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

[Luben Tuikov]

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@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 KÃnig <christian.koenig@xxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
>  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,
>