should become a parameter to |
| cache_add(). |
| |
| Exposing Objects Outside This File |
| ---------------------------------- |
| |
| If our objects contained more information, it might not be sufficient to |
| copy the information in and out: other parts of the code might want to |
| keep pointers to these objects, for example, rather than looking up the |
| id every time. This produces two problems. |
| |
| The first problem is that we use the ``cache_lock`` to protect objects: |
| we'd need to make this non-static so the rest of the code can use it. |
| This makes locking trickier, as it is no longer all in one place. |
| |
| The second problem is the lifetime problem: if another structure keeps a |
| pointer to an object, it presumably expects that pointer to remain |
| valid. Unfortunately, this is only guaranteed while you hold the lock, |
| otherwise someone might call cache_delete() and even |
| worse, add another object, re-using the same address. |
| |
| As there is only one lock, you can't hold it forever: no-one else would |
| get any work done. |
| |
| The solution to this problem is to use a reference count: everyone who |
| has a pointer to the object increases it when they first get the object, |
| and drops the reference count when they're finished with it. Whoever |
| drops it to zero knows it is unused, and can actually delete it. |
| |
| Here is the code:: |
| |
| --- cache.c.interrupt 2003-12-09 14:25:43.000000000 +1100 |
| +++ cache.c.refcnt 2003-12-09 14:33:05.000000000 +1100 |
| @@ -7,6 +7,7 @@ |
| struct object |
| { |
| struct list_head list; |
| + unsigned int refcnt; |
| int id; |
| char name[32]; |
| int popularity; |
| @@ -17,6 +18,35 @@ |
| static unsigned int cache_num = 0; |
| #define MAX_CACHE_SIZE 10 |
| |
| +static void __object_put(struct object *obj) |
| +{ |
| + if (--obj->refcnt == 0) |
| + kfree(obj); |
| +} |
| + |
| +static void __object_get(struct object *obj) |
| +{ |
| + obj->refcnt++; |
| +} |
| + |
| +void object_put(struct object *obj) |
| +{ |
| + unsigned long flags; |
| + |
| + spin_lock_irqsave(&cache_lock, flags); |
| + __object_put(obj); |
| + spin_unlock_irqrestore(&cache_lock, flags); |
| +} |
| + |
| +void object_get(struct object *obj) |
| +{ |
| + unsigned long flags; |
| + |
| + spin_lock_irqsave(&cache_lock, flags); |
| + __object_get(obj); |
| + spin_unlock_irqrestore(&cache_lock, flags); |
| +} |
| + |
| /* Must be holding cache_lock */ |
| static struct object *__cache_find(int id) |
| { |
| @@ -35,6 +65,7 @@ |
| { |
| BUG_ON(!obj); |
| list_del(&obj->list); |
| + __object_put(obj); |
| cache_num--; |
| } |
| |
| @@ -63,6 +94,7 @@ |
| strscpy(obj->name, name, sizeof(obj->name)); |
| obj->id = id; |
| obj->popularity = 0; |
| + obj->refcnt = 1; /* The cache holds a reference */ |
| |
| spin_lock_irqsave(&cache_lock, flags); |
| __cache_add(obj); |
| @@ -79,18 +111,15 @@ |
| spin_unlock_irqrestore(&cache_lock, flags); |
| } |
| |
| -int cache_find(int id, char *name) |
| +struct object *cache_find(int id) |
| { |
| struct object *obj; |
| - int ret = -ENOENT; |
| unsigned long flags; |
| |
| spin_lock_irqsave(&cache_lock, flags); |
| obj = __cache_find(id); |
| - if (obj) { |
| - ret = 0; |
| - strcpy(name, obj->name); |
| - } |
| + if (obj) |
| + __object_get(obj); |
| spin_unlock_irqrestore(&cache_lock, flags); |
| - return ret; |
| + return obj; |
| } |
| |
| We encapsulate the reference counting in the standard 'get' and 'put' |
| functions. Now we can return the object itself from |
| cache_find() which has the advantage that the user can |
| now sleep holding the object (eg. to copy_to_user() to |
| name to userspace). |
| |
| The other point to note is that I said a reference should be held for |
| every pointer to the object: thus the reference count is 1 when first |
| inserted into the cache. In some versions the framework does not hold a |
| reference count, but they are more complicated. |
| |
