[Intel-gfx] [RFC 02/17] dma-fence: basic lockdep annotations

Thomas Hellström (Intel) thomas_os at shipmail.org
Thu May 28 13:36:56 UTC 2020

On 2020-05-12 10:59, Daniel Vetter wrote:
> Design is similar to the lockdep annotations for workers, but with
> some twists:
> - We use a read-lock for the execution/worker/completion side, so that
>    this explicit annotation can be more liberally sprinkled around.
>    With read locks lockdep isn't going to complain if the read-side
>    isn't nested the same way under all circumstances, so ABBA deadlocks
>    are ok. Which they are, since this is an annotation only.
> - We're using non-recursive lockdep read lock mode, since in recursive
>    read lock mode lockdep does not catch read side hazards. And we
>    _very_ much want read side hazards to be caught. For full details of
>    this limitation see
>    commit e91498589746065e3ae95d9a00b068e525eec34f
>    Author: Peter Zijlstra <peterz at infradead.org>
>    Date:   Wed Aug 23 13:13:11 2017 +0200
>        locking/lockdep/selftests: Add mixed read-write ABBA tests
> - To allow nesting of the read-side explicit annotations we explicitly
>    keep track of the nesting. lock_is_held() allows us to do that.
> - The wait-side annotation is a write lock, and entirely done within
>    dma_fence_wait() for everyone by default.
> - To be able to freely annotate helper functions I want to make it ok
>    to call dma_fence_begin/end_signalling from soft/hardirq context.
>    First attempt was using the hardirq locking context for the write
>    side in lockdep, but this forces all normal spinlocks nested within
>    dma_fence_begin/end_signalling to be spinlocks. That bollocks.
>    The approach now is to simple check in_atomic(), and for these cases
>    entirely rely on the might_sleep() check in dma_fence_wait(). That
>    will catch any wrong nesting against spinlocks from soft/hardirq
>    contexts.
> The idea here is that every code path that's critical for eventually
> signalling a dma_fence should be annotated with
> dma_fence_begin/end_signalling. The annotation ideally starts right
> after a dma_fence is published (added to a dma_resv, exposed as a
> sync_file fd, attached to a drm_syncobj fd, or anything else that
> makes the dma_fence visible to other kernel threads), up to and
> including the dma_fence_wait(). Examples are irq handlers, the
> scheduler rt threads, the tail of execbuf (after the corresponding
> fences are visible), any workers that end up signalling dma_fences and
> really anything else. Not annotated should be code paths that only
> complete fences opportunistically as the gpu progresses, like e.g.
> shrinker/eviction code.
> The main class of deadlocks this is supposed to catch are:
> Thread A:
> 	mutex_lock(A);
> 	mutex_unlock(A);
> 	dma_fence_signal();
> Thread B:
> 	mutex_lock(A);
> 	dma_fence_wait();
> 	mutex_unlock(A);
> Thread B is blocked on A signalling the fence, but A never gets around
> to that because it cannot acquire the lock A.
> Note that dma_fence_wait() is allowed to be nested within
> dma_fence_begin/end_signalling sections. To allow this to happen the
> read lock needs to be upgraded to a write lock, which means that any
> other lock is acquired between the dma_fence_begin_signalling() call and
> the call to dma_fence_wait(), and still held, this will result in an
> immediate lockdep complaint. The only other option would be to not
> annotate such calls, defeating the point. Therefore these annotations
> cannot be sprinkled over the code entirely mindless to avoid false
> positives.
> v2: handle soft/hardirq ctx better against write side and dont forget
> EXPORT_SYMBOL, drivers can't use this otherwise.
> Cc: linux-media at vger.kernel.org
> Cc: linaro-mm-sig at lists.linaro.org
> Cc: linux-rdma at vger.kernel.org
> Cc: amd-gfx at lists.freedesktop.org
> Cc: intel-gfx at lists.freedesktop.org
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Cc: Christian König <christian.koenig at amd.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>

LGTM. Perhaps some in-code documentation on how to use the new functions 
are called.

Otherwise for patch 2 and 3,

Reviewed-by: Thomas Hellstrom <thomas.hellstrom at intel.com>

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