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

Tvrtko Ursulin
2020-06-11T10:36:47Z

On 10/06/2020 16:17, Daniel Vetter wrote:
> On Wed, Jun 10, 2020 at 4:22 PM Tvrtko Ursulin
> <tvrtko.ursulin@linux.intel.com> 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@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.
>>>
>>> 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@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"
>>> +};
>>
>> 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@mail.gmail.com/

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.

>>> +
>>> +/**
>>> + * 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.

Regards,

Tvrtko