Re: [PATCH v3] drm/i915: Ensure associated VMAs are inactive when contexts are destroyed

[Daniel Vetter <daniel@ffwll.ch>]

On Tue, Nov 17, 2015 at 05:24:01PM +0000, Tvrtko Ursulin wrote:
> 
> On 17/11/15 17:08, Daniel Vetter wrote:
> >On Tue, Nov 17, 2015 at 04:54:50PM +0000, Tvrtko Ursulin wrote:
> >>
> >>On 17/11/15 16:39, Daniel Vetter wrote:
> >>>On Tue, Nov 17, 2015 at 04:27:12PM +0000, Tvrtko Ursulin wrote:
> >>>>From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>>>
> >>>>In the following commit:
> >>>>
> >>>>     commit e9f24d5fb7cf3628b195b18ff3ac4e37937ceeae
> >>>>     Author: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>>>     Date:   Mon Oct 5 13:26:36 2015 +0100
> >>>>
> >>>>         drm/i915: Clean up associated VMAs on context destruction
> >>>>
> >>>>I added a WARN_ON assertion that VM's active list must be empty
> >>>>at the time of owning context is getting freed, but that turned
> >>>>out to be a wrong assumption.
> >>>>
> >>>>Due ordering of operations in i915_gem_object_retire__read, where
> >>>>contexts are unreferenced before VMAs are moved to the inactive
> >>>>list, the described situation can in fact happen.
> >>>>
> >>>>It feels wrong to do things in such order so this fix makes sure
> >>>>a reference to context is held until the move to inactive list
> >>>>is completed.
> >>>>
> >>>>v2: Rather than hold a temporary context reference move the
> >>>>     request unreference to be the last operation. (Daniel Vetter)
> >>>>
> >>>>v3: Fix use after free. (Chris Wilson)
> >>>>
> >>>>Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> >>>>Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92638
> >>>>Cc: Michel Thierry <michel.thierry@intel.com>
> >>>>Cc: Chris Wilson <chris@chris-wilson.co.uk>
> >>>>Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> >>>>---
> >>>>  drivers/gpu/drm/i915/i915_gem.c | 33 ++++++++++++++++++---------------
> >>>>  1 file changed, 18 insertions(+), 15 deletions(-)
> >>>>
> >>>>diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> >>>>index 98c83286ab68..094ac17a712d 100644
> >>>>--- a/drivers/gpu/drm/i915/i915_gem.c
> >>>>+++ b/drivers/gpu/drm/i915/i915_gem.c
> >>>>@@ -2404,29 +2404,32 @@ i915_gem_object_retire__read(struct drm_i915_gem_object *obj, int ring)
> >>>>  	RQ_BUG_ON(!(obj->active & (1 << ring)));
> >>>>
> >>>>  	list_del_init(&obj->ring_list[ring]);
> >>>>-	i915_gem_request_assign(&obj->last_read_req[ring], NULL);
> >>>>
> >>>>  	if (obj->last_write_req && obj->last_write_req->ring->id == ring)
> >>>>  		i915_gem_object_retire__write(obj);
> >>>>
> >>>>  	obj->active &= ~(1 << ring);
> >>>>-	if (obj->active)
> >>>>-		return;
> >>>
> >>>	if (obj->active) {
> >>>		i915_gem_request_assign(&obj->last_read_req[ring], NULL);
> >>>		return;
> >>>	}
> >>>
> >>>Would result in less churn in the code and drop the unecessary indent
> >>>level. Also comment is missing as to why we need to do things in a
> >>>specific order.
> >>
> >>Actually I think I changed my mind and that v1 is the way to go.
> >>
> >>Just re-ordering the code here still makes it possible for the context
> >>destructor to run with VMAs on the active list I think.
> >>
> >>If we hold the context then it is 100% clear it is not possible.
> >
> >request_assign _is_ the function which adjust the refcounts for us, which
> >means if we drop that reference too early then grabbing a temp reference
> >is just papering over the real bug.
> >
> >Written out your patch looks something like
> >
> >	a_reference(a);
> >	a_unreference(a);
> >
> >	/* more cleanup code that should get run before a_unreference but isn't */
> >
> >	a_unrefernce(a); /* for real this time */
> >
> >Unfortunately foo_assign is a new pattern and not well-established, so
> >that connection isn't clear. Maybe we should rename it to
> >foo_reference_assign to make it more obvious. Or just drop the pretense
> >and open-code it since we unconditionally assign NULL as the new pointer
> >value, and we know the current value of the pointer is non-NULL. So
> >there's really no benefit to the helper here, it only obfuscates. And
> >since that obfuscation tripped you up it's time to remove it ;-)
> 
> Then foo_reference_unreference_assign. :)
> 
> But seriously, I think it is more complicated that..
> 
> The thing it trips over is that moving VMAs to inactive does not correspond
> in time to request retirement. But in fact VMAs are moved to inactive only
> when all requests associated with an object are done.
> 
> This is the unintuitive thing I was working around. To make sure when
> context destructor runs there are not active VMAs for that VM.
> 
> I don't know how to guarantee that with what you propose. Perhaps I am
> missing something?

Ok, my example was slightly off, since we have 2 objects:

	b_reference(a->b);
	a_unreference(a); /* might unref a->b if it's the last reference */

	/* more cleanup code that should get run before a_unreference but isn't */

	b_unrefernce(a->b); /* for real this time */

Holding the ref to a makes sure that b doesn't disappear. We rely on that
in a fundamental way (a request really needs the ctx to stick around), and
the bug really is that we drop the ref to a too early. That it's the
releasing of a->b which is eventually blowing things up doesn't really
matter.

Btw would it be possible to have an igt for this? I should be possible to
hit this with some varian of gem_unref_active_buffers.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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