| Using Atomic Operations For The Reference Count |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| In practice, :c:type:`atomic_t` would usually be used for refcnt. There are a |
| number of atomic operations defined in ``include/asm/atomic.h``: these |
| are guaranteed to be seen atomically from all CPUs in the system, so no |
| lock is required. In this case, it is simpler than using spinlocks, |
| although for anything non-trivial using spinlocks is clearer. The |
| atomic_inc() and atomic_dec_and_test() |
| are used instead of the standard increment and decrement operators, and |
| the lock is no longer used to protect the reference count itself. |
| |
| :: |
| |
| --- cache.c.refcnt 2003-12-09 15:00:35.000000000 +1100 |
| +++ cache.c.refcnt-atomic 2003-12-11 15:49:42.000000000 +1100 |
| @@ -7,7 +7,7 @@ |
| struct object |
| { |
| struct list_head list; |
| - unsigned int refcnt; |
| + atomic_t refcnt; |
| int id; |
| char name[32]; |
| int popularity; |
| @@ -18,33 +18,15 @@ |
| static unsigned int cache_num = 0; |
| #define MAX_CACHE_SIZE 10 |
| |
| -static void __object_put(struct object *obj) |
| -{ |
| - if (--obj->refcnt == 0) |
| - kfree(obj); |
| -} |
| - |
| -static void __object_get(struct object *obj) |
| -{ |
| - obj->refcnt++; |
| -} |
| - |
| void object_put(struct object *obj) |
| { |
| - unsigned long flags; |
| - |
| - spin_lock_irqsave(&cache_lock, flags); |
| - __object_put(obj); |
| - spin_unlock_irqrestore(&cache_lock, flags); |
| + if (atomic_dec_and_test(&obj->refcnt)) |
| + kfree(obj); |
| } |
| |
| void object_get(struct object *obj) |
| { |
| - unsigned long flags; |
| - |
| - spin_lock_irqsave(&cache_lock, flags); |
| - __object_get(obj); |
| - spin_unlock_irqrestore(&cache_lock, flags); |
| + atomic_inc(&obj->refcnt); |
| } |
| |
| /* Must be holding cache_lock */ |
| @@ -65,7 +47,7 @@ |
| { |
| BUG_ON(!obj); |
| list_del(&obj->list); |
| - __object_put(obj); |
| + object_put(obj); |
| cache_num--; |
| } |
| |
| @@ -94,7 +76,7 @@ |
| strscpy(obj->name, name, sizeof(obj->name)); |
| obj->id = id; |
| obj->popularity = 0; |
| - obj->refcnt = 1; /* The cache holds a reference */ |
| + atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ |
| |
| spin_lock_irqsave(&cache_lock, flags); |
| __cache_add(obj); |
| @@ -119,7 +101,7 @@ |
| spin_lock_irqsave(&cache_lock, flags); |
| obj = __cache_find(id); |
| if (obj) |
| - __object_get(obj); |
| + object_get(obj); |
| spin_unlock_irqrestore(&cache_lock, flags); |
| return obj; |
| } |
| |
| Protecting The Objects Themselves |
| --------------------------------- |
| |
| In these examples, we assumed that the objects (except the reference |
| counts) never changed once they are created. If we wanted to allow the |
| name to change, there are three possibilities: |
| |
| - You can make ``cache_lock`` non-static, and tell people to grab that |
| lock before changing the name in any object. |
| |
| - You can provide a cache_obj_rename() which grabs this |
| lock and changes the name for the caller, and tell everyone to use |
| that function. |
| |
| - You can make the ``cache_lock`` protect only the cache itself, and |
| use another lock to protect the name. |
| |
| Theoretically, you can make the locks as fine-grained as one lock for |
| every field, for every object. In practice, the most common variants |
| are: |
| |
| - One lock which protects the infrastructure (the ``cache`` list in |
| this example) and all the objects. This is what we have done so far. |
| |
| - One lock which protects the infrastructure (including the list |
| pointers inside the objects), and one lock inside the object which |
| protects the rest of that object. |
| |
| - Multiple locks to protect the infrastructure (eg. one lock per hash |
| chain), possibly with a separate per-object lock. |
| |
| Here is the "lock-per-object" implementation: |
| |
| :: |
| |
| --- cache.c.refcnt-atomic 2003-12-11 15:50:54.000000000 +1100 |
| +++ cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 |
| @@ -6,11 +6,17 @@ |
| |
| struct object |
| { |
| + /* These two protected by cache_lock. */ |
