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    Subject[PATCH 5.5 086/151] drm/i915: Return early for await_start on same timeline
    Date
    From: Chris Wilson <chris@chris-wilson.co.uk>

    commit c951b0af2dddbb1f34be103029eb9030392d5554 upstream.

    Requests within a timeline are ordered by that timeline, so awaiting for
    the start of a request within the timeline is a no-op. This used to work
    by falling out of the mutex_trylock() as the signaler and waiter had the
    same timeline and not returning an error.

    Fixes: 6a79d848403d ("drm/i915: Lock signaler timeline while navigating")
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
    Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
    Cc: <stable@vger.kernel.org> # v5.5+
    Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20200305134822.2750496-1-chris@chris-wilson.co.uk
    (cherry picked from commit ab7a69020fb5d5c7ba19fba60f62fd6f9ca9f779)
    Signed-off-by: Jani Nikula <jani.nikula@intel.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/gpu/drm/i915/i915_request.c | 4 ++--
    1 file changed, 2 insertions(+), 2 deletions(-)

    --- a/drivers/gpu/drm/i915/i915_request.c
    +++ b/drivers/gpu/drm/i915/i915_request.c
    @@ -759,8 +759,8 @@ i915_request_await_start(struct i915_req
    struct dma_fence *fence;
    int err;

    - GEM_BUG_ON(i915_request_timeline(rq) ==
    - rcu_access_pointer(signal->timeline));
    + if (i915_request_timeline(rq) == rcu_access_pointer(signal->timeline))
    + return 0;

    rcu_read_lock();
    tl = rcu_dereference(signal->timeline);

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