[PATCH] drm/i915/request: fix early tracepoints

[Daniel Vetter]

On Fri, Sep 03, 2021 at 12:24:05PM +0100, Matthew Auld wrote:
> Currently we blow up in trace_dma_fence_init, when calling into
> get_driver_name or get_timeline_name, since both the engine and context
> might be NULL(or contain some garbage address) in the case of newly
> allocated slab objects via the request ctor. Note that we also use
> SLAB_TYPESAFE_BY_RCU here, which allows requests to be immediately
> freed, but delay freeing the underlying page by an RCU grace period.
> With this scheme requests can be re-allocated, at the same time as they
> are also being read by some lockless RCU lookup mechanism.
> 
> One possible fix, since we don't yet have a fully initialised request
> when in the ctor, is just setting the context/engine as NULL and adding
> some extra handling in get_driver_name etc. And since the ctor is only
> called for new slab objects(i.e allocate new page and call the ctor for
> each object) it's safe to reset the context/engine prior to calling into
> dma_fence_init, since we can be certain that no one is doing an RCU
> lookup which might depend on peeking at the engine/context, like in
> active_engine(), since the object can't yet be externally visible.
> 
> In the recycled case(which might also be externally visible) the request
> refcount always transitions from 0->1 after we set the context/engine
> etc, which should ensure it's valid to dereference the engine for
> example, when doing an RCU list-walk, so long as we can also increment
> the refcount first. If the refcount is already zero, then the request is
> considered complete/released.  If it's non-zero, then the request might
> be in the process of being re-allocated, or potentially still in flight,
> however after successfully incrementing the refcount, it's possible to
> carefully inspect the request state, to determine if the request is
> still what we were looking for. Note that all externally visible
> requests returned to the cache must have zero refcount.

The commit message here is a bit confusing, since you start out with
describing a solution that you're not actually implementing it. I usually
do this by putting alternate solutions at the bottom, starting with "An
alternate solution would be ..." or so.

And then closing with why we don't do that, here it would be that we do
no longer have a need for these partially set up i915_requests, and
therefore just reverting that complication is the simplest solution.

> An alternative fix then is to instead move the dma_fence_init out from
> the request ctor. Originally this was how it was done, but it was moved
> in:
> 
> commit 855e39e65cfc33a73724f1cc644ffc5754864a20
> Author: Chris Wilson <chris at chris-wilson.co.uk>
> Date:   Mon Feb 3 09:41:48 2020 +0000
> 
>     drm/i915: Initialise basic fence before acquiring seqno
> 
> where it looks like intel_timeline_get_seqno() relied on some of the
> rq->fence state, but that is no longer the case since:
> 
> commit 12ca695d2c1ed26b2dcbb528b42813bd0f216cfc
> Author: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> Date:   Tue Mar 23 16:49:50 2021 +0100
> 
>     drm/i915: Do not share hwsp across contexts any more, v8.
> 
> intel_timeline_get_seqno() could also be cleaned up slightly by dropping
> the request argument.
> 
> Moving dma_fence_init back out of the ctor, should ensure we have enough
> of the request initialised in case of trace_dma_fence_init.
> Functionally this should be the same, and is effectively what we were
> already open coding before, except now we also assign the fence->lock
> and fence->ops, but since these are invariant for recycled
> requests(which might be externally visible), and will therefore already
> hold the same value, it shouldn't matter. We still leave the
> spin_lock_init() in the ctor, since we can't re-init the rq->lock in
> case it is already held.

Holding rq->lock without having a full reference to it sounds like really
bad taste. I think it would be good to have a (kerneldoc) comment next to
i915_request.lock about this, with a FIXME. But separate patch.

> Fixes: 855e39e65cfc ("drm/i915: Initialise basic fence before acquiring seqno")
> Signed-off-by: Matthew Auld <matthew.auld at intel.com>
> Cc: Michael Mason <michael.w.mason at intel.com>
> Cc: Daniel Vetter <daniel at ffwll.ch>

With the commit message restructured a bit, and assuming this one actually
works:

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

But I'm really not confident :-(
-Daniel

> ---
>  drivers/gpu/drm/i915/i915_request.c | 11 ++---------
>  1 file changed, 2 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index ce446716d092..79da5eca60af 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -829,8 +829,6 @@ static void __i915_request_ctor(void *arg)
>  	i915_sw_fence_init(&rq->submit, submit_notify);
>  	i915_sw_fence_init(&rq->semaphore, semaphore_notify);
>  
> -	dma_fence_init(&rq->fence, &i915_fence_ops, &rq->lock, 0, 0);
> -
>  	rq->capture_list = NULL;
>  
>  	init_llist_head(&rq->execute_cb);
> @@ -905,17 +903,12 @@ __i915_request_create(struct intel_context *ce, gfp_t gfp)
>  	rq->ring = ce->ring;
>  	rq->execution_mask = ce->engine->mask;
>  
> -	kref_init(&rq->fence.refcount);
> -	rq->fence.flags = 0;
> -	rq->fence.error = 0;
> -	INIT_LIST_HEAD(&rq->fence.cb_list);
> -
>  	ret = intel_timeline_get_seqno(tl, rq, &seqno);
>  	if (ret)
>  		goto err_free;
>  
> -	rq->fence.context = tl->fence_context;
> -	rq->fence.seqno = seqno;
> +	dma_fence_init(&rq->fence, &i915_fence_ops, &rq->lock,
> +		       tl->fence_context, seqno);
>  
>  	RCU_INIT_POINTER(rq->timeline, tl);
>  	rq->hwsp_seqno = tl->hwsp_seqno;
> -- 
> 2.26.3
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch