Quoting Mika Kuoppala (2020-03-09 16:38:49) > Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes: > > > 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); > > This is the crux of it, needing rmb counterpart. I suspect this is overkill, but if we really do want the change in active to be visible before the memcpy, that wmb is strictly required. -Chris _______________________________________________ Intel-gfx mailing list Intel-gfx@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/intel-gfx
