Re: [PATCH v4 3/3] drm/xe: Synchronously wait for queue kill on FD close

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 4 potential issue(s) to consider:
- [High] drm/xe: premature return in guc_exec_queue_kill_sync() before disable
- [High] drm/xe: missing ct.wq wakeup in handle_sched_done() on queue kill
- [High] drm/xe: __guc_exec_queue_process_msg_kill() disables live multi-queue primary
- [Medium] drm/xe: missing unplug check in guc_exec_queue_kill_sync()
--

commit 79b6d795ea1a2b4401f8f9f859fbe92a958b14ab
Author: Daniel Charles <daniel.charles@intel.com>

drm/xe: Synchronously wait for queue kill on FD close

This commit splits FD close exec queue teardown into two passes so that all
queues are killed first, and then waits synchronously via
xe_exec_queue_kill_sync() on a dedicated kill_wq before dropping queue
references and tearing down VMs.

> diff --git a/drivers/gpu/drm/xe/xe_device.c b/drivers/gpu/drm/xe/xe_device.c
> index 4caf86be8c29c..537cd7800f267 100644
> --- a/drivers/gpu/drm/xe/xe_device.c
> +++ b/drivers/gpu/drm/xe/xe_device.c
> @@ -190,8 +190,13 @@ static void xe_file_close(struct drm_device *dev, struct drm_file *file)
>  		if (q->vm && q->hwe->hw_engine_group)
>  			xe_hw_engine_group_del_exec_queue(q->hwe->hw_engine_group, q);
>  		xe_exec_queue_kill(q);
> +	}
> +
> +	xa_for_each(&xef->exec_queue.xa, idx, q) {
> +		xe_exec_queue_kill_sync(q);
>  		xe_exec_queue_put(q);
>  	}

[Severity: High]
Because xe_file_close() now kills all queues in a first pass before dropping
any queue references in the second pass, kref_read(&q->refcount) remains
non-zero when STATIC_MSG_KILL is processed for every secondary queue in a
multi-queue group.

In __guc_exec_queue_process_msg_kill(), primary is resolved unconditionally via
xe_exec_queue_multi_queue_primary(q):

drivers/gpu/drm/xe/xe_guc_submit.c:__guc_exec_queue_process_msg_kill() {
	struct xe_exec_queue *q = msg->private_data;
	struct xe_exec_queue *primary = xe_exec_queue_multi_queue_primary(q);
	...
	if (!kref_read(&q->refcount) || !bound || exec_queue_wedged(q))
		return;

	ret = wait_event_timeout(guc->ct.wq,
				 (!exec_queue_pending_enable(primary) &&
				  !exec_queue_pending_disable(primary)) ||
				 xe_guc_read_stopped(guc) ||
				 vf_recovery(guc), HZ * 5);
	...
	if (exec_queue_enabled(primary))
		disable_scheduling(primary, true);
}

When a single secondary queue is destroyed via xe_exec_queue_destroy_ioctl(),
xe_exec_queue_kill(q) queues STATIC_MSG_KILL before xe_exec_queue_put(q) drops
the reference. Doesn't __guc_exec_queue_process_msg_kill() then overwrite the
live primary queue's preemption timeout to 1 us and disable GuC scheduling on
the active primary queue (which can also trip the !exec_queue_pending_disable()
assertion in guc_exec_queue_run_job() if the primary queue runs a job)?

Additionally, during xe_file_close(), after the primary queue's KILL message
sets EXEC_QUEUE_STATE_PENDING_DISABLE on primary, each secondary queue's
__guc_exec_queue_process_msg_kill() waits on guc->ct.wq for
!exec_queue_pending_disable(primary). Since handle_sched_done() only wakes
kill_wq for a killed (non-banned, non-destroyed) primary queue, won't the
secondary queue's KILL handler stall on guc->ct.wq for the 5-second timeout
and trigger a GT reset?

