| From: Steven Rostedt <rostedt@goodmis.org> |
| Date: Fri, 8 Sep 2017 16:27:56 -0400 |
| Subject: [PATCH] tracing: Add trace_event_buffer_reserve() variant that allows |
| recursion |
| |
| On Tue, 5 Sep 2017 16:57:52 -0500 |
| Tom Zanussi <tom.zanussi@linux.intel.com> wrote: |
| |
| > Synthetic event generation requires the reservation of a second event |
| > while the reservation of a previous event is still in progress. The |
| > trace_recursive_lock() check in ring_buffer_lock_reserve() prevents |
| > this however. |
| > |
| > This sets up a special reserve pathway for this particular case, |
| > leaving existing pathways untouched, other than an additional check in |
| > ring_buffer_lock_reserve() and trace_event_buffer_reserve(). These |
| > checks could be gotten rid of as well, with copies of those functions, |
| > but for now try to avoid that unless necessary. |
| > |
| > Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com> |
| |
| I've been planing on changing that lock, which may help you here |
| without having to mess around with parameters. That is to simply add a |
| counter. Would this patch help you. You can add a patch to increment |
| the count to 5 with an explanation of handling synthetic events, but |
| even getting to 4 is extremely unlikely. |
| |
| I'll make this into an official patch if this works for you, and then |
| you can include it in your series. |
| |
| -- Steve |
| |
| Link: https://lkml.kernel.org/r/20170908162756.74c6be8a@gandalf.local.home |
| Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> |
| --- |
| kernel/trace/ring_buffer.c | 64 ++++++++++++---------------------------------- |
| 1 file changed, 17 insertions(+), 47 deletions(-) |
| |
| diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c |
| index ac34ea2b2a10..29430b81c674 100644 |
| --- a/kernel/trace/ring_buffer.c |
| +++ b/kernel/trace/ring_buffer.c |
| @@ -2581,61 +2581,29 @@ rb_wakeups(struct ring_buffer *buffer, struct ring_buffer_per_cpu *cpu_buffer) |
| * The lock and unlock are done within a preempt disable section. |
| * The current_context per_cpu variable can only be modified |
| * by the current task between lock and unlock. But it can |
| - * be modified more than once via an interrupt. To pass this |
| - * information from the lock to the unlock without having to |
| - * access the 'in_interrupt()' functions again (which do show |
| - * a bit of overhead in something as critical as function tracing, |
| - * we use a bitmask trick. |
| + * be modified more than once via an interrupt. There are four |
| + * different contexts that we need to consider. |
| * |
| - * bit 0 = NMI context |
| - * bit 1 = IRQ context |
| - * bit 2 = SoftIRQ context |
| - * bit 3 = normal context. |
| + * Normal context. |
| + * SoftIRQ context |
| + * IRQ context |
| + * NMI context |
| * |
| - * This works because this is the order of contexts that can |
| - * preempt other contexts. A SoftIRQ never preempts an IRQ |
| - * context. |
| - * |
| - * When the context is determined, the corresponding bit is |
| - * checked and set (if it was set, then a recursion of that context |
| - * happened). |
| - * |
| - * On unlock, we need to clear this bit. To do so, just subtract |
| - * 1 from the current_context and AND it to itself. |
| - * |
| - * (binary) |
| - * 101 - 1 = 100 |
| - * 101 & 100 = 100 (clearing bit zero) |
| - * |
| - * 1010 - 1 = 1001 |
| - * 1010 & 1001 = 1000 (clearing bit 1) |
| - * |
| - * The least significant bit can be cleared this way, and it |
| - * just so happens that it is the same bit corresponding to |
| - * the current context. |
| + * If for some reason the ring buffer starts to recurse, we |
| + * only allow that to happen at most 4 times (one for each |
| + * context). If it happens 5 times, then we consider this a |
| + * recusive loop and do not let it go further. |
| */ |
| |
| static __always_inline int |
| trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer) |
| { |
| - unsigned int val = cpu_buffer->current_context; |
| - int bit; |
| - |
| - if (in_interrupt()) { |
| - if (in_nmi()) |
| - bit = RB_CTX_NMI; |
| - else if (in_irq()) |
| - bit = RB_CTX_IRQ; |
| - else |
| - bit = RB_CTX_SOFTIRQ; |
| - } else |
| - bit = RB_CTX_NORMAL; |
| - |
| - if (unlikely(val & (1 << bit))) |
| + if (cpu_buffer->current_context >= 4) |
| return 1; |
| |
| - val |= (1 << bit); |
| - cpu_buffer->current_context = val; |
| + cpu_buffer->current_context++; |
| + /* Interrupts must see this update */ |
| + barrier(); |
| |
| return 0; |
| } |
| @@ -2643,7 +2611,9 @@ trace_recursive_lock(struct ring_buffer_per_cpu *cpu_buffer) |
| static __always_inline void |
| trace_recursive_unlock(struct ring_buffer_per_cpu *cpu_buffer) |
| { |
| - cpu_buffer->current_context &= cpu_buffer->current_context - 1; |
| + /* Don't let the dec leak out */ |
| + barrier(); |
| + cpu_buffer->current_context--; |
| } |
| |
| /** |
| -- |
| 2.14.1 |
| |