| struct list_head list; |
| + int popularity; |
| + |
| atomic_t refcnt; |
| + |
| + /* Doesn't change once created. */ |
| int id; |
| + |
| + spinlock_t lock; /* Protects the name */ |
| char name[32]; |
| - int popularity; |
| }; |
| |
| static DEFINE_SPINLOCK(cache_lock); |
| @@ -77,6 +84,7 @@ |
| obj->id = id; |
| obj->popularity = 0; |
| atomic_set(&obj->refcnt, 1); /* The cache holds a reference */ |
| + spin_lock_init(&obj->lock); |
| |
| spin_lock_irqsave(&cache_lock, flags); |
| __cache_add(obj); |
| |
| Note that I decide that the popularity count should be protected by the |
| ``cache_lock`` rather than the per-object lock: this is because it (like |
| the :c:type:`struct list_head <list_head>` inside the object) |
| is logically part of the infrastructure. This way, I don't need to grab |
| the lock of every object in __cache_add() when seeking |
| the least popular. |
| |
| I also decided that the id member is unchangeable, so I don't need to |
| grab each object lock in __cache_find() to examine the |
| id: the object lock is only used by a caller who wants to read or write |
| the name field. |
| |
| Note also that I added a comment describing what data was protected by |
| which locks. This is extremely important, as it describes the runtime |
| behavior of the code, and can be hard to gain from just reading. And as |
| Alan Cox says, “Lock data, not code”. |
| |
| Common Problems |
| =============== |
| |
| Deadlock: Simple and Advanced |
| ----------------------------- |
| |
| There is a coding bug where a piece of code tries to grab a spinlock |
| twice: it will spin forever, waiting for the lock to be released |
| (spinlocks, rwlocks and mutexes are not recursive in Linux). This is |
| trivial to diagnose: not a |
| stay-up-five-nights-talk-to-fluffy-code-bunnies kind of problem. |
| |
| For a slightly more complex case, imagine you have a region shared by a |
| softirq and user context. If you use a spin_lock() call |
| to protect it, it is possible that the user context will be interrupted |
| by the softirq while it holds the lock, and the softirq will then spin |
| forever trying to get the same lock. |
| |
| Both of these are called deadlock, and as shown above, it can occur even |
| with a single CPU (although not on UP compiles, since spinlocks vanish |
| on kernel compiles with ``CONFIG_SMP``\ =n. You'll still get data |
| corruption in the second example). |
| |
| This complete lockup is easy to diagnose: on SMP boxes the watchdog |
| timer or compiling with ``DEBUG_SPINLOCK`` set |
| (``include/linux/spinlock.h``) will show this up immediately when it |
| happens. |
| |
| A more complex problem is the so-called 'deadly embrace', involving two |
| or more locks. Say you have a hash table: each entry in the table is a |
| spinlock, and a chain of hashed objects. Inside a softirq handler, you |
| sometimes want to alter an object from one place in the hash to another: |
| you grab the spinlock of the old hash chain and the spinlock of the new |
| hash chain, and delete the object from the old one, and insert it in the |
| new one. |
| |
| There are two problems here. First, if your code ever tries to move the |
| object to the same chain, it will deadlock with itself as it tries to |
| lock it twice. Secondly, if the same softirq on another CPU is trying to |
| move another object in the reverse direction, the following could |
| happen: |
| |
| +-----------------------+-----------------------+ |
| | CPU 1 | CPU 2 | |
| +=======================+=======================+ |
| | Grab lock A -> OK | Grab lock B -> OK | |
| +-----------------------+-----------------------+ |
| | Grab lock B -> spin | Grab lock A -> spin | |
| +-----------------------+-----------------------+ |
| |
| Table: Consequences |
| |
| The two CPUs will spin forever, waiting for the other to give up their |
| lock. It will look, smell, and feel like a crash. |
| |
| Preventing Deadlock |
| ------------------- |
| |
