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SubjectRe: [RFC 02/17] dma-fence: basic lockdep annotations
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On 2020-05-12 4:59 a.m., 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@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@vger.kernel.org
> Cc: linaro-mm-sig@lists.linaro.org
> Cc: linux-rdma@vger.kernel.org
> Cc: amd-gfx@lists.freedesktop.org
> Cc: intel-gfx@lists.freedesktop.org
> Cc: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Christian König <christian.koenig@amd.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> ---
> drivers/dma-buf/dma-fence.c | 53 +++++++++++++++++++++++++++++++++++++
> include/linux/dma-fence.h | 12 +++++++++
> 2 files changed, 65 insertions(+)
>
> diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> index 6802125349fb..d5c0fd2efc70 100644
> --- a/drivers/dma-buf/dma-fence.c
> +++ b/drivers/dma-buf/dma-fence.c
> @@ -110,6 +110,52 @@ u64 dma_fence_context_alloc(unsigned num)
> }
> EXPORT_SYMBOL(dma_fence_context_alloc);
>
> +#ifdef CONFIG_LOCKDEP
> +struct lockdep_map dma_fence_lockdep_map = {
> + .name = "dma_fence_map"
> +};
> +
> +bool dma_fence_begin_signalling(void)
> +{
> + /* explicitly nesting ... */
> + if (lock_is_held_type(&dma_fence_lockdep_map, 1))
> + return true;
> +
> + /* rely on might_sleep check for soft/hardirq locks */
> + if (in_atomic())
> + return true;
> +
> + /* ... and non-recursive readlock */
> + lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _RET_IP_);
> +
> + return false;
> +}
> +EXPORT_SYMBOL(dma_fence_begin_signalling);

Hi Daniel,

This is great work and could help a lot.

If you invert the result of dma_fence_begin_signalling()
then it would naturally mean "locked", i.e. whether we need to
later release "dma_fence_lockedep_map". Then,
in dma_fence_end_signalling(), you can call the "cookie"
argument "locked" and simply do:

void dma_fence_end_signalling(bool locked)
{
if (locked)
lock_release(&dma_fence_lockdep_map, _RET_IP_);
}
EXPORT_SYMBOL(dma_fence_end_signalling);

It'll be more natural to understand as well.

Regards,
Luben

> +
> +void dma_fence_end_signalling(bool cookie)
> +{
> + if (cookie)
> + return;
> +
> + lock_release(&dma_fence_lockdep_map, _RET_IP_);
> +}
> +EXPORT_SYMBOL(dma_fence_end_signalling);
> +
> +void __dma_fence_might_wait(void)
> +{
> + bool tmp;
> +
> + tmp = lock_is_held_type(&dma_fence_lockdep_map, 1);
> + if (tmp)
> + lock_release(&dma_fence_lockdep_map, _THIS_IP_);
> + lock_map_acquire(&dma_fence_lockdep_map);
> + lock_map_release(&dma_fence_lockdep_map);
> + if (tmp)
> + lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _THIS_IP_);
> +}
> +#endif
> +
> +
> /**
> * dma_fence_signal_locked - signal completion of a fence
> * @fence: the fence to signal
> @@ -170,14 +216,19 @@ int dma_fence_signal(struct dma_fence *fence)
> {
> unsigned long flags;
> int ret;
> + bool tmp;
>
> if (!fence)
> return -EINVAL;
>
> + tmp = dma_fence_begin_signalling();
> +
> spin_lock_irqsave(fence->lock, flags);
> ret = dma_fence_signal_locked(fence);
> spin_unlock_irqrestore(fence->lock, flags);
>
> + dma_fence_end_signalling(tmp);
> +
> return ret;
> }
> EXPORT_SYMBOL(dma_fence_signal);
> @@ -211,6 +262,8 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout)
> if (timeout > 0)
> might_sleep();
>
> + __dma_fence_might_wait();
> +
> trace_dma_fence_wait_start(fence);
> if (fence->ops->wait)
> ret = fence->ops->wait(fence, intr, timeout);
> diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> index 3347c54f3a87..3f288f7db2ef 100644
> --- a/include/linux/dma-fence.h
> +++ b/include/linux/dma-fence.h
> @@ -357,6 +357,18 @@ dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep)
> } while (1);
> }
>
> +#ifdef CONFIG_LOCKDEP
> +bool dma_fence_begin_signalling(void);
> +void dma_fence_end_signalling(bool cookie);
> +#else
> +static inline bool dma_fence_begin_signalling(void)
> +{
> + return true;
> +}
> +static inline void dma_fence_end_signalling(bool cookie) {}
> +static inline void __dma_fence_might_wait(void) {}
> +#endif
> +
> int dma_fence_signal(struct dma_fence *fence);
> int dma_fence_signal_locked(struct dma_fence *fence);
> signed long dma_fence_default_wait(struct dma_fence *fence,
>

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