Re: [PATCH -v2 1/5] sched: Fix ttwu() race

[Chris Wilson]

Quoting Peter Zijlstra (2020-06-22 11:01:23)
> @@ -2378,6 +2385,9 @@ static inline bool ttwu_queue_cond(int c
>  static bool ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags)
>  {
>         if (sched_feat(TTWU_QUEUE) && ttwu_queue_cond(cpu, wake_flags)) {
> +               if (WARN_ON_ONCE(cpu == smp_processor_id()))
> +                       return false;
> +
>                 sched_clock_cpu(cpu); /* Sync clocks across CPUs */
>                 __ttwu_queue_wakelist(p, cpu, wake_flags);
>                 return true;

We've been hitting this warning frequently, but have never seen the
rcu-torture-esque oops ourselves.

<4> [181.766705] RIP: 0010:ttwu_queue_wakelist+0xbc/0xd0
<4> [181.766710] Code: 00 00 00 5b 5d 41 5c 41 5d c3 31 c0 5b 5d 41 5c 41 5d c3 31 c0 f6 c3 08 74 f2 48 c7 c2 00 ad 03 00 83 7c 11 40 01 77 e4 eb 80 <0f> 0b 31 c0 eb dc 0f 1f 40 00 66 2e 0f 1f 84 00 00 00 00 00 bf 17
<4> [181.766726] RSP: 0018:ffffc90000003e08 EFLAGS: 00010046
<4> [181.766733] RAX: 0000000000000000 RBX: 00000000ffffffff RCX: ffff888276a00000
<4> [181.766740] RDX: 000000000003ad00 RSI: ffffffff8232045b RDI: ffffffff8233103e
<4> [181.766747] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
<4> [181.766754] R10: 00000000d3fa25c3 R11: 0000000053712267 R12: ffff88825b912940
<4> [181.766761] R13: 0000000000000000 R14: 0000000000000087 R15: 000000000003ad00
<4> [181.766769] FS:  0000000000000000(0000) GS:ffff888276a00000(0000) knlGS:0000000000000000
<4> [181.766777] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
<4> [181.766783] CR2: 000055b8245814e0 CR3: 0000000005610003 CR4: 00000000003606f0
<4> [181.766790] Call Trace:
<4> [181.766794]  <IRQ>
<4> [181.766798]  try_to_wake_up+0x21b/0x690
<4> [181.766805]  autoremove_wake_function+0xc/0x50
<4> [181.766858]  __i915_sw_fence_complete+0x1ee/0x250 [i915]
<4> [181.766912]  dma_i915_sw_fence_wake+0x2d/0x40 [i915]

We are seeing this on the ttwu_queue() path, so with p->on_cpu=0, and the
warning is cleared up by

-               if (WARN_ON_ONCE(cpu == smp_processor_id()))
+               if (WARN_ON_ONCE(p->on_cpu && cpu == smp_processor_id()))

which would appear to restore the old behaviour for ttwu_queue() and
seem to be consistent with the intent of this patch. Hopefully this
helps identify the problem correctly.
-Chris