| Textbooks will tell you that if you always lock in the same order, you |
| will never get this kind of deadlock. Practice will tell you that this |
| approach doesn't scale: when I create a new lock, I don't understand |
| enough of the kernel to figure out where in the 5000 lock hierarchy it |
| will fit. |
| |
| The best locks are encapsulated: they never get exposed in headers, and |
| are never held around calls to non-trivial functions outside the same |
| file. You can read through this code and see that it will never |
| deadlock, because it never tries to grab another lock while it has that |
| one. People using your code don't even need to know you are using a |
| lock. |
| |
| A classic problem here is when you provide callbacks or hooks: if you |
| call these with the lock held, you risk simple deadlock, or a deadly |
| embrace (who knows what the callback will do?). |
| |
| Overzealous Prevention Of Deadlocks |
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
| |
| Deadlocks are problematic, but not as bad as data corruption. Code which |
| grabs a read lock, searches a list, fails to find what it wants, drops |
| the read lock, grabs a write lock and inserts the object has a race |
| condition. |
| |
| Racing Timers: A Kernel Pastime |
| ------------------------------- |
| |
| Timers can produce their own special problems with races. Consider a |
| collection of objects (list, hash, etc) where each object has a timer |
| which is due to destroy it. |
| |
| If you want to destroy the entire collection (say on module removal), |
| you might do the following:: |
| |
| /* THIS CODE BAD BAD BAD BAD: IF IT WAS ANY WORSE IT WOULD USE |
| HUNGARIAN NOTATION */ |
| spin_lock_bh(&list_lock); |
| |
| while (list) { |
| struct foo *next = list->next; |
| timer_delete(&list->timer); |
| kfree(list); |
| list = next; |
| } |
| |
| spin_unlock_bh(&list_lock); |
| |
| |
| Sooner or later, this will crash on SMP, because a timer can have just |
| gone off before the spin_lock_bh(), and it will only get |
| the lock after we spin_unlock_bh(), and then try to free |
| the element (which has already been freed!). |
| |
| This can be avoided by checking the result of |
| timer_delete(): if it returns 1, the timer has been deleted. |
| If 0, it means (in this case) that it is currently running, so we can |
| do:: |
| |
| retry: |
| spin_lock_bh(&list_lock); |
| |
| while (list) { |
| struct foo *next = list->next; |
| if (!timer_delete(&list->timer)) { |
| /* Give timer a chance to delete this */ |
| spin_unlock_bh(&list_lock); |
| goto retry; |
| } |
| kfree(list); |
| list = next; |
| } |
| |
| spin_unlock_bh(&list_lock); |
| |
| |
| Another common problem is deleting timers which restart themselves (by |
| calling add_timer() at the end of their timer function). |
| Because this is a fairly common case which is prone to races, you should |
| use timer_delete_sync() (``include/linux/timer.h``) to handle this case. |
| |
| Before freeing a timer, timer_shutdown() or timer_shutdown_sync() should be |
| called which will keep it from being rearmed. Any subsequent attempt to |
| rearm the timer will be silently ignored by the core code. |
| |
| |
| Locking Speed |
| ============= |
| |
| There are three main things to worry about when considering speed of |
| some code which does locking. First is concurrency: how many things are |
| going to be waiting while someone else is holding a lock. Second is the |
| time taken to actually acquire and release an uncontended lock. Third is |
| using fewer, or smarter locks. I'm assuming that the lock is used fairly |
| often: otherwise, you wouldn't be concerned about efficiency. |
| |
| Concurrency depends on how long the lock is usually held: you should |
| hold the lock for as long as needed, but no longer. In the cache |
| example, we always create the object without the lock held, and then |
| grab the lock only when we are ready to insert it in the list. |
| |
| Acquisition times depend on how much damage the lock operations do to |
