[Intel-gfx] [PATCH 2/2] drm/i915: Avoid unguarded reads from the request pointer

[Mika Kuoppala]

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

> In commit 86aa7e760a67 ("drm/i915: Assert that the context-switch
> completion matches our context") I added a read to the irq tasklet
> handler that compared the on-chip status with that of our sw tracking,
> using an unguarded read of the request pointer to get the context and
> beyond. Whilst we hold a reference to the request, we do not hold
> anything on the context and if we are unlucky it may be reaped from a
> second thread retiring the request (since it may retire the request as
> soon as the breadcrumb is complete, even before we finish processing the
> context switch) as we try to read from the context pointer.
>

Please add warning of the possibility of context vanishing beneath
our feet. Perhaps a good spot is when we store a bug on variable
context_id.

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

> Avoid the racy read from underneath the request by storing the expected
> result in the execlist_port[].
>
> Fixes: 86aa7e760a67 ("drm/i915: Assert that the context-switch completion matches our context")
> Reported-by: Mika Kuoppala <mika.kuoppala at intel.com>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_lrc.c        | 7 ++++---
>  drivers/gpu/drm/i915/intel_ringbuffer.h | 1 +
>  2 files changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index 754f77c394fb..ba39c2952438 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -350,6 +350,7 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
>  		execlists_context_status_change(port[0].request,
>  						INTEL_CONTEXT_SCHEDULE_IN);
>  	desc[0] = execlists_update_context(port[0].request);
> +	GEM_BUG_ONLY(port[0].context_id = upper_32_bits(desc[0]));
>  	port[0].count++;
>  
>  	if (port[1].request) {
> @@ -357,6 +358,7 @@ static void execlists_submit_ports(struct intel_engine_cs *engine)
>  		execlists_context_status_change(port[1].request,
>  						INTEL_CONTEXT_SCHEDULE_IN);
>  		desc[1] = execlists_update_context(port[1].request);
> +		GEM_BUG_ONLY(port[1].context_id = upper_32_bits(desc[1]));
>  		port[1].count = 1;
>  	} else {
>  		desc[1] = 0;
> @@ -563,9 +565,8 @@ static void intel_lrc_irq_handler(unsigned long data)
>  				continue;
>  
>  			/* Check the context/desc id for this event matches */
> -			GEM_BUG_ON(readl(buf + 2 * idx + 1) !=
> -				   upper_32_bits(intel_lr_context_descriptor(port[0].request->ctx,
> -									     engine)));
> +			GEM_BUG_ONLY_ON(readl(buf + 2 * idx + 1) !=
> +					port[0].context_id);
>  
>  			GEM_BUG_ON(port[0].count == 0);
>  			if (--port[0].count == 0) {
> diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
> index 2c6d3655985e..896838ca502c 100644
> --- a/drivers/gpu/drm/i915/intel_ringbuffer.h
> +++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
> @@ -381,6 +381,7 @@ struct intel_engine_cs {
>  	struct execlist_port {
>  		struct drm_i915_gem_request *request;
>  		unsigned int count;
> +		GEM_BUG_ONLY_DECLARE(u32 context_id);
>  	} execlist_port[2];
>  	struct rb_root execlist_queue;
>  	struct rb_node *execlist_first;
> -- 
> 2.11.0
>
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