Re: [Intel-gfx] [PATCH v2] drm/i915/gt: Defend against concurrent updates to execlists->active

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

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

> [  206.875637] BUG: KCSAN: data-race in __i915_schedule+0x7fc/0x930 [i915]
> [  206.875654]
> [  206.875666] race at unknown origin, with read to 0xffff8881f7644480 of 8 bytes by task 703 on cpu 3:
> [  206.875901]  __i915_schedule+0x7fc/0x930 [i915]
> [  206.876130]  __bump_priority+0x63/0x80 [i915]
> [  206.876361]  __i915_sched_node_add_dependency+0x258/0x300 [i915]
> [  206.876593]  i915_sched_node_add_dependency+0x50/0xa0 [i915]
> [  206.876824]  i915_request_await_dma_fence+0x1da/0x530 [i915]
> [  206.877057]  i915_request_await_object+0x2fe/0x470 [i915]
> [  206.877287]  i915_gem_do_execbuffer+0x45dc/0x4c20 [i915]
> [  206.877517]  i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915]
> [  206.877535]  drm_ioctl_kernel+0xe4/0x120
> [  206.877549]  drm_ioctl+0x297/0x4c7
> [  206.877563]  ksys_ioctl+0x89/0xb0
> [  206.877577]  __x64_sys_ioctl+0x42/0x60
> [  206.877591]  do_syscall_64+0x6e/0x2c0
> [  206.877606]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> v2: Be safe and include mb
>
> References: https://gitlab.freedesktop.org/drm/intel/issues/1318
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
> ---
>  drivers/gpu/drm/i915/gt/intel_engine.h | 15 ++++++++++++++-
>  drivers/gpu/drm/i915/gt/intel_lrc.c    | 12 +++++++-----
>  2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h b/drivers/gpu/drm/i915/gt/intel_engine.h
> index 29c8c03c5caa..3644fd2b8877 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine.h
> +++ b/drivers/gpu/drm/i915/gt/intel_engine.h
> @@ -107,7 +107,20 @@ execlists_num_ports(const struct intel_engine_execlists * const execlists)
>  static inline struct i915_request *
>  execlists_active(const struct intel_engine_execlists *execlists)
>  {
> -	return *READ_ONCE(execlists->active);
> +	struct i915_request * const *cur, * const *old, *active;
> +
> +	cur = READ_ONCE(execlists->active);
> +	smp_rmb(); /* pairs with overwrite protection in process_csb() */
> +	do {
> +		old = cur;
> +
> +		active = READ_ONCE(*cur);
> +		cur = READ_ONCE(execlists->active);
> +
> +		smp_rmb(); /* and complete the seqlock retry */
> +	} while (cur != old);
> +
> +	return active;
>  }
>  
>  static inline void
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index 3eb7adc4e057..f4b7fdfa2d63 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -2389,6 +2389,7 @@ static void process_csb(struct intel_engine_cs *engine)
>  
>  			/* Point active to the new ELSP; prevent overwriting */
>  			WRITE_ONCE(execlists->active, execlists->pending);
> +			smp_wmb(); /* notify execlists_active() */
>  
>  			/* cancel old inflight, prepare for switch */
>  			trace_ports(execlists, "preempted", old);
> @@ -2396,11 +2397,12 @@ static void process_csb(struct intel_engine_cs *engine)
>  				execlists_schedule_out(*old++);
>  
>  			/* switch pending to inflight */
> -			WRITE_ONCE(execlists->active,
> -				   memcpy(execlists->inflight,
> -					  execlists->pending,
> -					  execlists_num_ports(execlists) *
> -					  sizeof(*execlists->pending)));
> +			memcpy(execlists->inflight,
> +			       execlists->pending,
> +			       execlists_num_ports(execlists) *
> +			       sizeof(*execlists->pending));
> +			smp_wmb(); /* complete the seqlock */
> +			WRITE_ONCE(execlists->active, execlists->inflight);

Now the memcpy bit is nicely in order,
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>

>  
>  			WRITE_ONCE(execlists->pending[0], NULL);
>  		} else {
> -- 
> 2.20.1
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