drm/i915: Reduce nested prepare_remote_context() to a trylock (3cc44feb) · Commits · 戴 / test

On context retiring, we may invoke the kernel_context to unpin this context. Elsewhere, we may use the kernel_context to modify this context. This currently leads to an AB-BA lock inversion, so we need to back-off from the contended lock, and repeat. Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111732 Signed-off-by:

Chris Wilson <chris@chris-wilson.co.uk> Fixes: ("drm/i915/oa: Reconfigure contexts on the fly") Reviewed-by:

Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20191126065521.2331017-1-chris@chris-wilson.co.uk (cherry picked from commit ) Signed-off-by:

Joonas Lahtinen <joonas.lahtinen@linux.intel.com>

drivers/gpu/drm/i915/gt/intel_context.c

+17 −4

Original line number	Diff line number	Diff line
		@@ -310,10 +310,23 @@ int intel_context_prepare_remote_request(struct intel_context *ce,
		GEM_BUG_ON(rq->hw_context == ce);

		if (rcu_access_pointer(rq->timeline) != tl) { /* timeline sharing! */
		err = mutex_lock_interruptible_nested(&tl->mutex,
		SINGLE_DEPTH_NESTING);
		if (err)
		return err;
		/*
		* Ideally, we just want to insert our foreign fence as
		* a barrier into the remove context, such that this operation
		* occurs after all current operations in that context, and
		* all future operations must occur after this.
		*
		* Currently, the timeline->last_request tracking is guarded
		* by its mutex and so we must obtain that to atomically
		* insert our barrier. However, since we already hold our
		* timeline->mutex, we must be careful against potential
		* inversion if we are the kernel_context as the remote context
		* will itself poke at the kernel_context when it needs to
		* unpin. Ergo, if already locked, we drop both locks and
		* try again (through the magic of userspace repeating EAGAIN).
		*/
		if (!mutex_trylock(&tl->mutex))
		return -EAGAIN;

		/* Queue this switch after current activity by this context. */
		err = i915_active_fence_set(&tl->last_request, rq);

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