| the pipeline (pipeline stalls) and how likely it is that this CPU was |
| the last one to grab the lock (ie. is the lock cache-hot for this CPU): |
| on a machine with more CPUs, this likelihood drops fast. Consider a |
| 700MHz Intel Pentium III: an instruction takes about 0.7ns, an atomic |
| increment takes about 58ns, a lock which is cache-hot on this CPU takes |
| 160ns, and a cacheline transfer from another CPU takes an additional 170 |
| to 360ns. (These figures from Paul McKenney's `Linux Journal RCU |
| article <http://www.linuxjournal.com/article.php?sid=6993>`__). |
| |
| These two aims conflict: holding a lock for a short time might be done |
| by splitting locks into parts (such as in our final per-object-lock |
| example), but this increases the number of lock acquisitions, and the |
| results are often slower than having a single lock. This is another |
| reason to advocate locking simplicity. |
| |
| The third concern is addressed below: there are some methods to reduce |
| the amount of locking which needs to be done. |
| |
| Read/Write Lock Variants |
| ------------------------ |
| |
| Both spinlocks and mutexes have read/write variants: ``rwlock_t`` and |
| :c:type:`struct rw_semaphore <rw_semaphore>`. These divide |
| users into two classes: the readers and the writers. If you are only |
| reading the data, you can get a read lock, but to write to the data you |
| need the write lock. Many people can hold a read lock, but a writer must |
| be sole holder. |
| |
| If your code divides neatly along reader/writer lines (as our cache code |
| does), and the lock is held by readers for significant lengths of time, |
| using these locks can help. They are slightly slower than the normal |
| locks though, so in practice ``rwlock_t`` is not usually worthwhile. |
| |
| Avoiding Locks: Read Copy Update |
| -------------------------------- |
| |
| There is a special method of read/write locking called Read Copy Update. |
| Using RCU, the readers can avoid taking a lock altogether: as we expect |
| our cache to be read more often than updated (otherwise the cache is a |
| waste of time), it is a candidate for this optimization. |
| |
| How do we get rid of read locks? Getting rid of read locks means that |
| writers may be changing the list underneath the readers. That is |
| actually quite simple: we can read a linked list while an element is |
| being added if the writer adds the element very carefully. For example, |
| adding ``new`` to a single linked list called ``list``:: |
| |
| new->next = list->next; |
| wmb(); |
| list->next = new; |
| |
| |
| The wmb() is a write memory barrier. It ensures that the |
| first operation (setting the new element's ``next`` pointer) is complete |
| and will be seen by all CPUs, before the second operation is (putting |
| the new element into the list). This is important, since modern |
| compilers and modern CPUs can both reorder instructions unless told |
| otherwise: we want a reader to either not see the new element at all, or |
| see the new element with the ``next`` pointer correctly pointing at the |
| rest of the list. |
| |
| Fortunately, there is a function to do this for standard |
| :c:type:`struct list_head <list_head>` lists: |
| list_add_rcu() (``include/linux/list.h``). |
| |
| Removing an element from the list is even simpler: we replace the |
| pointer to the old element with a pointer to its successor, and readers |
| will either see it, or skip over it. |
| |
| :: |
| |
| list->next = old->next; |
| |
| |
| There is list_del_rcu() (``include/linux/list.h``) which |
| does this (the normal version poisons the old object, which we don't |
| want). |
| |
| The reader must also be careful: some CPUs can look through the ``next`` |
| pointer to start reading the contents of the next element early, but |
| don't realize that the pre-fetched contents is wrong when the ``next`` |
| pointer changes underneath them. Once again, there is a |
