Chris Wilson <chris@xxxxxxxxxxxxxxxxxx> writes:
> Since we are using bitops on context_tag to allow us to reserve and
> release inflight tags concurrently, the scan for the next bit is
> intentionally racy.
Now it truely sinks in why you started with a full mask.
Reviewed-by: Mika Kuoppala <mika.kuoppala@xxxxxxxxxxxxxxx>
-Mika
>
> [ 516.446854] BUG: KCSAN: data-race in execlists_schedule_in.isra.0 [i915] / execlists_schedule_out [i915]
> [ 516.446874]
> [ 516.446886] write (marked) to 0xffff8881f7644048 of 8 bytes by interrupt on cpu 2:
> [ 516.447076] execlists_schedule_out+0x538/0x6a0 [i915]
> [ 516.447263] process_csb+0x10b/0x3d0 [i915]
> [ 516.447449] execlists_submission_tasklet+0x30/0x170 [i915]
> [ 516.447468] tasklet_action_common.isra.0+0x42/0x90
> [ 516.447484] __do_softirq+0xc8/0x206
> [ 516.447498] irq_exit+0xcd/0xe0
> [ 516.447516] do_IRQ+0x44/0xc0
> [ 516.447535] ret_from_intr+0x0/0x1c
> [ 516.447550] cpuidle_enter_state+0x199/0x400
> [ 516.447572] cpuidle_enter+0x50/0x90
> [ 516.447587] do_idle+0x197/0x1e0
> [ 516.447600] cpu_startup_entry+0x14/0x20
> [ 516.447619] start_secondary+0xf9/0x130
> [ 516.447643] secondary_startup_64+0xa4/0xb0
> [ 516.447655]
> [ 516.447671] read to 0xffff8881f7644048 of 8 bytes by task 460 on cpu 1:
> [ 516.447863] execlists_schedule_in.isra.0+0x3cf/0x5a0 [i915]
> [ 516.448064] execlists_dequeue+0xf8f/0x1690 [i915]
> [ 516.448252] __execlists_submission_tasklet+0x48/0x60 [i915]
> [ 516.448440] execlists_submit_request+0x2e2/0x310 [i915]
> [ 516.448634] submit_notify+0x8f/0xc8 [i915]
> [ 516.448820] __i915_sw_fence_complete+0x61/0x420 [i915]
> [ 516.449005] i915_sw_fence_complete+0x58/0x80 [i915]
> [ 516.449208] i915_sw_fence_commit+0x16/0x20 [i915]
> [ 516.449399] __i915_request_queue+0x60/0x70 [i915]
> [ 516.449590] i915_gem_do_execbuffer+0x33f1/0x4a00 [i915]
> [ 516.449782] i915_gem_execbuffer2_ioctl+0x2a2/0x550 [i915]
> [ 516.449800] drm_ioctl_kernel+0xe9/0x130
> [ 516.449814] drm_ioctl+0x27d/0x45e
> [ 516.449827] ksys_ioctl+0x89/0xb0
> [ 516.449842] __x64_sys_ioctl+0x42/0x60
> [ 516.449864] do_syscall_64+0x6e/0x2c0
> [ 516.449878] entry_SYSCALL_64_after_hwframe+0x44/0xa9
>
> Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
> ---
> drivers/gpu/drm/i915/gt/intel_lrc.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> index 8e254f639751..ed45fc40f884 100644
> --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> @@ -1367,7 +1367,7 @@ __execlists_schedule_in(struct i915_request *rq)
> ce->lrc.ccid = ce->tag;
> } else {
> /* We don't need a strict matching tag, just different values */
> - unsigned int tag = ffs(engine->context_tag);
> + unsigned int tag = ffs(READ_ONCE(engine->context_tag));
>
> GEM_BUG_ON(tag == 0 || tag >= BITS_PER_LONG);
> clear_bit(tag - 1, &engine->context_tag);
> --
> 2.20.1
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