[Intel-gfx] [PATCH v6 63/64] drm/i915: Move gt_revoke() slightly
thomas.hellstrom at linux.intel.com
Mon Jan 18 13:22:58 UTC 2021
On 1/18/21 1:01 PM, Maarten Lankhorst wrote:
> Op 18-01-2021 om 12:11 schreef Thomas Hellström:
>> On 1/5/21 4:35 PM, Maarten Lankhorst wrote:
>>> We get a lockdep splat when the reset mutex is held, because it can be
>>> taken from fence_wait. This conflicts with the mmu notifier we have,
>>> because we recurse between reset mutex and mmap lock -> mmu notifier.
>>> Remove this recursion by calling revoke_mmaps before taking the lock.
>> Hmm. Is the mmap se taken from gt_revoke()?
>> If so, isn't the real problem that the mmap_sem is taken in the dma_fence critical path (where the reset code sits)?
> The gpu reset code specifically needs to revoke all gtt mappings, and the fault handler uses intel_gt_reset_trylock(),
> so this change should be ok since all those mappings are invalidated correctly and completed before this point.
> The reset mutex isn't actually taken inside fence code, but used for lockdep validation, so this should be ok.
Hmm, OK but then we still have the following established locking order.
But in the notifier
So gt_revoke() is violating dma-fence rules.
BTW it looks to me like the reset mutex notation is actually doing much
the same as the dma-fence annotations; While we can move gt_revoke() out
of the reset mutex, that only gives us false hopes since it moves it out
of the equivalent dma-fence annotation. I figure the reason this was not
seen before the new code is that the reset mutex lockdep isn't taken
when waiting for active. Only when waiting for dma-fence, but IMO the
root problem is pre-existing.
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