| list_for_each_entry_rcu() (``include/linux/list.h``) |
| to help you. Of course, writers can just use |
| list_for_each_entry(), since there cannot be two |
| simultaneous writers. |
| |
| Our final dilemma is this: when can we actually destroy the removed |
| element? Remember, a reader might be stepping through this element in |
| the list right now: if we free this element and the ``next`` pointer |
| changes, the reader will jump off into garbage and crash. We need to |
| wait until we know that all the readers who were traversing the list |
| when we deleted the element are finished. We use |
| call_rcu() to register a callback which will actually |
| destroy the object once all pre-existing readers are finished. |
| Alternatively, synchronize_rcu() may be used to block |
| until all pre-existing are finished. |
| |
| But how does Read Copy Update know when the readers are finished? The |
| method is this: firstly, the readers always traverse the list inside |
| rcu_read_lock()/rcu_read_unlock() pairs: |
| these simply disable preemption so the reader won't go to sleep while |
| reading the list. |
| |
| RCU then waits until every other CPU has slept at least once: since |
| readers cannot sleep, we know that any readers which were traversing the |
| list during the deletion are finished, and the callback is triggered. |
| The real Read Copy Update code is a little more optimized than this, but |
| this is the fundamental idea. |
| |
| :: |
| |
| --- cache.c.perobjectlock 2003-12-11 17:15:03.000000000 +1100 |
| +++ cache.c.rcupdate 2003-12-11 17:55:14.000000000 +1100 |
| @@ -1,15 +1,18 @@ |
| #include <linux/list.h> |
| #include <linux/slab.h> |
| #include <linux/string.h> |
| +#include <linux/rcupdate.h> |
| #include <linux/mutex.h> |
| #include <asm/errno.h> |
| |
| struct object |
| { |
| - /* These two protected by cache_lock. */ |
| + /* This is protected by RCU */ |
| struct list_head list; |
| int popularity; |
| |
| + struct rcu_head rcu; |
| + |
| atomic_t refcnt; |
| |
| /* Doesn't change once created. */ |
| @@ -40,7 +43,7 @@ |
| { |
| struct object *i; |
| |
| - list_for_each_entry(i, &cache, list) { |
| + list_for_each_entry_rcu(i, &cache, list) { |
| if (i->id == id) { |
| i->popularity++; |
| return i; |
| @@ -49,19 +52,25 @@ |
| return NULL; |
| } |
| |
| +/* Final discard done once we know no readers are looking. */ |
| +static void cache_delete_rcu(void *arg) |
| +{ |
| + object_put(arg); |
| +} |
| + |
| /* Must be holding cache_lock */ |
| static void __cache_delete(struct object *obj) |
| { |
| BUG_ON(!obj); |
| - list_del(&obj->list); |
| - object_put(obj); |
| + list_del_rcu(&obj->list); |
| cache_num--; |
| + call_rcu(&obj->rcu, cache_delete_rcu); |
| } |
| |
| /* Must be holding cache_lock */ |
| static void __cache_add(struct object *obj) |
| { |
| - list_add(&obj->list, &cache); |
| + list_add_rcu(&obj->list, &cache); |
| if (++cache_num > MAX_CACHE_SIZE) { |
| struct object *i, *outcast = NULL; |
| list_for_each_entry(i, &cache, list) { |
| @@ -104,12 +114,11 @@ |
| struct object *cache_find(int id) |
| { |
| struct object *obj; |
| - unsigned long flags; |
| |
| - spin_lock_irqsave(&cache_lock, flags); |
| + rcu_read_lock(); |
| obj = __cache_find(id); |
| if (obj) |
| object_get(obj); |
| - spin_unlock_irqrestore(&cache_lock, flags); |
| + rcu_read_unlock(); |
| return obj; |
| } |
| |
| Note that the reader will alter the popularity member in |
| __cache_find(), and now it doesn't hold a lock. One |
| solution would be to make it an ``atomic_t``, but for this usage, we |
| don't really care about races: an approximate result is good enough, so |
| I didn't change it. |
| |
| The result is that cache_find() requires no |
| synchronization with any other functions, so is almost as fast on SMP as |
| it would be on UP. |
| |
| There is a further optimization possible here: remember our original |
| cache code, where there were no reference counts and the caller simply |
| held the lock whenever using the object? This is still possible: if you |
