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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxx
Cc: linaro-mm-sig@xxxxxxxxxxxxxxxx
Cc: linux-rdma@xxxxxxxxxxxxxxx
Cc: amd-gfx@xxxxxxxxxxxxxxxxxxxxx
Cc: intel-gfx@xxxxxxxxxxxxxxxxxxxxx
Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
Cc: Christian König <christian.koenig@xxxxxxx>
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
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@xxxxxxxxx>
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