On 1/18/21 2:22 PM, Thomas Hellström wrote:
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)?
Hey,
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.
~Maarten
Hmm, OK but then we still have the following established locking order.
lock(fence_signaling)
lock(i_mmap_lock)
But in the notifier
lock(i_mmap_lock)
fence_signaling(within 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.
/Thomas
The interesting scenario is
thread 1:
take i_mmap_lock()
enter_mmu_notifier()
wait_fence()
thread 2:
need_to_reset_gpu_for_the_above_fence();
take i_mmap_lock()
Deadlock.
/Thomas
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