> +
>  	xa_for_each(&xef->vm.xa, idx, vm)
>  		xe_vm_close_and_put(vm);
>  

[ ... ]

> diff --git a/drivers/gpu/drm/xe/xe_guc_submit.c b/drivers/gpu/drm/xe/xe_guc_submit.c
> index 1fbc8cd767675..40537ad12d729 100644
> --- a/drivers/gpu/drm/xe/xe_guc_submit.c
> +++ b/drivers/gpu/drm/xe/xe_guc_submit.c

[ ... ]

> @@ -2304,6 +2326,28 @@ static void guc_exec_queue_kill(struct xe_exec_queue *q)
>  	xe_guc_exec_queue_trigger_cleanup(q);
>  }
>  
> +static void guc_exec_queue_kill_sync(struct xe_exec_queue *q)
> +{
> +	struct xe_exec_queue *primary = xe_exec_queue_multi_queue_primary(q);
> +	struct xe_guc *guc = exec_queue_to_guc(primary);
> +	long ret;
> +
> +	xe_assert(guc_to_xe(guc), exec_queue_killed(q));
> +
> +	ret = wait_event_timeout(guc->submission_state.kill_wq,
> +				 (!exec_queue_enabled(primary) &&
> +				  !exec_queue_pending_disable(primary) &&
> +				  !exec_queue_pending_enable(primary)) ||
> +				 xe_guc_read_stopped(guc) ||
> +				 vf_recovery(guc) ||
> +				 exec_queue_wedged(primary), HZ * 5);

[Severity: High]
Can guc_exec_queue_kill_sync() return prematurely before GuC scheduling is
actually disabled in the following scenarios?

1. In disable_scheduling() and disable_scheduling_deregister(),
EXEC_QUEUE_STATE_ENABLED is cleared before EXEC_QUEUE_STATE_PENDING_DISABLE is
set and before the GUC_CONTEXT_DISABLE H2G message is sent:

drivers/gpu/drm/xe/xe_guc_submit.c:disable_scheduling() {
	...
	clear_exec_queue_enabled(q);
	set_exec_queue_pending_disable(q);
}

And guc_exec_queue_kill_sync() reads the three state flags via separate
atomic_read() calls. If guc_exec_queue_kill_sync() evaluates the condition
between clear_exec_queue_enabled() and set_exec_queue_pending_disable() (either
upon entry in the second pass of xe_file_close() or when woken on the shared
kill_wq by another queue's SCHED_DONE), won't it observe all three flags as 0
and return before GUC_CONTEXT_DISABLE has even been sent or acknowledged?

2. guc_exec_queue_kill() sets EXEC_QUEUE_STATE_KILLED and queues
STATIC_MSG_KILL on submit_wq without stopping or flushing submit_wq. If
guc_exec_queue_run_job() or __guc_exec_queue_process_msg_resume() is already
running on submit_wq and passed its killed check just before
set_exec_queue_killed(q), primary can still have ENABLED, PENDING_DISABLE, and
PENDING_ENABLE all clear while register_exec_queue() or emit_job() is in
progress (with STATIC_MSG_KILL queued behind it on submit_wq):

drivers/gpu/drm/xe/xe_guc_submit.c:guc_exec_queue_run_job() {
	...
	bool killed_or_banned_or_wedged_or_error =
		exec_queue_killed_or_banned_or_wedged(q) ||
		xe_sched_job_is_error(job);
	...
	if (!killed_or_banned_or_wedged_or_error) {
		...
		submit_exec_queue(q, job);
	}
}

drivers/gpu/drm/xe/xe_guc_submit.c:__guc_exec_queue_process_msg_resume() {
	if (guc_exec_queue_allowed_to_change_state(q) && bound) {
		clear_exec_queue_suspended(q);
		if (!exec_queue_enabled(q)) {
			q->guc->resume_time = RESUME_PENDING;
			set_exec_queue_pending_resume(q);
			enable_scheduling(q);
		}
	}
}

Won't guc_exec_queue_kill_sync() immediately see (!enabled && !pending_disable
&& !pending_enable) and return, after which the in-flight worker calls
submit_exec_queue() or enable_scheduling() while xe_vm_close_and_put() is
tearing down page tables? In the resume case where no job holds a reference on
q, xe_exec_queue_put(q) also drops q->refcount to 0, so the queued
__guc_exec_queue_process_msg_kill() returns early at !kref_read(&q->refcount)
without disabling scheduling.

3. In xe_vm_close_and_put(), vm->q is killed and put without calling
xe_exec_queue_kill_sync(vm->q):

drivers/gpu/drm/xe/xe_vm.c:xe_vm_close_and_put() {
	...
	if (vm->q) {
		...
		xe_exec_queue_kill(vm->q);
		xe_exec_queue_put(vm->q);
		vm->q = NULL;
	}
}

Should xe_vm_close_and_put() also call xe_exec_queue_kill_sync(vm->q) before
destroying the VM's page tables?

> +	if (!ret) {
> +		xe_gt_warn(q->gt, "Queue kill failed to respond, guc_id=%d\n",
> +			   primary->guc->id);
> +		xe_gt_reset_async(q->gt);
> +	}
> +}

[Severity: Medium]
What happens in guc_exec_queue_kill_sync() if the FD is closed after
drm_dev_unplug() has been called in xe_device_remove(), before the devm action
guc_submit_fini() sets guc->submission_state.stopped?

Once unplugged, drm_dev_enter() returns false, so
__guc_exec_queue_process_msg_kill(), __guc_exec_queue_process_msg_cleanup(),
and guc_exec_queue_timedout_job() all bail out early without clearing
EXEC_QUEUE_STATE_ENABLED or waking kill_wq. Without a drm_dev_enter() guard in
guc_exec_queue_kill_sync(), won't wait_event_timeout() block for 5 seconds per
enabled queue and then call xe_gt_reset_async() on the unplugged device?

[ ... ]

> @@ -3460,6 +3507,7 @@ static void handle_sched_done(struct xe_guc *guc, struct xe_exec_queue *q,
>  				clear_exec_queue_pending_disable(q);
>  			}
>  		}
> +		guc_wake_kill_waiters(guc);
>  	}
>  }

[Severity: High]
When a scheduling-disable (runnable_state == 0) triggered by STATIC_MSG_KILL
completes in handle_sched_done(), the queue is typically neither suspended,
banned, nor marked destroyed yet, so handle_sched_done() clears
EXEC_QUEUE_STATE_PENDING_DISABLE and only wakes kill_wq via
guc_wake_kill_waiters(guc) without waking guc->ct.wq:

drivers/gpu/drm/xe/xe_guc_submit.c:handle_sched_done() {
	...
		if (!was_pending) {
			if (exec_queue_banned(q)) {
				smp_wmb();
				wake_up_all(&guc->ct.wq);
			}
			if (exec_queue_destroyed(q)) {
				clear_exec_queue_pending_disable(q);
				deregister_exec_queue(guc, q);
			} else {
				clear_exec_queue_pending_disable(q);
			}
		}
		guc_wake_kill_waiters(guc);
}

However, several paths still wait on guc->ct.wq for
!exec_queue_pending_disable() while a KILL-initiated disable is in flight:

- disable_scheduling_deregister(), when STATIC_MSG_CLEANUP runs after the last
  reference to a killed queue is dropped while KILL's PENDING_DISABLE is still
  in flight (for example via xe_exec_queue_destroy_ioctl(), vm->q in
  xe_vm_close_and_put(), or when a job drops the last reference during
  xe_file_close()), before set_exec_queue_destroyed(q) is reached:

drivers/gpu/drm/xe/xe_guc_submit.c:disable_scheduling_deregister() {
	...
	ret = wait_event_timeout(guc->ct.wq,
				 (!exec_queue_pending_enable(q) &&
				  !exec_queue_pending_disable(q)) ||
					 xe_guc_read_stopped(guc) ||
					 vf_recovery(guc),
				 HZ * 5);
	if (!ret && !vf_recovery(guc)) {
		struct xe_gpu_scheduler *sched = &q->guc->sched;

		xe_gt_warn(q->gt, "Pending enable/disable failed to respond\n");
		xe_sched_submission_start(sched);
		xe_gt_reset_async(q->gt);
		xe_sched_tdr_queue_imm(sched);
		return;
	}

	clear_exec_queue_enabled(q);
	set_exec_queue_pending_disable(q);
	set_exec_queue_destroyed(q);
	...
}

- __guc_exec_queue_process_msg_kill() on a secondary queue in a multi-queue
  group waiting for !exec_queue_pending_disable(primary).
- guc_exec_queue_timedout_job() on a secondary queue in a multi-queue group,
  where EXEC_QUEUE_STATE_BANNED is set on the secondary queue q rather than on
  primary while waiting on guc->ct.wq for
  !exec_queue_pending_disable(primary).

Since handle_sched_done() does not wake guc->ct.wq when clearing
EXEC_QUEUE_STATE_PENDING_DISABLE (and even when exec_queue_banned(q) is true,
wake_up_all(&guc->ct.wq) happens before clear_exec_queue_pending_disable(q)),
won't these waiters on guc->ct.wq sleep for the full 5-second timeout and
trigger a spurious GT reset? Moreover, when disable_scheduling_deregister()
times out and returns early before calling set_exec_queue_destroyed(q), doesn't
that leave exec_queue_destroyed(q) false after STATIC_MSG_CLEANUP has already
been consumed, leaking struct xe_exec_queue and its guc_id across the reset?

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-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261016195904.1377149-1-daniel.charles@intel.com?part=3