Re: [PATCH v2] drm/i915/active: Fix misuse of non-idle barriers as fence trackers

[Janusz Krzysztofik]

Hi Andy,

Thanks for review.

On Thursday, 2 March 2023 01:42:05 CET Andi Shyti wrote:
> Hi Janusz,
> 
> On Sat, Feb 25, 2023 at 11:12:18PM +0100, Janusz Krzysztofik wrote:
> > Users reported oopses on list corruptions when using i915 perf with a
> > number of concurrently running graphics applications.  Root cause analysis
> > pointed at an issue in barrier processing code -- a race among perf open /
> > close replacing active barriers with perf requests on kernel context and
> > concurrent barrier preallocate / acquire operations performed during user
> > context first pin / last unpin.
> > 
> > When adding a request to a composite tracker, we try to reuse an existing
> > fence tracker, already allocated and registered with that composite.  The
> > tracker we obtain may already track another fence, may be an idle barrier,
> > or an active barrier.
> > 
> > If the tracker we get occurs a non-idle barrier then we try to delete that
> > barrier from a list of barrier tasks it belongs to.  However, while doing
> > that we don't respect return value from a function that performs the
> > barrier deletion.  Should the deletion ever failed, we would end up
> > reusing the tracker still registered as a barrier task.  Since the same
> > structure field is reused with both fence callback lists and barrier
> > tasks list, list corruptions would likely occur.
> > 
> > Barriers are now deleted from a barrier tasks list by temporarily removing
> > the list content, traversing that content with skip over the node to be
> > deleted, then populating the list back with the modified content.  Should
> > that intentionally racy concurrent deletion attempts be not serialized,
> > one or more of those may fail because of the list being temporary empty.
> > 
> > Related code that ignores the results of barrier deletion was initially
> > introduced in v5.4 by commit d8af05ff38ae ("drm/i915: Allow sharing the
> > idle-barrier from other kernel requests").  However, all users of the
> > barrier deletion routine were apparently serialized at that time, then the
> > issue didn't exhibit itself.  Results of git bisect with help of a newly
> > developed igt@gem_barrier_race@remote-request IGT test indicate that list
> > corruptions might start to appear after commit 311770173fac ("drm/i915/gt:
> > Schedule request retirement when timeline idles"), introduced in v5.5.
> > 
> > Respect results of barrier deletion attempts -- mark the barrier as idle
> > only if successfully deleted from the list.  Then, before proceeding with
> > setting our fence as the one currently tracked, make sure that the tracker
> > we've got is not a non-idle barrier.  If that check fails then don't use
> > that tracker but go back and try to acquire a new, usable one.
> > 
> > v2: no code changes,
> >   - blame commit 311770173fac ("drm/i915/gt: Schedule request retirement
> > when timeline idles"), v5.5, not commit d8af05ff38ae ("drm/i915: Allow
> > sharing the idle-barrier from other kernel requests"), v5.4,
> >   - reword commit description.
> 
> That's a very good explanation and very much needed for such a
> catch. Thanks!
> 
> > Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6333
> > Fixes: 311770173fac ("drm/i915/gt: Schedule request retirement when 
timeline idles")
> > Cc: Chris Wilson 
> > Cc: sta...@vger.kernel.org # v5.5
> > Signed-off-by: Janusz Krzysztofik 
> > ---
> >  drivers/gpu/drm/i915/i915_active.c | 25 ++---
> >  1 file changed, 14 insertions(+), 11 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/
i915_active.c
> > index 7412abf166a8c..f9282b8c87c1c 100644
> > --- a/drivers/gpu/drm/i915/i915_active.c
> > +++ b/drivers/gpu/drm/i915/i915_active.c
> > @@ -422,12 +422,12 @@ replace_barrier(struct i915_active *ref, struct 
i915_active_fence *active)
> >  * we can use it to substitute for the pending idle-barrer
> >  * request that we want to emit on the kernel_context.
> >  */
> > -   __active_del_barrier(ref, node_from_active(active));
> > -   return true;
> > +   return __active_del_barrier(ref, node_from_active(active));
> 
> In general, I support the idea of always checking the return
> value, even if we expect a certain outcome. In these cases, the
> likely/unlikely macros can be helpful. 

OK.

> Given this change, I
> believe the patch deserves an ack.
> 
> That being said, I was curious whether using an explicit lock
> and a normal list of active barriers, rather than a lockless
> list, could have solved the problem.  It seems like using a
> lockless list and iterating over it could be overkill, unless
> there are specific scenarios where the lockless properties are
> necessary.
> 
> Of course, this may be something to consider in a future cleanup,
> as it may be outside the scope of this particular patch.

