Re: [Intel-gfx] [PATCH 03/18] dma-fence: basic lockdep annotations

From: Daniel Vetter
Date: Thu Jun 11 2020 - 07:29:36 EST

On Thu, Jun 11, 2020 at 12:36 PM Tvrtko Ursulin
<tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
>
>
> On 10/06/2020 16:17, Daniel Vetter wrote:
> > On Wed, Jun 10, 2020 at 4:22 PM Tvrtko Ursulin
> > <tvrtko.ursulin@xxxxxxxxxxxxxxx> wrote:
> >>
> >>
> >> On 04/06/2020 09:12, 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.
> >>>
> >>> v3: Kerneldoc.
> >>>
> >>> v4: Some spelling fixes from Mika
> >>>
> >>> 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 KÃnig <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"
> >>> +};
> >>
> >> Maybe a stupid question because this is definitely complicated, but.. If
> >> you have a single/static/global lockdep map, doesn't this mean _all_
> >> locks, from _all_ drivers happening to use dma-fences will get recorded
> >> in it. Will this work and not cause false positives?
> >>
> >> Sounds like it could create a common link between two completely
> >> unconnected usages. Because below you do add annotations to generic
> >> dma_fence_signal and dma_fence_wait.
> >
> > This is fully intentional. dma-fence is a cross-driver interface, if
> > every driver invents its own rules about how this should work we have
> > an unmaintainable and unreviewable mess.
> >
> > I've typed up the full length rant already here:
> >
> > https://lore.kernel.org/dri-devel/CAKMK7uGnFhbpuurRsnZ4dvRV9gQ_3-rmSJaoqSFY=+Kvepz_CA@xxxxxxxxxxxxxx/
>
> But "perfect storm" of:
>
> + global fence lockmap
> + mmu notifiers
> + fs reclaim
> + default annotations in dma_fence_signal / dma_fence_wait
>
> Equals to anything ever using dma_fence will be in impossible chains with random other drivers, even if neither driver has code to export/share that fence.
>
> Example from the CI run:
>
> [25.918788] Chain exists of:
> fs_reclaim --> mmu_notifier_invalidate_range_start --> dma_fence_map
> [25.918794] Possible unsafe locking scenario:
> [25.918797] CPU0 CPU1
> [25.918799] ---- ----
> [25.918801] lock(dma_fence_map);
> [25.918803] lock(mmu_notifier_invalidate_range_start);
> [25.918807] lock(dma_fence_map);
> [25.918809] lock(fs_reclaim);
>
> What about a dma_fence_export helper which would "arm" the annotations? It would be called as soon as the fence is exported. Maybe when added to dma_resv, or exported via sync_file, etc. Before that point begin/end_signaling and so would be no-ops.

Run CI without the i915 annotation patch, nothing breaks.

So we can gradually fix up existing code that doesn't quite get it
right and move on.

> >>> +
> >>> +/**
> >>> + * 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_);
> >>
> >> Would it work if signalling path would mark itself as a write lock? I am
> >> thinking it would be nice to see in lockdep splats what are signals and
> >> what are waits.
> >
> > Yeah it'd be nice to have a read vs write name for the lock. But we
> > already have this problem for e.g. flush_work(), from which I've
> > stolen this idea. So it's not really new. Essentially look at the
> > backtraces lockdep gives you, and reconstruct the deadlock. I'm hoping
> > that people will notice the special functions on the backtrace, e.g.
> > dma_fence_begin_signalling will be listed as offending function/lock
> > holder, and then read the kerneldoc.
> >
> >> The recursive usage wouldn't work then right? Would write annotation on
> >> the wait path work?
> >
> > Wait path is write annotations already, but yeah annotating the
> > signalling side as write would cause endless amounts of alse
> > positives. Also it makes composability of these e.g. what I've done in
> > amdgpu with annotations in tdr work in drm/scheduler, annotations in
> > the amdgpu gpu reset code and then also annotations in atomic code,
> > which all nest within each other in some call chains, but not others.
> > Dropping the recursion would break that and make it really awkward to
> > annotate such cases correctly.
> >
> > And the recursion only works if it's read locks, otherwise lockdep
> > complains if you have inconsistent annotations on the signalling side
> > (which again would make it more or less impossible to annotate the
> > above case fully).
>
> How do I see in lockdep splats if it was a read or write user? Your patch appears to have:
>
> dma_fence_signal:
> + lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _RET_IP_);
>
> __dma_fence_might_wait:
> + lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _THIS_IP_);
>
> Which both seem like read lock. I don't fully understand the lockdep API so I might be wrong, not sure. But neither I see a difference in splats telling me which path is which.

I think you got tricked by the implementation, this isn't quite what's
going on. There's two things which make the annotations special:

- we want a recursive read lock on the signalling critical section.
The problem is that lockdep doesn't implement full validation for
recursive read locks, only non-recursive read/write locks fully
validated. There's some checks for recursive read locks, but exactly
the checks we need to catch common dma_fence_wait deadlocks aren't
done. That's why we need to implement manual lock recursion on the
reader side

- now on the write side we additionally need to implement an
read2write upgrade, and a write2read downgrade. Lockdep doesn't
implement that, so again we have to hand-roll this.

Let's go through the code line-by-line:

bool tmp;

tmp = lock_is_held_type(&dma_fence_lockdep_map, 1);

We check whether someone is holding the non-recursive read lock already.

if (tmp)
lock_release(&dma_fence_lockdep_map, _THIS_IP_);

If that's the case, we drop that read lock.

lock_map_acquire(&dma_fence_lockdep_map);

Then we do the actual might_wait annotation, the above takes the full
write lock ...

lock_map_release(&dma_fence_lockdep_map);

... and now we release the write lock again.


if (tmp)
lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _THIS_IP_);

Finally we need to re-acquire the read lock, if we've held that when
entering this function. This annotation naturally has to exactly match
what begin_signalling would do, otherwise the hand-rolled nesting
would fall apart.

I hope that explains what's going on here, and assures you that
might_wait() is indeed a write lock annotation, but with a big pile of
complications.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch