drm/i915/fence: Avoid del_timer_sync() from inside a timer
A fence may be signaled from any context, including from inside a timer. One example is timer_i915_sw_fence_wake() which is used to provide a safety-net when waiting on an external fence. If the external fence is not signaled within a timely fashion, we signal our fence on its behalf, and so we then may process subsequent fences in the chain from within that timer context. Given that dma_i915_sw_fence_wake() may be from inside a timer, we cannot then use del_timer_sync() as that requires the timer lock for itself. To circumvent this, while trying to keep the signal propagation as low latency as possible, move the completion into a worker and use a bit of atomic switheroo to serialise the timer-callback and the dma-callback. Testcase: igt/gem_eio/in-flight-external Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20170911084135.22903-3-chris@chris-wilson.co.ukReviewed-by: NTvrtko Ursulin <tvrtko.ursulin@intel.com>
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