Yes, I think so.

> 
> >  }
> >  
> >  int i915_active_add_request(struct i915_active *ref, struct i915_request 
*rq)
> >  {
> > +   u64 

Re: [PATCH v2] drm/i915/active: Fix misuse of non-idle barriers as fence trackers

[Andi Shyti]

Hi Janusz,

On Sat, Feb 25, 2023 at 11:12:18PM +0100, Janusz Krzysztofik wrote:
> Users reported oopses on list corruptions when using i915 perf with a
> number of concurrently running graphics applications.  Root cause analysis
> pointed at an issue in barrier processing code -- a race among perf open /
> close replacing active barriers with perf requests on kernel context and
> concurrent barrier preallocate / acquire operations performed during user
> context first pin / last unpin.
> 
> When adding a request to a composite tracker, we try to reuse an existing
> fence tracker, already allocated and registered with that composite.  The
> tracker we obtain may already track another fence, may be an idle barrier,
> or an active barrier.
> 
> If the tracker we get occurs a non-idle barrier then we try to delete that
> barrier from a list of barrier tasks it belongs to.  However, while doing
> that we don't respect return value from a function that performs the
> barrier deletion.  Should the deletion ever failed, we would end up
> reusing the tracker still registered as a barrier task.  Since the same
> structure field is reused with both fence callback lists and barrier
> tasks list, list corruptions would likely occur.
> 
> Barriers are now deleted from a barrier tasks list by temporarily removing
> the list content, traversing that content with skip over the node to be
> deleted, then populating the list back with the modified content.  Should
> that intentionally racy concurrent deletion attempts be not serialized,
> one or more of those may fail because of the list being temporary empty.
> 
> Related code that ignores the results of barrier deletion was initially
> introduced in v5.4 by commit d8af05ff38ae ("drm/i915: Allow sharing the
> idle-barrier from other kernel requests").  However, all users of the
> barrier deletion routine were apparently serialized at that time, then the
> issue didn't exhibit itself.  Results of git bisect with help of a newly
> developed igt@gem_barrier_race@remote-request IGT test indicate that list
> corruptions might start to appear after commit 311770173fac ("drm/i915/gt:
> Schedule request retirement when timeline idles"), introduced in v5.5.
> 
> Respect results of barrier deletion attempts -- mark the barrier as idle
> only if successfully deleted from the list.  Then, before proceeding with
> setting our fence as the one currently tracked, make sure that the tracker
> we've got is not a non-idle barrier.  If that check fails then don't use
> that tracker but go back and try to acquire a new, usable one.
> 
> v2: no code changes,
>   - blame commit 311770173fac ("drm/i915/gt: Schedule request retirement
> when timeline idles"), v5.5, not commit d8af05ff38ae ("drm/i915: Allow
> sharing the idle-barrier from other kernel requests"), v5.4,
>   - reword commit description.

That's a very good explanation and very much needed for such a
catch. Thanks!

> Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6333
> Fixes: 311770173fac ("drm/i915/gt: Schedule request retirement when timeline 
> idles")
> Cc: Chris Wilson 
> Cc: sta...@vger.kernel.org # v5.5
> Signed-off-by: Janusz Krzysztofik 
> ---
>  drivers/gpu/drm/i915/i915_active.c | 25 ++---
>  1 file changed, 14 insertions(+), 11 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_active.c 
> b/drivers/gpu/drm/i915/i915_active.c
> index 7412abf166a8c..f9282b8c87c1c 100644
> --- a/drivers/gpu/drm/i915/i915_active.c
> +++ b/drivers/gpu/drm/i915/i915_active.c
> @@ -422,12 +422,12 @@ replace_barrier(struct i915_active *ref, struct 
> i915_active_fence *active)
>* we can use it to substitute for the pending idle-barrer
>* request that we want to emit on the kernel_context.
>*/
> - __active_del_barrier(ref, node_from_active(active));
> - return true;
> + return __active_del_barrier(ref, node_from_active(active));

In general, I support the idea of always checking the return
value, even if we expect a certain outcome. In these cases, the
likely/unlikely macros can be helpful. Given this change, I
believe the patch deserves an ack.

That being said, I was curious whether using an explicit lock
and a normal list of active barriers, rather than a lockless
list, could have solved the problem. It seems like using a
lockless list and iterating over it could be overkill, unless
there are specific scenarios where the lockless properties are
necessary.

Of course, this may be something to consider in a future cleanup,
as it may be outside the scope of this particular patch.

