Re: [PATCH 07/25] drm/i915: Cancel context if it hangs after it is closed

[Mika Kuoppala <mika.kuoppala@linux.intel.com>]

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

> Quoting Mika Kuoppala (2019-11-11 10:54:14)
>> Chris Wilson <chris@chris-wilson.co.uk> writes:
>> 
>> > If we detect a hang in a closed context, just flush all of its requests
>> > and cancel any remaining execution along the context. Note that after
>> > closing the context, the last reference to the context may be dropped,
>> > leaving it only valid under RCU.
>> 
>> Sound good. But is there a window for userspace to start
>> to see -EIO if it resubmits to a closed context?
>
> Userspace can not submit to a closed context (-ENOENT) as that would be
> tantamount to a use-after-free kernel bug.
>  
>> In other words, after userspace doing gem_ctx_destroy(ctx_handle),
>> we would return -EINVAL due to ctx_handle being stale
>> earlier than we check for banned status and return -EIO?
>
> It's as simple as if the context is closed, it is removed from the
> file->context_idr and userspace cannot access it. If userspace is racing
> with itself, there's not much we can do other than protect our
> references. If userspace succeeds in submitting to the context prior to
> closing it in another thread, it has the context to continue (and if
> then hangs, it will be shot down immediately). If it loses that race, it
> gets an -ENOENT. If it loses that race so badly the context id is
> replace by a new context, it submits to that new context; which surely
> will end in tears and GPU hangs, but not our fault and nothing we can do
> to prevent that.

Let them shed tears if they bring it on themselves. I was concerned
on a behavioural change on close/resubmit race. But as you explained
racing on a different id, they deserve what they begged for.

We are in a business of protecting the state of all the sane
ones.

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