| hold the lock, no one can delete the object, so you don't need to get |
| and put the reference count. |
| |
| Now, because the 'read lock' in RCU is simply disabling preemption, a |
| caller which always has preemption disabled between calling |
| cache_find() and object_put() does not |
| need to actually get and put the reference count: we could expose |
| __cache_find() by making it non-static, and such |
| callers could simply call that. |
| |
| The benefit here is that the reference count is not written to: the |
| object is not altered in any way, which is much faster on SMP machines |
| due to caching. |
| |
| Per-CPU Data |
| ------------ |
| |
| Another technique for avoiding locking which is used fairly widely is to |
| duplicate information for each CPU. For example, if you wanted to keep a |
| count of a common condition, you could use a spin lock and a single |
| counter. Nice and simple. |
| |
| If that was too slow (it's usually not, but if you've got a really big |
| machine to test on and can show that it is), you could instead use a |
| counter for each CPU, then none of them need an exclusive lock. See |
| DEFINE_PER_CPU(), get_cpu_var() and |
| put_cpu_var() (``include/linux/percpu.h``). |
| |
| Of particular use for simple per-cpu counters is the ``local_t`` type, |
| and the cpu_local_inc() and related functions, which are |
| more efficient than simple code on some architectures |
| (``include/asm/local.h``). |
| |
| Note that there is no simple, reliable way of getting an exact value of |
| such a counter, without introducing more locks. This is not a problem |
| for some uses. |
| |
| Data Which Mostly Used By An IRQ Handler |
| ---------------------------------------- |
| |
| If data is always accessed from within the same IRQ handler, you don't |
| need a lock at all: the kernel already guarantees that the irq handler |
| will not run simultaneously on multiple CPUs. |
| |
| Manfred Spraul points out that you can still do this, even if the data |
| is very occasionally accessed in user context or softirqs/tasklets. The |
| irq handler doesn't use a lock, and all other accesses are done as so:: |
| |
| mutex_lock(&lock); |
| disable_irq(irq); |
| ... |
| enable_irq(irq); |
| mutex_unlock(&lock); |
| |
| The disable_irq() prevents the irq handler from running |
| (and waits for it to finish if it's currently running on other CPUs). |
| The spinlock prevents any other accesses happening at the same time. |
| Naturally, this is slower than just a spin_lock_irq() |
| call, so it only makes sense if this type of access happens extremely |
| rarely. |
| |
| What Functions Are Safe To Call From Interrupts? |
| ================================================ |
| |
| Many functions in the kernel sleep (ie. call schedule()) directly or |
| indirectly: you can never call them while holding a spinlock, or with |
| preemption disabled. This also means you need to be in user context: |
| calling them from an interrupt is illegal. |
| |
| Some Functions Which Sleep |
| -------------------------- |
| |
| The most common ones are listed below, but you usually have to read the |
| code to find out if other calls are safe. If everyone else who calls it |
| can sleep, you probably need to be able to sleep, too. In particular, |
| registration and deregistration functions usually expect to be called |
| from user context, and can sleep. |
| |
| - Accesses to userspace: |
| |
| - copy_from_user() |
| |
| - copy_to_user() |
| |
| - get_user() |
| |
| - put_user() |
| |
| - kmalloc(GP_KERNEL) <kmalloc>` |
| |
| - mutex_lock_interruptible() and |
| mutex_lock() |
| |
| There is a mutex_trylock() which does not sleep. |
| Still, it must not be used inside interrupt context since its |
| implementation is not safe for that. mutex_unlock() |
| will also never sleep. It cannot be used in interrupt context either |
| since a mutex must be released by the same task that acquired it. |
| |
| Some Functions Which Don't Sleep |
