From: Nicolai Hähnle <Nicolai.Haehnle@xxxxxxx>
When ww_mutex_set_context_slowpath runs, we are in one of two situations:
1. The current task was woken up by ww_mutex_unlock.
2. The current task is racing with ww_mutex_unlock: We entered the slow
path while lock->base.count <= 0, but skipped the wait in
__mutex_lock_common.
In both cases, all tasks that are currently contending for the lock are
either racing as in point 2 and blocked on lock->wait_lock, or they have
been or will be woken up by ww_mutex_unlock.
Either way, they will check their stamp against ours without having to be
woken up again.
This improvement is possible only after the changed behavior of
ww_mutex_unlock from the previous patch ("locking/ww_mutex: Fix a deadlock
affecting ww_mutexes").
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Nicolai Hähnle <nicolai.haehnle@xxxxxxx>
---
kernel/locking/mutex.c | 17 ++++-------------
1 file changed, 4 insertions(+), 13 deletions(-)
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 7fbf9b4..7c09376 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -192,8 +192,10 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock,
}
/*
- * After acquiring lock in the slowpath set ctx and wake up any
- * waiters so they can recheck.
+ * After acquiring lock in the slowpath set ctx.
+ *
+ * Unlike the fast path, other waiters are already woken up by ww_mutex_unlock,
+ * so we don't have to do it again here.
*
* Callers must hold the mutex wait_lock.
*/
@@ -201,19 +203,8 @@ static __always_inline void
ww_mutex_set_context_slowpath(struct ww_mutex *lock,
struct ww_acquire_ctx *ctx)
{
- struct mutex_waiter *cur;
-
ww_mutex_lock_acquired(lock, ctx);
lock->ctx = ctx;
-
- /*
- * Give any possible sleeping processes the chance to wake up,
- * so they can recheck if they have to back off.
- */
- list_for_each_entry(cur, &lock->base.wait_list, list) {
- debug_mutex_wake_waiter(&lock->base, cur);
- wake_up_process(cur->task);
- }
}
#ifdef CONFIG_MUTEX_SPIN_ON_OWNER
--
2.7.4
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