[PATCH v2 02/11] locking/ww_mutex: Re-check ww->ctx in the inner optimistic spin loop

[Nicolai HÃhnle]

From: Nicolai HÃhnle <Nicolai.Haehnle@xxxxxxx>

In the following scenario, thread #1 should back off its attempt to lock
ww1 and unlock ww2 (assuming the acquire context stamps are ordered
accordingly).

    Thread #0               Thread #1
    ---------               ---------
                            successfully lock ww2
    set ww1->base.owner
                            attempt to lock ww1
                            confirm ww1->ctx == NULL
                            enter mutex_spin_on_owner
    set ww1->ctx

What was likely to happen previously is:

    attempt to lock ww2
    refuse to spin because
      ww2->ctx != NULL
    schedule()
                            detect thread #0 is off CPU
                            stop optimistic spin
                            return -EDEADLK
                            unlock ww2
                            wakeup thread #0
    lock ww2

Now, we are more likely to see:

                            detect ww1->ctx != NULL
                            stop optimistic spin
                            return -EDEADLK
                            unlock ww2
    successfully lock ww2

... because thread #1 will stop its optimistic spin as soon as possible.

The whole scenario is quite unlikely, since it requires thread #1 to get
between thread #0 setting the owner and setting the ctx. But since we're
idling here anyway, the additional check is basically free.

Found by inspection.

Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Maarten Lankhorst <dev@xxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel@xxxxxxxx>
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Nicolai HÃhnle <Nicolai.Haehnle@xxxxxxx>
---
 kernel/locking/mutex.c | 44 ++++++++++++++++++++++++++------------------
 1 file changed, 26 insertions(+), 18 deletions(-)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 9b34961..0afa998 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -350,7 +350,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
  * access and not reliable.
  */
 static noinline
-bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
+bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
+			 bool use_ww_ctx, struct ww_acquire_ctx *ww_ctx)
 {
 	bool ret = true;
 
@@ -373,6 +374,28 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
 			break;
 		}
 
+		if (use_ww_ctx && ww_ctx->acquired > 0) {
+			struct ww_mutex *ww;
+
+			ww = container_of(lock, struct ww_mutex, base);
+
+			/*
+			 * If ww->ctx is set the contents are undefined, only
+			 * by acquiring wait_lock there is a guarantee that
+			 * they are not invalid when reading.
+			 *
+			 * As such, when deadlock detection needs to be
+			 * performed the optimistic spinning cannot be done.
+			 *
+			 * Check this in every inner iteration because we may
+			 * be racing against another thread's ww_mutex_lock.
+			 */
+			if (READ_ONCE(ww->ctx)) {
+				ret = false;
+				break;
+			}
+		}
+
 		cpu_relax();
 	}
 	rcu_read_unlock();
@@ -460,22 +483,6 @@ static bool mutex_optimistic_spin(struct mutex *lock,
 	for (;;) {
 		struct task_struct *owner;
 
-		if (use_ww_ctx && ww_ctx->acquired > 0) {
-			struct ww_mutex *ww;
-
-			ww = container_of(lock, struct ww_mutex, base);
-			/*
-			 * If ww->ctx is set the contents are undefined, only
-			 * by acquiring wait_lock there is a guarantee that
-			 * they are not invalid when reading.
-			 *
-			 * As such, when deadlock detection needs to be
-			 * performed the optimistic spinning cannot be done.
-			 */
-			if (READ_ONCE(ww->ctx))
-				goto fail_unlock;
-		}
-
 		/*
 		 * If there's an owner, wait for it to either
 		 * release the lock or go to sleep.
@@ -487,7 +494,8 @@ static bool mutex_optimistic_spin(struct mutex *lock,
 				break;
 			}
 
-			if (!mutex_spin_on_owner(lock, owner))
+			if (!mutex_spin_on_owner(lock, owner, use_ww_ctx,
+						 ww_ctx))
 				goto fail_unlock;
 		}
 
-- 
2.7.4