On Wed, Aug 14, 2019 at 02:49:32PM +0200, Daniel Vetter wrote:
> Discussed this a bit with Chris, I think a comment here is warranted
> that this will be bad once we have more than one i915 instance. And
> lockdep won't catch it.
>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
> Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
> ---
> drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> index 74da35611d7c..70dc506a5426 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> @@ -135,6 +135,12 @@ userptr_mn_invalidate_range_start(struct mmu_notifier *_mn,
> switch (mutex_trylock_recursive(unlock)) {
> default:
> case MUTEX_TRYLOCK_FAILED:
> + /*
> + * NOTE: This only works because there's only
> + * ever one i915-style struct_mutex in the
> + * entire system. If we could have two i915
> + * instances, this would deadlock.
> + */
While fixing up annotations for the 2nd patch I though more about this,
and I'm not sold that "there's only one" makes sense. Scenario:
thread A:
get_pages
-> mutex_lock(obj->mm.lock)
-> fs_reclaim
-> mmu_notifier picks range of memory we're interested in
-> mutex_lock_killable(struct_mutex)
Why can this not deadlock against any other thread which does:
mutex_lock(struct_mutex)
-> get_pages
-> mutex_lock(obj->mm.lock)
They would both need to pick the same object, that's right now at a 0->1
transition for pages_pin_count. Plus a long list of other unhappy
circumstances ...
Note that this is different from the same annotation in shrinker_lock:
That one is only used if current_is_kswapd is, which guarantees we're not
holding a few unfortunate locks.
-Daniel
> if (mutex_lock_killable_nested(unlock, I915_MM_SHRINKER)) {
> i915_gem_object_put(obj);
> return -EINTR;
> --
> 2.22.0
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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