Re: [Intel-gfx] [PATCH 4/5] drm/i915/execlists: Mark up read of i915_request.fence.flags

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

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

> [  145.927961] BUG: KCSAN: data-race in can_merge_rq [i915] / signal_irq_work [i915]
> [  145.927980]
> [  145.927992] write (marked) to 0xffff8881e513fab0 of 8 bytes by interrupt on cpu 2:
> [  145.928250]  signal_irq_work+0x134/0x640 [i915]
> [  145.928268]  irq_work_run_list+0xd7/0x120
> [  145.928283]  irq_work_run+0x1d/0x50
> [  145.928300]  smp_irq_work_interrupt+0x21/0x30
> [  145.928328]  irq_work_interrupt+0xf/0x20
> [  145.928356]  _raw_spin_unlock_irqrestore+0x34/0x40
> [  145.928596]  execlists_submission_tasklet+0xde/0x170 [i915]
> [  145.928616]  tasklet_action_common.isra.0+0x42/0xa0
> [  145.928632]  __do_softirq+0xd7/0x2cd
> [  145.928646]  irq_exit+0xbe/0xe0
> [  145.928665]  do_IRQ+0x51/0x100
> [  145.928684]  ret_from_intr+0x0/0x1c
> [  145.928699]  schedule+0x0/0xb0
> [  145.928719]  worker_thread+0x194/0x670
> [  145.928743]  kthread+0x19a/0x1e0
> [  145.928765]  ret_from_fork+0x1f/0x30
> [  145.928784]
> [  145.928796] read to 0xffff8881e513fab0 of 8 bytes by task 738 on cpu 1:
> [  145.929046]  can_merge_rq+0xb1/0x100 [i915]
> [  145.929282]  __execlists_submission_tasklet+0x866/0x25a0 [i915]
> [  145.929518]  execlists_submit_request+0x2a4/0x2b0 [i915]
> [  145.929758]  submit_notify+0x8f/0xc0 [i915]
> [  145.929989]  __i915_sw_fence_complete+0x5d/0x3e0 [i915]
> [  145.930221]  i915_sw_fence_complete+0x58/0x80 [i915]
> [  145.930453]  i915_sw_fence_commit+0x16/0x20 [i915]
> [  145.930698]  __i915_request_queue+0x60/0x70 [i915]
> [  145.930935]  i915_gem_do_execbuffer+0x3997/0x4c20 [i915]
> [  145.931175]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
> [  145.931194]  drm_ioctl_kernel+0xe4/0x120
> [  145.931208]  drm_ioctl+0x297/0x4c7
> [  145.931222]  ksys_ioctl+0x89/0xb0
> [  145.931238]  __x64_sys_ioctl+0x42/0x60
> [  145.931260]  do_syscall_64+0x6e/0x2c0
> [  145.931275]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> ---
>  drivers/gpu/drm/i915/gt/intel_lrc.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index a9d77b0e4e27..20dd3c2cfa2f 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -1597,6 +1597,11 @@ static bool can_merge_ctx(const struct intel_context *prev,
>  	return true;
>  }
>  
> +static unsigned long i915_request_flags(const struct i915_request *rq)
> +{
> +	return READ_ONCE(rq->fence.flags);

Bitmasks and atomicity through read/write once is bad idea.
But the write side was by atomic bitops.

Race between comparing two requests is still there tho.
The flags compared against tho are well established
apriori request queueing.

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

> +}
> +
>  static bool can_merge_rq(const struct i915_request *prev,
>  			 const struct i915_request *next)
>  {
> @@ -1614,7 +1619,7 @@ static bool can_merge_rq(const struct i915_request *prev,
>  	if (i915_request_completed(next))
>  		return true;
>  
> -	if (unlikely((prev->fence.flags ^ next->fence.flags) &
> +	if (unlikely((i915_request_flags(prev) ^ i915_request_flags(next)) &
>  		     (BIT(I915_FENCE_FLAG_NOPREEMPT) |
>  		      BIT(I915_FENCE_FLAG_SENTINEL))))
>  		return false;
> -- 
> 2.20.1
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