[Intel-gfx] [PATCH v2] drm/i915: Mark up racy read of active rq->engine

[Mika Kuoppala]

Chris Wilson <chris at chris-wilson.co.uk> writes:

> As a virtual engine may change the rq->engine to point to the active
> request in flight, we need to warn the compiler that an active request's
> engine is volatile.
>
> [   95.017686] write (marked) to 0xffff8881e8386b10 of 8 bytes by interrupt on cpu 2:
> [   95.018123]  execlists_dequeue+0x762/0x2150 [i915]
> [   95.018539]  __execlists_submission_tasklet+0x48/0x60 [i915]
> [   95.018955]  execlists_submission_tasklet+0xd3/0x170 [i915]
> [   95.018986]  tasklet_action_common.isra.0+0x42/0xa0
> [   95.019016]  __do_softirq+0xd7/0x2cd
> [   95.019043]  irq_exit+0xbe/0xe0
> [   95.019068]  irq_work_interrupt+0xf/0x20
> [   95.019491]  i915_request_retire+0x2c5/0x670 [i915]
> [   95.019937]  retire_requests+0xa1/0xf0 [i915]
> [   95.020348]  engine_retire+0xa1/0xe0 [i915]
> [   95.020376]  process_one_work+0x3b1/0x690
> [   95.020403]  worker_thread+0x80/0x670
> [   95.020429]  kthread+0x19a/0x1e0
> [   95.020454]  ret_from_fork+0x1f/0x30
> [   95.020476]
> [   95.020498] read to 0xffff8881e8386b10 of 8 bytes by task 8909 on cpu 3:
> [   95.020918]  __i915_request_commit+0x177/0x220 [i915]
> [   95.021329]  i915_gem_do_execbuffer+0x38c4/0x4e50 [i915]
> [   95.021750]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
> [   95.021784]  drm_ioctl_kernel+0xe4/0x120
> [   95.021809]  drm_ioctl+0x297/0x4c7
> [   95.021832]  ksys_ioctl+0x89/0xb0
> [   95.021865]  __x64_sys_ioctl+0x42/0x60
> [   95.021901]  do_syscall_64+0x6e/0x2c0
> [   95.021927]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Reviewed-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>

> ---
>  drivers/gpu/drm/i915/i915_request.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 74169671afb5..c0df71d7d0ff 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -977,6 +977,8 @@ emit_semaphore_wait(struct i915_request *to,
>  		    struct i915_request *from,
>  		    gfp_t gfp)
>  {
> +	const intel_engine_mask_t mask = READ_ONCE(from->engine)->mask;
> +
>  	if (!intel_context_use_semaphores(to->context))
>  		goto await_fence;
>  
> @@ -984,7 +986,7 @@ emit_semaphore_wait(struct i915_request *to,
>  		goto await_fence;
>  
>  	/* Just emit the first semaphore we see as request space is limited. */
> -	if (already_busywaiting(to) & from->engine->mask)
> +	if (already_busywaiting(to) & mask)
>  		goto await_fence;
>  
>  	if (i915_request_await_start(to, from) < 0)
> @@ -997,7 +999,7 @@ emit_semaphore_wait(struct i915_request *to,
>  	if (__emit_semaphore_wait(to, from, from->fence.seqno))
>  		goto await_fence;
>  
> -	to->sched.semaphores |= from->engine->mask;
> +	to->sched.semaphores |= mask;
>  	to->sched.flags |= I915_SCHED_HAS_SEMAPHORE_CHAIN;
>  	return 0;
>  
> @@ -1338,7 +1340,7 @@ __i915_request_add_to_timeline(struct i915_request *rq)
>  			   i915_seqno_passed(prev->fence.seqno,
>  					     rq->fence.seqno));
>  
> -		if (is_power_of_2(prev->engine->mask | rq->engine->mask))
> +		if (is_power_of_2(READ_ONCE(prev->engine)->mask | rq->engine->mask))
>  			i915_sw_fence_await_sw_fence(&rq->submit,
>  						     &prev->submit,
>  						     &rq->submitq);
> -- 
> 2.20.1
>
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