dma-buf: Restart reservation_object_test_signaled_rcu() after writes (b68d8379) · Commits · 戴 / test

In order to be completely generic, we have to double check the read seqlock after acquiring a reference to the fence. If the driver is allocating fences from a SLAB_DESTROY_BY_RCU, or similar freelist, then within an RCU grace period a fence may be freed and reallocated. The RCU read side critical section does not prevent this reallocation, instead we have to inspect the reservation's seqlock to double check if the fences have been reassigned as we were acquiring our reference. Signed-off-by:

Chris Wilson <chris@chris-wilson.co.uk> Cc: Daniel Vetter <daniel.vetter@ffwll.ch> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com> Cc: Christian König <christian.koenig@amd.com> Cc: Alex Deucher <alexander.deucher@amd.com> Cc: Sumit Semwal <sumit.semwal@linaro.org> Cc: linux-media@vger.kernel.org Cc: dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org Reviewed-by:

Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by:

Sumit Semwal <sumit.semwal@linaro.org> Link: http://patchwork.freedesktop.org/patch/msgid/20160829070834.22296-9-chris@chris-wilson.co.uk

drivers/dma-buf/reservation.c

+10 −20

Original line number	Diff line number	Diff line
		@@ -474,12 +474,13 @@ bool reservation_object_test_signaled_rcu(struct reservation_object *obj,
		bool test_all)
		{
		unsigned seq, shared_count;
		int ret = true;
		int ret;

		rcu_read_lock();
		retry:
		ret = true;
		shared_count = 0;
		seq = read_seqcount_begin(&obj->seq);
		rcu_read_lock();

		if (test_all) {
		unsigned i;
		@@ -490,46 +491,35 @@ retry:
		if (fobj)
		shared_count = fobj->shared_count;

		if (read_seqcount_retry(&obj->seq, seq))
		goto unlock_retry;

		for (i = 0; i < shared_count; ++i) {
		struct fence *fence = rcu_dereference(fobj->shared[i]);

		ret = reservation_object_test_signaled_single(fence);
		if (ret < 0)
		goto unlock_retry;
		goto retry;
		else if (!ret)
		break;
		}

		/*
		* There could be a read_seqcount_retry here, but nothing cares
		* about whether it's the old or newer fence pointers that are
		* signaled. That race could still have happened after checking
		* read_seqcount_retry. If you care, use ww_mutex_lock.
		*/
		if (read_seqcount_retry(&obj->seq, seq))
		goto retry;
		}

		if (!shared_count) {
		struct fence *fence_excl = rcu_dereference(obj->fence_excl);

		if (read_seqcount_retry(&obj->seq, seq))
		goto unlock_retry;

		if (fence_excl) {
		ret = reservation_object_test_signaled_single(
		fence_excl);
		if (ret < 0)
		goto unlock_retry;
		goto retry;

		if (read_seqcount_retry(&obj->seq, seq))
		goto retry;
		}
		}

		rcu_read_unlock();
		return ret;

		unlock_retry:
		rcu_read_unlock();
		goto retry;
		}
		EXPORT_SYMBOL_GPL(reservation_object_test_signaled_rcu);

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