blob: 924c14e0bfdefd1bfccf040200f58f8ec86bd351 [file] [log] [blame]
/* rwsem.c: R/W semaphores: contention handling functions
*
* Written by David Howells (dhowells@redhat.com).
* Derived from arch/i386/kernel/semaphore.c
*/
#include <linux/rwsem.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/module.h>
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
unsigned int flags;
#define RWSEM_WAITING_FOR_READ 0x00000001
#define RWSEM_WAITING_FOR_WRITE 0x00000002
};
#if RWSEM_DEBUG
#undef rwsemtrace
void rwsemtrace(struct rw_semaphore *sem, const char *str)
{
printk("sem=%p\n", sem);
printk("(sem)=%08lx\n", sem->count);
if (sem->debug)
printk("[%d] %s({%08lx})\n", current->pid, str, sem->count);
}
#endif
/*
* handle the lock release when processes blocked on it that can now run
* - if we come here from up_xxxx(), then:
* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
* - there must be someone on the queue
* - the spinlock must be held by the caller
* - woken process blocks are discarded from the list after having task zeroed
* - writers are only woken if downgrading is false
*/
static inline struct rw_semaphore *
__rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
{
struct rwsem_waiter *waiter;
struct task_struct *tsk;
struct list_head *next;
signed long oldcount, woken, loop;
rwsemtrace(sem, "Entering __rwsem_do_wake");
if (downgrading)
goto dont_wake_writers;
/* if we came through an up_xxxx() call, we only only wake someone up
* if we can transition the active part of the count from 0 -> 1
*/
try_again:
oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
- RWSEM_ACTIVE_BIAS;
if (oldcount & RWSEM_ACTIVE_MASK)
goto undo;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
/* try to grant a single write lock if there's a writer at the front
* of the queue - note we leave the 'active part' of the count
* incremented by 1 and the waiting part incremented by 0x00010000
*/
if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
goto readers_only;
/* We must be careful not to touch 'waiter' after we set ->task = NULL.
* It is an allocated on the waiter's stack and may become invalid at
* any time after that point (due to a wakeup from another source).
*/
list_del(&waiter->list);
tsk = waiter->task;
mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
goto out;
/* don't want to wake any writers */
dont_wake_writers:
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
goto out;
/* grant an infinite number of read locks to the readers at the front
* of the queue
* - note we increment the 'active part' of the count by the number of
* readers before waking any processes up
*/
readers_only:
woken = 0;
do {
woken++;
if (waiter->list.next == &sem->wait_list)
break;
waiter = list_entry(waiter->list.next,
struct rwsem_waiter, list);
} while (waiter->flags & RWSEM_WAITING_FOR_READ);
loop = woken;
woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
if (!downgrading)
/* we'd already done one increment earlier */
woken -= RWSEM_ACTIVE_BIAS;
rwsem_atomic_add(woken, sem);
next = sem->wait_list.next;
for (; loop > 0; loop--) {
waiter = list_entry(next, struct rwsem_waiter, list);
next = waiter->list.next;
tsk = waiter->task;
mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
}
sem->wait_list.next = next;
next->prev = &sem->wait_list;
out:
rwsemtrace(sem, "Leaving __rwsem_do_wake");
return sem;
/* undo the change to count, but check for a transition 1->0 */
undo:
if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) != 0)
goto out;
goto try_again;
}
/*
* wait for a lock to be granted
*/
static inline struct rw_semaphore *
rwsem_down_failed_common(struct rw_semaphore *sem,
struct rwsem_waiter *waiter, signed long adjustment)
{
struct task_struct *tsk = current;
signed long count;
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
/* set up my own style of waitqueue */
spin_lock(&sem->wait_lock);
waiter->task = tsk;
get_task_struct(tsk);
list_add_tail(&waiter->list, &sem->wait_list);
/* we're now waiting on the lock, but no longer actively read-locking */
count = rwsem_atomic_update(adjustment, sem);
/* if there are no active locks, wake the front queued process(es) up */
if (!(count & RWSEM_ACTIVE_MASK))
sem = __rwsem_do_wake(sem, 0);
spin_unlock(&sem->wait_lock);
/* wait to be given the lock */
for (;;) {
if (!waiter->task)
break;
schedule();
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
}
tsk->state = TASK_RUNNING;
return sem;
}
/*
* wait for the read lock to be granted
*/
struct rw_semaphore fastcall __sched *
rwsem_down_read_failed(struct rw_semaphore *sem)
{
struct rwsem_waiter waiter;
rwsemtrace(sem, "Entering rwsem_down_read_failed");
waiter.flags = RWSEM_WAITING_FOR_READ;
rwsem_down_failed_common(sem, &waiter,
RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
rwsemtrace(sem, "Leaving rwsem_down_read_failed");
return sem;
}
/*
* wait for the write lock to be granted
*/
struct rw_semaphore fastcall __sched *
rwsem_down_write_failed(struct rw_semaphore *sem)
{
struct rwsem_waiter waiter;
rwsemtrace(sem, "Entering rwsem_down_write_failed");
waiter.flags = RWSEM_WAITING_FOR_WRITE;
rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
rwsemtrace(sem, "Leaving rwsem_down_write_failed");
return sem;
}
/*
* handle waking up a waiter on the semaphore
* - up_read/up_write has decremented the active part of count if we come here
*/
struct rw_semaphore fastcall *rwsem_wake(struct rw_semaphore *sem)
{
rwsemtrace(sem, "Entering rwsem_wake");
spin_lock(&sem->wait_lock);
/* do nothing if list empty */
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, 0);
spin_unlock(&sem->wait_lock);
rwsemtrace(sem, "Leaving rwsem_wake");
return sem;
}
/*
* downgrade a write lock into a read lock
* - caller incremented waiting part of count and discovered it still negative
* - just wake up any readers at the front of the queue
*/
struct rw_semaphore fastcall *rwsem_downgrade_wake(struct rw_semaphore *sem)
{
rwsemtrace(sem, "Entering rwsem_downgrade_wake");
spin_lock(&sem->wait_lock);
/* do nothing if list empty */
if (!list_empty(&sem->wait_list))
sem = __rwsem_do_wake(sem, 1);
spin_unlock(&sem->wait_lock);
rwsemtrace(sem, "Leaving rwsem_downgrade_wake");
return sem;
}
EXPORT_SYMBOL(rwsem_down_read_failed);
EXPORT_SYMBOL(rwsem_down_write_failed);
EXPORT_SYMBOL(rwsem_wake);
EXPORT_SYMBOL(rwsem_downgrade_wake);
#if RWSEM_DEBUG
EXPORT_SYMBOL(rwsemtrace);
#endif