[Intel-gfx] [PATCH] dma-buf/fence: Fix lock inversion within dma-fence-array

[Chris Wilson]

Quoting Chris Wilson (2017-11-14 16:27:19)
> Ages ago Rob Clark noted,
> 
> "Currently with fence-array, we have a potential deadlock situation.  If
> we fence_add_callback() on an array-fence, the array-fence's lock is
> acquired first, and in it's ->enable_signaling() callback, it will install
> cbs on it's array-member fences, so the array-member's lock is acquired
> second.
> 
> But in the signal path, the array-member's lock is acquired first, and
> the array-fence's lock acquired second."
> 
> Rob proposed either extensive changes to dma-fence to unnest the
> fence-array signaling, or to defer the signaling onto a workqueue. This
> is a more refined version of the later, that should keep the latency
> of the fence signaling to a minimum by using an irq-work, which is
> executed asap.
> 
> Reported-by: Rob Clark <robdclark at gmail.com>
> Suggested-by: Rob Clark <robdclark at gmail.com>

Testcase: igt/sw_sync/sync_multi_timeline_wait

> References: 1476635975-21981-1-git-send-email-robdclark at gmail.com
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Rob Clark <robdclark at gmail.com>
> Cc: Gustavo Padovan <gustavo.padovan at collabora.co.uk>
> Cc: Sumit Semwal <sumit.semwal at linaro.org>
> Cc: Christian König <christian.koenig at amd.com>