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This reverts commit6c943de668("drm/i915: Skip execlists_dequeue() early if the list is empty"). The validity of using READ_ONCE there depends upon having a mb to coordinate the assignment of engine->execlist_first inside submit_request() and checking prior to taking the spinlock in execlists_dequeue(). We wrote "the update to TASKLET_SCHED incurs a memory barrier making this cross-cpu checking safe", but failed to notice that this mb was *conditional* on the execlists being ready, i.e. there wasn't the required mb when it was most necessary! We could install an unconditional memory barrier to fixup the READ_ONCE(): diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c index 7dd732cb9f57..1ed164b16d44 100644 --- a/drivers/gpu/drm/i915/intel_lrc.c +++ b/drivers/gpu/drm/i915/intel_lrc.c @@ -616,6 +616,7 @@ static void execlists_submit_request(struct drm_i915_gem_request *request) if (insert_request(&request->priotree, &engine->execlist_queue)) { engine->execlist_first = &request->priotree.node; + smp_wmb(); if (execlists_elsp_ready(engine)) But we have opted to remove the race as it should be rarely effective, and saves us having to explain the necessary memory barriers which we quite clearly failed at. Reported-and-tested-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> Fixes:6c943de668("drm/i915: Skip execlists_dequeue() early if the list is empty") Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com> Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com> Cc: Michał Winiarski <michal.winiarski@intel.com> Link: http://patchwork.freedesktop.org/patch/msgid/20170329100052.29505-1-chris@chris-wilson.co.uk Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.