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From: Joakim Hernberg <jhernberg@alchemy.lu>
Date: Sat, 19 Mar 2016 13:03:55 +0100
Subject: [PATCH] Fix a race in the PRT wait for completion simple wait code
for NVIDIA on the rt patch
-Note refactored again 160319.
-NOTE: this patch is a rebase of John Blackwood's patch. On his kernel, he must be using
-an older simple wait patch - as his applies to kernel/sched/core.c, while the simple wait
-completion code lives in kernel/sched/completion.c ... I have ported this to test with
-nvidia, as i would like to see if it fixes the semaphore issues i have seen.
-I've kept the original patch comment in tact;
I'm not 100% sure that the patch below will fix your problem, but we
saw something that sounds pretty familiar to your issue involving the
nvidia driver and the preempt-rt patch. The nvidia driver uses the
completion support to create their own driver's notion of an internally
used semaphore.
Fix a race in the PRT wait for completion simple wait code.
A wait_for_completion() waiter task can be awoken by a task calling
complete(), but fail to consume the 'done' completion resource if it
looses a race with another task calling wait_for_completion() just as
it is waking up.
In this case, the awoken task will call schedule_timeout() again
without being in the simple wait queue.
So if the awoken task is unable to claim the 'done' completion resource,
check to see if it needs to be re-inserted into the wait list before
waiting again in schedule_timeout().
Fix-by: John Blackwood <john.blackwood@ccur.com>
---
kernel/sched/completion.c | 9 +++++++++
1 file changed, 9 insertions(+)
diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index b62cf64..c01bbd8 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -61,11 +61,19 @@ static inline long __sched
do_wait_for_common(struct completion *x,
long (*action)(long), long timeout, int state)
{
+ int again = 0;
+
if (!x->done) {
DECLARE_SWAITQUEUE(wait);
__prepare_to_swait(&x->wait, &wait);
do {
+ /* Check to see if we lost race for 'done' and are
+ * no longer in the wait list.
+ */
+ if (unlikely(again) && list_empty(&wait.task_list))
+ __prepare_to_swait(&x->wait, &wait);
+
if (signal_pending_state(state, current)) {
timeout = -ERESTARTSYS;
break;
@@ -74,6 +82,7 @@ do_wait_for_common(struct completion *x,
raw_spin_unlock_irq(&x->wait.lock);
timeout = action(timeout);
raw_spin_lock_irq(&x->wait.lock);
+ again = 1;
} while (!x->done && timeout);
__finish_swait(&x->wait, &wait);
if (!x->done)
--
2.7.2
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