Re: [Intel-gfx] [PATCH] drm/i915: Check for rq->hwsp validity after acquiring RCU lock

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 18 Jan 2021, Chris Wilson <chris@chris-wilson.co.uk> wrote:
> Since we allow removing the timeline map at runtime, there is a risk
> that rq->hwsp points into a stale page. To control that risk, we hold
> the RCU read lock while reading *rq->hwsp, but we missed a couple of
> important barriers. First, the unpinning / removal of the timeline map
> must be after all RCU readers into that map are complete, i.e. after an
> rcu barrier (in this case courtesy of call_rcu()). Secondly, we must
> make sure that the rq->hwsp we are about to dereference under the RCU
> lock is valid. In this case, we make the rq->hwsp pointer safe during
> i915_request_retire() and so we know that rq->hwsp may become invalid
> only after the request has been signaled. Therefore is the request is
> not yet signaled when we acquire rq->hwsp under the RCU, we know that
> rq->hwsp will remain valid for the duration of the RCU read lock.
>
> This is a very small window that may lead to either considering the
> request not completed (causing a delay until the request is checked
> again, any wait for the request is not affected) or dereferencing an
> invalid pointer.
>
> Fixes: 3adac4689f58 ("drm/i915: Introduce concept of per-timeline (context) HWSP")
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Cc: <stable@vger.kernel.org> # v5.1+
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Link: https://patchwork.freedesktop.org/patch/msgid/20201218122421.18344-1-chris@chris-wilson.co.uk
> (cherry picked from commit 9bb36cf66091ddf2d8840e5aa705ad3c93a6279b)

Thanks for the backports, all three pushed to drm-intel-fixes.

BR,
Jani.


> ---
>  drivers/gpu/drm/i915/gt/intel_breadcrumbs.c |  9 ++---
>  drivers/gpu/drm/i915/gt/intel_timeline.c    | 10 +++---
>  drivers/gpu/drm/i915/i915_request.h         | 37 ++++++++++++++++++---
>  3 files changed, 38 insertions(+), 18 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> index a24cc1ff08a0..0625cbb3b431 100644
> --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> @@ -134,11 +134,6 @@ static bool remove_signaling_context(struct intel_breadcrumbs *b,
>  	return true;
>  }
>  
> -static inline bool __request_completed(const struct i915_request *rq)
> -{
> -	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
> -}
> -
>  __maybe_unused static bool
>  check_signal_order(struct intel_context *ce, struct i915_request *rq)
>  {
> @@ -257,7 +252,7 @@ static void signal_irq_work(struct irq_work *work)
>  		list_for_each_entry_rcu(rq, &ce->signals, signal_link) {
>  			bool release;
>  
> -			if (!__request_completed(rq))
> +			if (!__i915_request_is_complete(rq))
>  				break;
>  
>  			if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL,
> @@ -379,7 +374,7 @@ static void insert_breadcrumb(struct i915_request *rq)
>  	 * straight onto a signaled list, and queue the irq worker for
>  	 * its signal completion.
>  	 */
> -	if (__request_completed(rq)) {
> +	if (__i915_request_is_complete(rq)) {
>  		if (__signal_request(rq) &&
>  		    llist_add(&rq->signal_node, &b->signaled_requests))
>  			irq_work_queue(&b->irq_work);
> diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
> index 7ea94d201fe6..8015964043eb 100644
> --- a/drivers/gpu/drm/i915/gt/intel_timeline.c
> +++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
> @@ -126,6 +126,10 @@ static void __rcu_cacheline_free(struct rcu_head *rcu)
>  	struct intel_timeline_cacheline *cl =
>  		container_of(rcu, typeof(*cl), rcu);
>  
> +	/* Must wait until after all *rq->hwsp are complete before removing */
> +	i915_gem_object_unpin_map(cl->hwsp->vma->obj);
> +	__idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
> +
>  	i915_active_fini(&cl->active);
>  	kfree(cl);
>  }
> @@ -133,11 +137,6 @@ static void __rcu_cacheline_free(struct rcu_head *rcu)
>  static void __idle_cacheline_free(struct intel_timeline_cacheline *cl)
>  {
>  	GEM_BUG_ON(!i915_active_is_idle(&cl->active));
> -
> -	i915_gem_object_unpin_map(cl->hwsp->vma->obj);
> -	i915_vma_put(cl->hwsp->vma);
> -	__idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
> -
>  	call_rcu(&cl->rcu, __rcu_cacheline_free);
>  }
>  
> @@ -179,7 +178,6 @@ cacheline_alloc(struct intel_timeline_hwsp *hwsp, unsigned int cacheline)
>  		return ERR_CAST(vaddr);
>  	}
>  
> -	i915_vma_get(hwsp->vma);
>  	cl->hwsp = hwsp;
>  	cl->vaddr = page_pack_bits(vaddr, cacheline);
>  
> diff --git a/drivers/gpu/drm/i915/i915_request.h b/drivers/gpu/drm/i915/i915_request.h
> index 620b6fab2c5c..92adfee30c7c 100644
> --- a/drivers/gpu/drm/i915/i915_request.h
> +++ b/drivers/gpu/drm/i915/i915_request.h
> @@ -434,7 +434,7 @@ static inline u32 hwsp_seqno(const struct i915_request *rq)
>  
>  static inline bool __i915_request_has_started(const struct i915_request *rq)
>  {
> -	return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno - 1);
> +	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno - 1);
>  }
>  
>  /**
> @@ -465,11 +465,19 @@ static inline bool __i915_request_has_started(const struct i915_request *rq)
>   */
>  static inline bool i915_request_started(const struct i915_request *rq)
>  {
> +	bool result;
> +
>  	if (i915_request_signaled(rq))
>  		return true;
>  
> -	/* Remember: started but may have since been preempted! */
> -	return __i915_request_has_started(rq);
> +	result = true;
> +	rcu_read_lock(); /* the HWSP may be freed at runtime */
> +	if (likely(!i915_request_signaled(rq)))
> +		/* Remember: started but may have since been preempted! */
> +		result = __i915_request_has_started(rq);
> +	rcu_read_unlock();
> +
> +	return result;
>  }
>  
>  /**
> @@ -482,10 +490,16 @@ static inline bool i915_request_started(const struct i915_request *rq)
>   */
>  static inline bool i915_request_is_running(const struct i915_request *rq)
>  {
> +	bool result;
> +
>  	if (!i915_request_is_active(rq))
>  		return false;
>  
> -	return __i915_request_has_started(rq);
> +	rcu_read_lock();
> +	result = __i915_request_has_started(rq) && i915_request_is_active(rq);
> +	rcu_read_unlock();
> +
> +	return result;
>  }
>  
>  /**
> @@ -509,12 +523,25 @@ static inline bool i915_request_is_ready(const struct i915_request *rq)
>  	return !list_empty(&rq->sched.link);
>  }
>  
> +static inline bool __i915_request_is_complete(const struct i915_request *rq)
> +{
> +	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
> +}
> +
>  static inline bool i915_request_completed(const struct i915_request *rq)
>  {
> +	bool result;
> +
>  	if (i915_request_signaled(rq))
>  		return true;
>  
> -	return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno);
> +	result = true;
> +	rcu_read_lock(); /* the HWSP may be freed at runtime */
> +	if (likely(!i915_request_signaled(rq)))
> +		result = __i915_request_is_complete(rq);
> +	rcu_read_unlock();
> +
> +	return result;
>  }
>  
>  static inline void i915_request_mark_complete(struct i915_request *rq)

-- 
Jani Nikula, Intel Open Source Graphics Center