From 25f13b4040b68dfc5a2a22e7234894e718e3f4c5 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?Nicolai=20H=C3=A4hnle?= Date: Wed, 21 Dec 2016 19:46:37 +0100 Subject: locking/ww_mutex: Re-check ww->ctx in the inner optimistic spin loop MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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. Signed-off-by: Nicolai Hähnle Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Chris Wilson Cc: Daniel Vetter Cc: Linus Torvalds Cc: Maarten Lankhorst Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: dri-devel@lists.freedesktop.org Link: http://lkml.kernel.org/r/1482346000-9927-10-git-send-email-nhaehnle@gmail.com Signed-off-by: Ingo Molnar --- kernel/locking/mutex.c | 49 +++++++++++++++++++++++++++++-------------------- 1 file changed, 29 insertions(+), 20 deletions(-) (limited to 'kernel') diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c index 43ff6110014c..41b0406069e8 100644 --- a/kernel/locking/mutex.c +++ b/kernel/locking/mutex.c @@ -372,11 +372,14 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx) #ifdef CONFIG_MUTEX_SPIN_ON_OWNER /* - * Look out! "owner" is an entirely speculative pointer - * access and not reliable. + * Look out! "owner" is an entirely speculative pointer access and not + * reliable. + * + * "noinline" so that this function shows up on perf profiles. */ 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, + struct ww_acquire_ctx *ww_ctx) { bool ret = true; @@ -399,6 +402,28 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner) break; } + if (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(); @@ -484,22 +509,6 @@ mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx, for (;;) { struct task_struct *owner; - if (use_ww_ctx && 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; - } - /* Try to acquire the mutex... */ owner = __mutex_trylock_or_owner(lock); if (!owner) @@ -509,7 +518,7 @@ mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx, * There's an owner, wait for it to either * release the lock or go to sleep. */ - if (!mutex_spin_on_owner(lock, owner)) + if (!mutex_spin_on_owner(lock, owner, ww_ctx)) goto fail_unlock; /* -- cgit v1.2.1