Chris Wilson <ch...@chris-wilson.co.uk> 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
>
> v2: Be safe and include mb
>
> References: https://gitlab.freedesktop.org/drm/intel/issues/1318
> Signed-off-by: Chris Wilson <ch...@chris-wilson.co.uk>
> Cc: Mika Kuoppala <mika.kuopp...@linux.intel.com>
> ---
> drivers/gpu/drm/i915/gt/intel_engine.h | 15 ++++++++++++++-
> drivers/gpu/drm/i915/gt/intel_lrc.c | 12 +++++++-----
> 2 files changed, 21 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_engine.h
> b/drivers/gpu/drm/i915/gt/intel_engine.h
> index 29c8c03c5caa..3644fd2b8877 100644
> --- a/drivers/gpu/drm/i915/gt/intel_engine.h
> +++ b/drivers/gpu/drm/i915/gt/intel_engine.h
> @@ -107,7 +107,20 @@ 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, * const *old, *active;
> +
> + cur = READ_ONCE(execlists->active);
> + smp_rmb(); /* pairs with overwrite protection in process_csb() */
> + do {
> + old = cur;
> +
> + active = READ_ONCE(*cur);
> + cur = READ_ONCE(execlists->active);
> +
> + smp_rmb(); /* and complete the seqlock retry */
> + } while (cur != old);
> +
> + return active;
> }
>
> static inline void
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c
> b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index 3eb7adc4e057..f4b7fdfa2d63 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -2389,6 +2389,7 @@ static void process_csb(struct intel_engine_cs *engine)
>
> /* Point active to the new ELSP; prevent overwriting */
> WRITE_ONCE(execlists->active, execlists->pending);
> + smp_wmb(); /* notify execlists_active() */
>
> /* cancel old inflight, prepare for switch */
> trace_ports(execlists, "preempted", old);
> @@ -2396,11 +2397,12 @@ static void process_csb(struct intel_engine_cs
> *engine)
> execlists_schedule_out(*old++);
>
> /* switch pending to inflight */
> - WRITE_ONCE(execlists->active,
> - memcpy(execlists->inflight,
> - execlists->pending,
> - execlists_num_ports(execlists) *
> - sizeof(*execlists->pending)));
> + memcpy(execlists->inflight,
> + execlists->pending,
> + execlists_num_ports(execlists) *
> + sizeof(*execlists->pending));
> + smp_wmb(); /* complete the seqlock */
> + WRITE_ONCE(execlists->active, execlists->inflight);
Now the memcpy bit is nicely in order,
Reviewed-by: Mika Kuoppala <mika.kuopp...@linux.intel.com>
>
> WRITE_ONCE(execlists->pending[0], NULL);
> } else {
> --
> 2.20.1
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