From 331bf90591573dfe6c8e892239713ef9702f1396 Mon Sep 17 00:00:00 2001 From: Chris Wilson Date: Mon, 25 Nov 2019 09:43:18 +0000 Subject: drm/i915/gt: Mark the execlists->active as the primary volatile access Since we want to do a lockless read of the current active request, and that request is written to by process_csb also without serialisation, we need to instruct gcc to take care in reading the pointer itself. Otherwise, we have observed execlists_active() to report 0x40. [ 2400.760381] igt/para-4098 1..s. 2376479300us : process_csb: rcs0 cs-irq head=3, tail=4 [ 2400.760826] igt/para-4098 1..s. 2376479303us : process_csb: rcs0 csb[4]: status=0x00000001:0x00000000 [ 2400.761271] igt/para-4098 1..s. 2376479306us : trace_ports: rcs0: promote { b9c59:2622, b9c55:2624 } [ 2400.761726] igt/para-4097 0d... 2376479311us : __i915_schedule: rcs0: -2147483648->3, inflight:0000000000000040, rq:ffff888208c1e940 which is impossible! The answer is that as we keep the existing execlists->active pointing into the array as we copy over that array, the unserialised read may see a partial pointer value. Fixes: df403069029d ("drm/i915/execlists: Lift process_csb() out of the irq-off spinlock") Signed-off-by: Chris Wilson Reviewed-by: Mika Kuoppala Link: https://patchwork.freedesktop.org/patch/msgid/20191125094318.1630806-1-chris@chris-wilson.co.uk --- drivers/gpu/drm/i915/gt/intel_engine.h | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) (limited to 'drivers/gpu/drm/i915/gt/intel_engine.h') diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h b/drivers/gpu/drm/i915/gt/intel_engine.h index bc3b72bfa9e3..01765a7ec18f 100644 --- a/drivers/gpu/drm/i915/gt/intel_engine.h +++ b/drivers/gpu/drm/i915/gt/intel_engine.h @@ -100,9 +100,7 @@ execlists_num_ports(const struct intel_engine_execlists * const execlists) static inline struct i915_request * execlists_active(const struct intel_engine_execlists *execlists) { - GEM_BUG_ON(execlists->active - execlists->inflight > - execlists_num_ports(execlists)); - return READ_ONCE(*execlists->active); + return *READ_ONCE(execlists->active); } static inline void -- cgit