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Tue, 12 May 2020 02:08:49 -0700 (PDT) Date: Tue, 12 May 2020 11:08:47 +0200 From: Daniel Vetter To: Chris Wilson Message-ID: <20200512090847.GF206103@phenom.ffwll.local> Mail-Followup-To: Chris Wilson , DRI Development , LKML , linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, linux-rdma@vger.kernel.org, amd-gfx@lists.freedesktop.org, intel-gfx@lists.freedesktop.org, Maarten Lankhorst , Christian =?iso-8859-1?Q?K=F6nig?= , Daniel Vetter References: <20200512085944.222637-1-daniel.vetter@ffwll.ch> <20200512085944.222637-3-daniel.vetter@ffwll.ch> <158927426244.15653.14406159524439944950@build.alporthouse.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <158927426244.15653.14406159524439944950@build.alporthouse.com> X-Operating-System: Linux phenom 5.6.0-1-amd64 Subject: Re: [Intel-gfx] [RFC 02/17] dma-fence: basic lockdep annotations X-BeenThere: intel-gfx@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel graphics driver community testing & development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-rdma@vger.kernel.org, Daniel Vetter , intel-gfx@lists.freedesktop.org, LKML , DRI Development , linaro-mm-sig@lists.linaro.org, amd-gfx@lists.freedesktop.org, Daniel Vetter , Christian =?iso-8859-1?Q?K=F6nig?= , linux-media@vger.kernel.org Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Errors-To: intel-gfx-bounces@lists.freedesktop.org Sender: "Intel-gfx" On Tue, May 12, 2020 at 10:04:22AM +0100, Chris Wilson wrote: > Quoting Daniel Vetter (2020-05-12 09:59:29) > > 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 > > 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 > > Cc: Maarten Lankhorst > > Cc: Christian K=F6nig > > Signed-off-by: Daniel Vetter > > --- > > 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 =3D { > > + .name =3D "dma_fence_map" > > +}; > = > Not another false global sharing lockmap. It's a global contract, it needs a global lockdep map. And yes a big reason for the motivation here is that i915-gem has a tremendous urge to just redefine all these global locks to fit to some local interpretation of what's going on. That doesn't make the resulting real&existing deadlocks go away. -Daniel -- = Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch _______________________________________________ Intel-gfx mailing list Intel-gfx@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/intel-gfx