On 13/04/16 15:21, John Harrison wrote:
On 13/04/2016 10:57, Daniel Vetter wrote:
On Tue, Apr 12, 2016 at 09:03:09PM +0100, Chris Wilson wrote:
Conceptually, each request is a record of a hardware transaction - we
build up a list of pending commands and then either commit them to
hardware, or cancel them. However, whilst building up the list of
pending commands, we may modify state outside of the request and make
references to the pending request. If we do so and then cancel that
request, external objects then point to the deleted request leading to
both graphical and memory corruption.
The easiest example is to consider object/VMA tracking. When we mark an
object as active in a request, we store a pointer to this, the most
recent request, in the object. Then we want to free that object, we wait
for the most recent request to be idle before proceeding (otherwise the
hardware will write to pages now owned by the system, or we will attempt
to read from those pages before the hardware is finished writing). If
the request was cancelled instead, that wait completes immediately. As a
result, all requests must be committed and not cancelled if the external
state is unknown.
All that remains of i915_gem_request_cancel() users are just a couple of
extremely unlikely allocation failures, so remove the API entirely.
A consequence of committing all incomplete requests is that we generate
excess breadcrumbs and fill the ring much more often with dummy work. We
have completely undone the outstanding_last_seqno optimisation.
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=93907
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: John Harrison <John.C.Harrison@xxxxxxxxx>
I'd like John's ack on this on too, but patch itself looks sound. Fast
r-b
since we've discussed this a while ago already ...
I think this is going to cause a problem with the scheduler. You are
effectively saying that the execbuf call cannot fail beyond the point of
allocating a request. If it gets that far then it must go all the way
and submit the request to the hardware. With a scheduler, that means
adding it to the scheduler's queues and tracking it all the way through
the system to completion. If nothing else, that sounds like a lot of
extra overhead for no actual work. Or worse if the failure is at a point
where the request cannot be sent further through the system because it
was something critical that failed then you are really stuffed.
I'm not sure what the other option would be though, short of being able
to undo the last read/write object tracking updates.
With the chained-ownership code we have in the scheduler, it becomes
perfectly possible to undo the last-read/write tracking changes.
I'd much rather see any failure during submission rewound and undone, so
we can just return -EAGAIN at any point and let someone retry if required.
This just looks like a hack to work around not having a properly
transactional model of request submission :(
.Dave.
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