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    • Daniel Vetter's avatar
      drm/i915: clarify concurrent hang detect/gpu reset consistency · 7db0ba24
      Daniel Vetter authored 
      Damien Lespiau wondered how race the gpu reset/hang detection code is
against concurrent gpu resets/hang detections or combinations thereof.
Luckily the single work item is guranteed to never run concurrently,
so reset handling is already single-threaded.

Hence we only have to worry about concurrent hang detections, or a
hang detection firing off while we're still processing an older gpu
reset request. Due to the new mechanism of setting the reset in
progress flag and the ordering guaranteed by the schedule_work
function there's nothing to do but add a comment explaining why we're
safe.

The only thing I've noticed is that we still try to reset the gpu now,
even when it is declared terminally wedged. Add a check for that to
avoid continous warnings about failed resets, in case the hangcheck
timer ever gets stuck.
Reviewed-by: default avatarDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
      7db0ba24
    • Daniel Vetter's avatar
      drm/i915: create a race-free reset detection · f69061be
      Daniel Vetter authored 
      With the previous patch the state transition handling of the reset
code itself is now (hopefully) race free and solid. But that still
leaves out everyone else - with the various lock-free wait paths
we have there's the possibility that the reset happens between the
point where we read the seqno we should wait on and the actual wait.

And if __wait_seqno then never sees the RESET_IN_PROGRESS state, we'll
happily wait for a seqno which will in all likelyhood never signal.

In practice this is not a big problem since the X server gets
constantly interrupted, and can then submit more work (hopefully) to
unblock everyone else: As soon as a new seqno write lands, all waiters
will unblock. But running the i-g-t reset testcase ZZ_hangman can
expose this race, especially on slower hw with fewer cpu cores.

Now looking forward to ARB_robustness and friends that's not the best
possible behaviour, hence this patch adds a reset_counter to be able
to detect any reset, even if a given thread never observed the
in-progress state.

The important part is to correctly order things:
- The write side needs to increment the counter after any seqno gets
  reset.  Hence we need to do that at the end of the reset work, and
  again wake everyone up. We also need to place a barrier in between
  any possible seqno changes and the counter increment, since any
  unlock operations only guarantee that nothing leaks out, but not
  that at later load operation gets moved ahead.
- On the read side we need to ensure that no reset can sneak in and
  invalidate the seqno. In all cases we can use the one-sided barrier
  that unlock operations guarantee (of the lock protecting the
  respective seqno/ring pair) to ensure correct ordering. Hence it is
  sufficient to place the atomic read before the mutex/spin_unlock and
  no additional barriers are required.

The end-result of all this is that we need to wake up everyone twice
in a reset operation:
- First, before the reset starts, to get any lockholders of the locks,
  so that the reset can proceed.
- Second, after the reset is completed, to allow waiters to properly
  and reliably detect the reset condition and bail out.

I admit that this entire reset_counter thing smells a bit like
overkill, but I think it's justified since it makes it really explicit
what the bail-out condition is. And we need a reset counter anyway to
implement ARB_robustness, and imo with finer-grained locking on the
horizont this is the most resilient scheme I could think of.

v2: Drop spurious change in the wait_for_error EXIT_COND - we only
need to wait until we leave the reset-in-progress wedged state.

v3: Don't play tricks with barriers in the throttle ioctl, the
spin_unlock is barrier enough.

I've also considered using a little helper to grab the current
reset_counter, but then decided that hiding the atomic_read isn't a
great idea, since having it explicitly show up in the code is a nice
remainder to reviews to check the memory barriers.

v4: Add a comment to explain why we need to fall through in
__wait_seqno in the end variable assignments.

v5: Review from Damien:
- s/smb/smp/ in a comment
- don't increment the reset counter after we've set it to WEDGED. Now
  we (again) properly wedge the gpu when the reset fails.
Reviewed-by: default avatarDamien Lespiau <damien.lespiau@intel.com>
Signed-off-by: default avatarDaniel Vetter <daniel.vetter@ffwll.ch>
      f69061be
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