Messages in this thread | | | Subject | Re: [PATCH] dma-fence: basic lockdep annotations | From | Thomas Hellström (Intel) <> | Date | Fri, 5 Jun 2020 16:30:39 +0200 |
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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@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@kernel.org> > Date: Tue Dec 12 12:31:16 2017 +0100 > > locking/lockdep: Remove the cross-release locking checks > > This code (CONFIG_LOCKDEP_CROSSRELEASE=y and CONFIG_LOCKDEP_COMPLETIONS=y), > 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@intel.com> > Cc: Thomas Hellstrom <thomas.hellstrom@intel.com> > 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> > ---
Reviewed-by: Thomas Hellström <thomas.hellstrom@intel.com>
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