Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider: - [Critical] Dropping `ce->signal_lock` to avoid lock inversion in `i915_request_enable_breadcrumb` allows concurrent threads to double-add a context to the `b->signalers` list, corrupting the list and leaking `intel_context` references. --
commit 51101492996df475bcd91b7f36bd0af9680441f1 Author: Sebastian Andrzej Siewior <[email protected]> drm/i915/gt: Use signalers_lock to prevent starvation of irq_work. This patch adds a global spinlock to prevent starvation of irq_work by the threaded interrupt and removes the existing RCU handling. > diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c > b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c > index c10ac0ab3bfa8..0ae7759dc74a3 100644 > --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c > +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c > @@ -88,24 +88,25 @@ static void add_signaling_context(struct > intel_breadcrumbs *b, > struct intel_context *ce) > { > lockdep_assert_held(&ce->signal_lock); > + lockdep_assert_held(&b->signalers_lock); > > - spin_lock(&b->signalers_lock); > - list_add_rcu(&ce->signal_link, &b->signalers); > - spin_unlock(&b->signalers_lock); > + if (list_empty(&ce->signals)) > + return; > + > + intel_context_get(ce); > + list_add(&ce->signal_link, &b->signalers); > } [Severity: Critical] Could this unconditionally corrupt the signalers list? If add_signaling_context() is called concurrently for the same context, it will call list_add() without verifying whether ce->signal_link is already linked, leading to list corruption and a leak of intel_context objects. [ ... ] > @@ -427,30 +424,87 @@ bool i915_request_enable_breadcrumb(struct i915_request > *rq) > return true; > > spin_lock(&ce->signal_lock); > + b = READ_ONCE(rq->engine)->breadcrumbs; > + > if (test_bit(I915_FENCE_FLAG_ACTIVE, &rq->fence.flags)) > - insert_breadcrumb(rq); > + add_context = insert_breadcrumb(rq, b); > + > + if (add_context && spin_trylock(&b->signalers_lock)) { > + add_signaling_context(b, ce); > + spin_unlock(&b->signalers_lock); > + add_context = false; > + } > spin_unlock(&ce->signal_lock); > > + if (add_context) { > + /* > + * Fast trylock didn't work, use slow locking. > + * > + * Dropping the lock to solve the inversion is safe, since > + * no race is possible against remove_signaling_context() > + * without being added as signaling context. > + */ > + spin_lock(&b->signalers_lock); > + spin_lock(&ce->signal_lock); > + add_signaling_context(b, ce); > + spin_unlock(&ce->signal_lock); > + spin_unlock(&b->signalers_lock); > + } [Severity: Critical] Does dropping ce->signal_lock here create a race condition that leads to the double-add in add_signaling_context()? If multiple threads attempt to enable breadcrumbs for the same context and fail the fast trylock, they will drop ce->signal_lock and enter the slow path. This can allow a sequence where: Thread 1 calls i915_request_enable_breadcrumb() for rq1, gets add_context = true, fails the trylock, drops ce->signal_lock, and waits on the slow path. Thread 2 cancels rq1, making ce->signals empty again. Thread 3 calls i915_request_enable_breadcrumb() for rq3. Because ce->signals is empty, it also gets add_context = true. It fails the trylock, drops the lock, and waits on the slow path. Thread 1 and Thread 3 sequentially acquire the locks in the slow path and both call add_signaling_context(). Since Thread 3 added rq3, ce->signals is not empty. Both threads will then execute intel_context_get() and list_add() unconditionally, permanently corrupting the doubly-linked list. Should add_signaling_context() verify that list_empty(&ce->signal_link) is true before calling list_add() to prevent this regression? -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=7
