Op 05-06-2020 om 15:29 schreef Daniel Vetter: > 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. > > 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@xxxxxxxxxx> > 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@xxxxxxxxx> > Cc: Thomas Hellstrom <thomas.hellstrom@xxxxxxxxx> > 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 König <christian.koenig@xxxxxxx> > Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx> > --- > Documentation/driver-api/dma-buf.rst | 12 +- > drivers/dma-buf/dma-fence.c | 161 +++++++++++++++++++++++++++ > include/linux/dma-fence.h | 12 ++ > 3 files changed, 182 insertions(+), 3 deletions(-) > > diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst > index 63dec76d1d8d..05d856131140 100644 > --- a/Documentation/driver-api/dma-buf.rst > +++ b/Documentation/driver-api/dma-buf.rst > @@ -100,11 +100,11 @@ CPU Access to DMA Buffer Objects > .. kernel-doc:: drivers/dma-buf/dma-buf.c > :doc: cpu access > > -Fence Poll Support > -~~~~~~~~~~~~~~~~~~ > +Implicit Fence Poll Support > +~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > .. kernel-doc:: drivers/dma-buf/dma-buf.c > - :doc: fence polling > + :doc: implicit fence polling > > Kernel Functions and Structures Reference > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > @@ -133,6 +133,12 @@ DMA Fences > .. kernel-doc:: drivers/dma-buf/dma-fence.c > :doc: DMA fences overview > > +DMA Fence Signalling Annotations > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +.. kernel-doc:: drivers/dma-buf/dma-fence.c > + :doc: fence signalling annotation > + > DMA Fences Functions Reference > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c > index 656e9ac2d028..0005bc002529 100644 > --- a/drivers/dma-buf/dma-fence.c > +++ b/drivers/dma-buf/dma-fence.c > @@ -110,6 +110,160 @@ u64 dma_fence_context_alloc(unsigned num) > } > EXPORT_SYMBOL(dma_fence_context_alloc); > > +/** > + * DOC: fence signalling annotation > + * > + * Proving correctness of all the kernel code around &dma_fence through code > + * review and testing is tricky for a few reasons: > + * > + * * It is a cross-driver contract, and therefore all drivers must follow the > + * same rules for lock nesting order, calling contexts for various functions > + * and anything else significant for in-kernel interfaces. But it is also > + * impossible to test all drivers in a single machine, hence brute-force N vs. > + * N testing of all combinations is impossible. Even just limiting to the > + * possible combinations is infeasible. > + * > + * * There is an enormous amount of driver code involved. For render drivers > + * there's the tail of command submission, after fences are published, > + * scheduler code, interrupt and workers to process job completion, > + * and timeout, gpu reset and gpu hang recovery code. Plus for integration > + * with core mm with have &mmu_notifier, respectively &mmu_interval_notifier, > + * and &shrinker. For modesetting drivers there's the commit tail functions > + * between when fences for an atomic modeset are published, and when the > + * corresponding vblank completes, including any interrupt processing and > + * related workers. Auditing all that code, across all drivers, is not > + * feasible. > + * > + * * Due to how many other subsystems are involved and the locking hierarchies > + * this pulls in there is extremely thin wiggle-room for driver-specific > + * differences. &dma_fence interacts with almost all of the core memory > + * handling through page fault handlers via &dma_resv, dma_resv_lock() and > + * dma_resv_unlock(). On the other side it also interacts through all > + * allocation sites through &mmu_notifier and &shrinker. > + * > + * Furthermore lockdep does not handle cross-release dependencies, which means > + * any deadlocks between dma_fence_wait() and dma_fence_signal() can't be caught > + * at runtime with some quick testing. The simplest example is one thread > + * waiting on a &dma_fence while holding a lock:: > + * > + * lock(A); > + * dma_fence_wait(B); > + * unlock(A); > + * > + * while the other thread is stuck trying to acquire the same lock, which > + * prevents it from signalling the fence the previous thread is stuck waiting > + * on:: > + * > + * lock(A); > + * unlock(A); > + * dma_fence_signal(B); > + * > + * By manually annotating all code relevant to signalling a &dma_fence we can > + * teach lockdep about these dependencies, which also helps with the validation > + * headache since now lockdep can check all the rules for us:: > + * > + * cookie = dma_fence_begin_signalling(); > + * lock(A); > + * unlock(A); > + * dma_fence_signal(B); > + * dma_fence_end_signalling(cookie); > + * > + * For using dma_fence_begin_signalling() and dma_fence_end_signalling() to > + * annotate critical sections the following rules need to be observed: > + * > + * * All code necessary to complete a &dma_fence must be annotated, from the > + * point where a fence is accessible to other threads, to the point where > + * dma_fence_signal() is called. Un-annotated code can contain deadlock issues, > + * and due to the very strict rules and many corner cases it is infeasible to > + * catch these just with review or normal stress testing. > + * > + * * &struct dma_resv deserves a special note, since the readers are only > + * protected by rcu. This means the signalling critical section starts as soon > + * as the new fences are installed, even before dma_resv_unlock() is called. > + * > + * * The only exception are fast paths and opportunistic signalling code, which > + * calls dma_fence_signal() purely as an optimization, but is not required to > + * guarantee completion of a &dma_fence. The usual example is a wait IOCTL > + * which calls dma_fence_signal(), while the mandatory completion path goes > + * through a hardware interrupt and possible job completion worker. > + * > + * * To aid composability of code, the annotations can be freely nested, as long > + * as the overall locking hierarchy is consistent. The annotations also work > + * both in interrupt and process context. Due to implementation details this > + * requires that callers pass an opaque cookie from > + * dma_fence_begin_signalling() to dma_fence_end_signalling(). > + * > + * * Validation against the cross driver contract is implemented by priming > + * lockdep with the relevant hierarchy at boot-up. This means even just > + * testing with a single device is enough to validate a driver, at least as > + * far as deadlocks with dma_fence_wait() against dma_fence_signal() are > + * concerned. > + */ > +#ifdef CONFIG_LOCKDEP > +struct lockdep_map dma_fence_lockdep_map = { > + .name = "dma_fence_map" > +}; > + > +/** > + * dma_fence_begin_signalling - begin a critical DMA fence signalling section > + * > + * Drivers should use this to annotate the beginning of any code section > + * required to eventually complete &dma_fence by calling dma_fence_signal(). > + * > + * The end of these critical sections are annotated with > + * dma_fence_end_signalling(). > + * > + * Returns: > + * > + * Opaque cookie needed by the implementation, which needs to be passed to > + * dma_fence_end_signalling(). > + */ > +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); > + > +/** > + * dma_fence_end_signalling - end a critical DMA fence signalling section > + * > + * Closes a critical section annotation opened by dma_fence_begin_signalling(). > + */ > +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 +324,19 @@ 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); > @@ -210,6 +369,8 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout) > > 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, As original author of dma-fence, I enjoy seeing more lockdep annotations. Fence was always meant to be cross-driver, so strict driver annotations that can be verified by lockdep are a good thing. Because drivers have to interact with other drivers that use dma-fence, the rules must be the same for everyone, and the above code makes sense. Reviewed-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
