From 7c34bb03983e3c1e42ad2749514dec9e5a19c336 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Sun, 26 Jan 2020 10:23:43 +0000 Subject: [PATCH] drm/i915: Tighten atomicity of i915_active_acquire vs i915_active_release As we use a mutex to serialise the first acquire (as it may be a lengthy operation), but only an atomic decrement for the release, we have to be careful in case a second thread races and completes both acquire/release as the first finishes its acquire. Thread A Thread B i915_active_acquire i915_active_acquire atomic_read() == 0 atomic_read() == 0 mutex_lock() mutex_lock() atomic_read() == 0 ref->active(); atomic_inc() mutex_unlock() atomic_read() == 1 i915_active_release atomic_dec_and_test() -> 0 ref->retire() atomic_inc() -> 1 mutex_unlock() So thread A has acquired the ref->active_count but since the ref was still active at the time, it did not initialise it. By switching the check inside the mutex to an atomic increment only if already active, we close the race. Fixes: c9ad602feabe ("drm/i915: Split i915_active.mutex into an irq-safe spinlock for the rbtree") Signed-off-by: Chris Wilson Reviewed-by: Tvrtko Ursulin Link: https://patchwork.freedesktop.org/patch/msgid/20200126102346.1877661-3-chris@chris-wilson.co.uk (cherry picked from commit ac0e331a628b5ded087eab09fad2ffb082ac61ba) Signed-off-by: Jani Nikula --- drivers/gpu/drm/i915/i915_active.c | 16 +++++++++------- 1 file changed, 9 insertions(+), 7 deletions(-) diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c index f3da5c06f331..4fcd567ff818 100644 --- a/drivers/gpu/drm/i915/i915_active.c +++ b/drivers/gpu/drm/i915/i915_active.c @@ -416,13 +416,15 @@ int i915_active_acquire(struct i915_active *ref) if (err) return err; - if (!atomic_read(&ref->count) && ref->active) - err = ref->active(ref); - if (!err) { - spin_lock_irq(&ref->tree_lock); /* vs __active_retire() */ - debug_active_activate(ref); - atomic_inc(&ref->count); - spin_unlock_irq(&ref->tree_lock); + if (likely(!i915_active_acquire_if_busy(ref))) { + if (ref->active) + err = ref->active(ref); + if (!err) { + spin_lock_irq(&ref->tree_lock); /* __active_retire() */ + debug_active_activate(ref); + atomic_inc(&ref->count); + spin_unlock_irq(&ref->tree_lock); + } } mutex_unlock(&ref->mutex); -- 2.11.0