Re: [PATCH] dma-fence: basic lockdep annotations

Maarten Lankhorst
2020-06-11T10:02:19Z

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@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>
> ---
> 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@linux.intel.com>