[PATCH] dma-fence: basic lockdep annotations

Thomas Hellström (Intel) thomas_os at shipmail.org
Fri Jun 5 14:30:39 UTC 2020

On 6/5/20 3:29 PM, 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.
> Originally I hope that the cross-release lockdep extensions would
> alleviate the need for explicit annotations:
> https://lwn.net/Articles/709849/
> But there's a few reasons why that's not an option:
> - It's not happening in upstream, since it got reverted due to too
>    many false positives:
> 	commit e966eaeeb623f09975ef362c2866fae6f86844f9
> 	Author: Ingo Molnar <mingo at kernel.org>
> 	Date:   Tue Dec 12 12:31:16 2017 +0100
> 	    locking/lockdep: Remove the cross-release locking checks
> 	    while it found a number of old bugs initially, was also causing too many
> 	    false positives that caused people to disable lockdep - which is arguably
> 	    a worse overall outcome.
> - cross-release uses the complete() call to annotate the end of
>    critical sections, for dma_fence that would be dma_fence_signal().
>    But we do not want all dma_fence_signal() calls to be treated as
>    critical, since many are opportunistic cleanup of gpu requests. If
>    these get stuck there's still the main completion interrupt and
>    workers who can unblock everyone. Automatically annotating all
>    dma_fence_signal() calls would hence cause false positives.
> - cross-release had some educated guesses for when a critical section
>    starts, like fresh syscall or fresh work callback. This would again
>    cause false positives without explicit annotations, since for
>    dma_fence the critical sections only starts when we publish a fence.
> - Furthermore there can be cases where a thread never does a
>    dma_fence_signal, but is still critical for reaching completion of
>    fences. One example would be a scheduler kthread which picks up jobs
>    and pushes them into hardware, where the interrupt handler or
>    another completion thread calls dma_fence_signal(). But if the
>    scheduler thread hangs, then all the fences hang, hence we need to
>    manually annotate it. cross-release aimed to solve this by chaining
>    cross-release dependencies, but the dependency from scheduler thread
>    to the completion interrupt handler goes through hw where
>    cross-release code can't observe it.
> In short, without manual annotations and careful review of the start
> and end of critical sections, cross-relese dependency tracking doesn't
> work. We need explicit annotations.
> v2: handle soft/hardirq ctx better against write side and dont forget
> EXPORT_SYMBOL, drivers can't use this otherwise.
> v3: Kerneldoc.
> v4: Some spelling fixes from Mika
> v5: Amend commit message to explain in detail why cross-release isn't
> the solution.
> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> Cc: Thomas Hellstrom <thomas.hellstrom at intel.com>
> 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>
> ---

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

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