[Bug 112402] [CI][BAT]igt at i915_selftest@live_gem_contexts - dmesg-fail - igt_ctx_sseu failed with error -\d+

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=112402

Chris Wilson <chris at chris-wilson.co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|---                         |FIXED

--- Comment #3 from Chris Wilson <chris at chris-wilson.co.uk> ---
commit df9f85d8582ebda052835c55ae940e4f866e1ef5
Author: Chris Wilson <chris at chris-wilson.co.uk>
Date:   Wed Nov 27 13:45:27 2019 +0000

    drm/i915: Serialise i915_active_fence_set() with itself

    The expected downside to commit 58b4c1a07ada ("drm/i915: Reduce nested
    prepare_remote_context() to a trylock") was that it would need to return
    -EAGAIN to userspace in order to resolve potential mutex inversion. Such
    an unsightly round trip is unnecessary if we could atomically insert a
    barrier into the i915_active_fence, so make it happen.

    Currently, we use the timeline->mutex (or some other named outer lock)
    to order insertion into the i915_active_fence (and so individual nodes
    of i915_active). Inside __i915_active_fence_set, we only need then
    serialise with the interrupt handler in order to claim the timeline for
    ourselves.

    However, if we remove the outer lock, we need to ensure the order is
    intact between not only multiple threads trying to insert themselves
    into the timeline, but also with the interrupt handler completing the
    previous occupant. We use xchg() on insert so that we have an ordered
    sequence of insertions (and each caller knows the previous fence on
    which to wait, preserving the chain of all fences in the timeline), but
    we then have to cmpxchg() in the interrupt handler to avoid overwriting
    the new occupant. The only nasty side-effect is having to temporarily
    strip off the RCU-annotations to apply the atomic operations, otherwise
    the rules are much more conventional!

    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=112402
    Fixes: 58b4c1a07ada ("drm/i915: Reduce nested prepare_remote_context() to a
trylock")
    Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
    Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
    Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
    Link:
https://patchwork.freedesktop.org/patch/msgid/20191127134527.3438410-1-chris@chris-wilson.co.uk

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