| -------------------------------- |
| |
| Some functions are safe to call from any context, or holding almost any |
| lock. |
| |
| - printk() |
| |
| - kfree() |
| |
| - add_timer() and timer_delete() |
| |
| Mutex API reference |
| =================== |
| |
| .. kernel-doc:: include/linux/mutex.h |
| :internal: |
| |
| .. kernel-doc:: kernel/locking/mutex.c |
| :export: |
| |
| Futex API reference |
| =================== |
| |
| .. kernel-doc:: kernel/futex/core.c |
| :internal: |
| |
| .. kernel-doc:: kernel/futex/futex.h |
| :internal: |
| |
| .. kernel-doc:: kernel/futex/pi.c |
| :internal: |
| |
| .. kernel-doc:: kernel/futex/requeue.c |
| :internal: |
| |
| .. kernel-doc:: kernel/futex/waitwake.c |
| :internal: |
| |
| Further reading |
| =============== |
| |
| - ``Documentation/locking/spinlocks.rst``: Linus Torvalds' spinlocking |
| tutorial in the kernel sources. |
| |
| - Unix Systems for Modern Architectures: Symmetric Multiprocessing and |
| Caching for Kernel Programmers: |
| |
| Curt Schimmel's very good introduction to kernel level locking (not |
| written for Linux, but nearly everything applies). The book is |
| expensive, but really worth every penny to understand SMP locking. |
| [ISBN: 0201633388] |
| |
| Thanks |
| ====== |
| |
| Thanks to Telsa Gwynne for DocBooking, neatening and adding style. |
| |
| Thanks to Martin Pool, Philipp Rumpf, Stephen Rothwell, Paul Mackerras, |
| Ruedi Aschwanden, Alan Cox, Manfred Spraul, Tim Waugh, Pete Zaitcev, |
| James Morris, Robert Love, Paul McKenney, John Ashby for proofreading, |
| correcting, flaming, commenting. |
| |
| Thanks to the cabal for having no influence on this document. |
| |
| Glossary |
| ======== |
| |
| preemption |
| Prior to 2.5, or when ``CONFIG_PREEMPT`` is unset, processes in user |
| context inside the kernel would not preempt each other (ie. you had that |
| CPU until you gave it up, except for interrupts). With the addition of |
| ``CONFIG_PREEMPT`` in 2.5.4, this changed: when in user context, higher |
| priority tasks can "cut in": spinlocks were changed to disable |
| preemption, even on UP. |
| |
| bh |
| Bottom Half: for historical reasons, functions with '_bh' in them often |
| now refer to any software interrupt, e.g. spin_lock_bh() |
| blocks any software interrupt on the current CPU. Bottom halves are |
| deprecated, and will eventually be replaced by tasklets. Only one bottom |
| half will be running at any time. |
| |
| Hardware Interrupt / Hardware IRQ |
| Hardware interrupt request. in_hardirq() returns true in a |
| hardware interrupt handler. |
| |
| Interrupt Context |
| Not user context: processing a hardware irq or software irq. Indicated |
| by the in_interrupt() macro returning true. |
| |
| SMP |
| Symmetric Multi-Processor: kernels compiled for multiple-CPU machines. |
| (``CONFIG_SMP=y``). |
| |
| Software Interrupt / softirq |
| Software interrupt handler. in_hardirq() returns false; |
| in_softirq() returns true. Tasklets and softirqs both |
| fall into the category of 'software interrupts'. |
| |
| Strictly speaking a softirq is one of up to 32 enumerated software |
| interrupts which can run on multiple CPUs at once. Sometimes used to |
| refer to tasklets as well (ie. all software interrupts). |
| |
| tasklet |
| A dynamically-registrable software interrupt, which is guaranteed to |
| only run on one CPU at a time. |
| |
| timer |
| A dynamically-registrable software interrupt, which is run at (or close |
| to) a given time. When running, it is just like a tasklet (in fact, they |
| are called from the ``TIMER_SOFTIRQ``). |
| |
| UP |
| Uni-Processor: Non-SMP. (``CONFIG_SMP=n``). |
| |
| User Context |
| The kernel executing on behalf of a particular process (ie. a system |
| call or trap) or kernel thread. You can tell which process with the |
| ``current`` macro.) Not to be confused with userspace. Can be |
| interrupted by software or hardware interrupts. |
| |
| Userspace |
| A process executing its own code outside the kernel. |