>  }
>  
>  int i915_active_add_request(struct i915_active *ref, struct i915_request *rq)
>  {
> + u64 idx = i915_request_timeline(rq)->fence_context;
>   struct dma_fence *fence = >fence;
>   struct i915_active_fence *active;
>   int err;
> @@ -437,16 +437,19 @@ int i915_active_add_request(struct i915_active *ref, 
> struct i915_request *rq)
>   if (err)
> 

[PATCH v2] drm/i915/active: Fix misuse of non-idle barriers as fence trackers

[Janusz Krzysztofik]

Users reported oopses on list corruptions when using i915 perf with a
number of concurrently running graphics applications.  Root cause analysis
pointed at an issue in barrier processing code -- a race among perf open /
close replacing active barriers with perf requests on kernel context and
concurrent barrier preallocate / acquire operations performed during user
context first pin / last unpin.

When adding a request to a composite tracker, we try to reuse an existing
fence tracker, already allocated and registered with that composite.  The
tracker we obtain may already track another fence, may be an idle barrier,
or an active barrier.

If the tracker we get occurs a non-idle barrier then we try to delete that
barrier from a list of barrier tasks it belongs to.  However, while doing
that we don't respect return value from a function that performs the
barrier deletion.  Should the deletion ever failed, we would end up
reusing the tracker still registered as a barrier task.  Since the same
structure field is reused with both fence callback lists and barrier
tasks list, list corruptions would likely occur.

Barriers are now deleted from a barrier tasks list by temporarily removing
the list content, traversing that content with skip over the node to be
deleted, then populating the list back with the modified content.  Should
that intentionally racy concurrent deletion attempts be not serialized,
one or more of those may fail because of the list being temporary empty.

Related code that ignores the results of barrier deletion was initially
introduced in v5.4 by commit d8af05ff38ae ("drm/i915: Allow sharing the
idle-barrier from other kernel requests").  However, all users of the
barrier deletion routine were apparently serialized at that time, then the
issue didn't exhibit itself.  Results of git bisect with help of a newly
developed igt@gem_barrier_race@remote-request IGT test indicate that list
corruptions might start to appear after commit 311770173fac ("drm/i915/gt:
Schedule request retirement when timeline idles"), introduced in v5.5.

Respect results of barrier deletion attempts -- mark the barrier as idle
only if successfully deleted from the list.  Then, before proceeding with
setting our fence as the one currently tracked, make sure that the tracker
we've got is not a non-idle barrier.  If that check fails then don't use
that tracker but go back and try to acquire a new, usable one.

v2: no code changes,
  - blame commit 311770173fac ("drm/i915/gt: Schedule request retirement
when timeline idles"), v5.5, not commit d8af05ff38ae ("drm/i915: Allow
sharing the idle-barrier from other kernel requests"), v5.4,
  - reword commit description.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6333
Fixes: 311770173fac ("drm/i915/gt: Schedule request retirement when timeline 
idles")
Cc: Chris Wilson 
Cc: sta...@vger.kernel.org # v5.5
Signed-off-by: Janusz Krzysztofik 
---
 drivers/gpu/drm/i915/i915_active.c | 25 ++---
 1 file changed, 14 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c 
b/drivers/gpu/drm/i915/i915_active.c
index 7412abf166a8c..f9282b8c87c1c 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -422,12 +422,12 @@ replace_barrier(struct i915_active *ref, struct 
i915_active_fence *active)
 * we can use it to substitute for the pending idle-barrer
 * request that we want to emit on the kernel_context.
 */
-   __active_del_barrier(ref, node_from_active(active));
-   return true;
+   return __active_del_barrier(ref, node_from_active(active));
 }
 
 int i915_active_add_request(struct i915_active *ref, struct i915_request *rq)
 {
+   u64 idx = i915_request_timeline(rq)->fence_context;
struct dma_fence *fence = >fence;
struct i915_active_fence *active;
int err;
@@ -437,16 +437,19 @@ int i915_active_add_request(struct i915_active *ref, 
struct i915_request *rq)
if (err)
return err;
 
-   active = active_instance(ref, i915_request_timeline(rq)->fence_context);
-   if (!active) {
-   err = -ENOMEM;
-   goto out;
-   }
+   do {
+   active = active_instance(ref, idx);
+   if (!active) {
+   err = -ENOMEM;
+   goto out;
+   }
+
+   if (replace_barrier(ref, active)) {
+   RCU_INIT_POINTER(active->fence, NULL);
+   atomic_dec(>count);
+   }
+   } while (is_barrier(active));
 
-   if (replace_barrier(ref, active)) {
-   RCU_INIT_POINTER(active->fence, NULL);
-   atomic_dec(>count);
-   }
if (!__i915_active_fence_set(active, fence))
__i915_active_acquire(ref);
 
-- 
2.25.1