Quoting Mika Kuoppala (2020-03-09 15:34:40) > Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes: > > > [ 206.875637] BUG: KCSAN: data-race in __i915_schedule+0x7fc/0x930 [i915] > > [ 206.875654] > > [ 206.875666] race at unknown origin, with read to 0xffff8881f7644480 of 8 bytes by task 703 on cpu 3: > > [ 206.875901] __i915_schedule+0x7fc/0x930 [i915] > > [ 206.876130] __bump_priority+0x63/0x80 [i915] > > [ 206.876361] __i915_sched_node_add_dependency+0x258/0x300 [i915] > > [ 206.876593] i915_sched_node_add_dependency+0x50/0xa0 [i915] > > [ 206.876824] i915_request_await_dma_fence+0x1da/0x530 [i915] > > [ 206.877057] i915_request_await_object+0x2fe/0x470 [i915] > > [ 206.877287] i915_gem_do_execbuffer+0x45dc/0x4c20 [i915] > > [ 206.877517] i915_gem_execbuffer2_ioctl+0x2c3/0x580 [i915] > > [ 206.877535] drm_ioctl_kernel+0xe4/0x120 > > [ 206.877549] drm_ioctl+0x297/0x4c7 > > [ 206.877563] ksys_ioctl+0x89/0xb0 > > [ 206.877577] __x64_sys_ioctl+0x42/0x60 > > [ 206.877591] do_syscall_64+0x6e/0x2c0 > > [ 206.877606] entry_SYSCALL_64_after_hwframe+0x44/0xa9 > > > > References: https://gitlab.freedesktop.org/drm/intel/issues/1318 > > Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> > > --- > > drivers/gpu/drm/i915/gt/intel_engine.h | 12 +++++++++++- > > 1 file changed, 11 insertions(+), 1 deletion(-) > > > > diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h b/drivers/gpu/drm/i915/gt/intel_engine.h > > index 29c8c03c5caa..f267f51c457c 100644 > > --- a/drivers/gpu/drm/i915/gt/intel_engine.h > > +++ b/drivers/gpu/drm/i915/gt/intel_engine.h > > @@ -107,7 +107,17 @@ execlists_num_ports(const struct intel_engine_execlists * const execlists) > > static inline struct i915_request * > > execlists_active(const struct intel_engine_execlists *execlists) > > { > > - return *READ_ONCE(execlists->active); > > + struct i915_request * const *cur = READ_ONCE(execlists->active); > > + struct i915_request * const *old; > > + struct i915_request *active; > > + > > + do { > > + old = cur; > > + active = READ_ONCE(*cur); > > + cur = READ_ONCE(execlists->active); > > + } while (cur != old); > > + > > + return active; > > The updated side is scary. We are updating the execlists->active > in two phases and handling the array copying in between. > > as WRITE_ONCE only guarantees ordering inside one context, due to > it is for compiler only, it makes me very suspicious about > how the memcpy of pending->inflight might unravel between two cpus. > > smb_store_mb(execlists->active, execlists->pending); > memcpy(inflight, pending) > smb_wmb(); > smb_store_mb(execlists->active, execlists->inflight); > smb_store_mb(execlists->pending[0], NULL); Not quite. You've overkill on the mb there. If you want to be pedantic, WRITE_ONCE(active, pending); smp_wmb(); memcpy(inflight, pending); smp_wmb(); WRITE_ONCE(active, inflight); The update of pending is not part of this sequence. But do we need that, and I still think we do not. > This in paired with: > > active = READ_ONCE(*cur); > smb_rmb(); > cur = READ_ONCE(execlists->active); > > With this, it should not matter at which point the execlists->active > is sampled as the pending would be guaranteed to be > immutable if it sampled early and inflight immutable if it > sampled late? Simply because we don't care about the sampling, just that the read dependency gives us a valid pointer. (We are not looking at a snapshot of several reads, but a _single_ read and the data dependency from that.) -Chris _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
