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SubjectRe: [PATCH 4/6] x86,kvm: Fix apf_task_wake_one() serialization
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On 05/09/2017 21:00, Davidlohr Bueso wrote:
> During code inspection, the following potential race was seen:
>
> CPU0 CPU1
> kvm_async_pf_task_wait apf_task_wake_one
> [S] prepare_to_swait(&n.wq)
> [L] swait_active(&n->wq)
> [S] hlist_del_init(&n->link);
> [L] if (!hlist_unhahed(&n.link))
> schedule()
>
> Properly serialize swait_active() checks such that a wakeup is
> not missed.
>
> Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
> ---
> arch/x86/kernel/kvm.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index 874827b0d7ca..aa60a08b65b1 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -180,7 +180,7 @@ static void apf_task_wake_one(struct kvm_task_sleep_node *n)
> hlist_del_init(&n->link);
> if (n->halted)
> smp_send_reschedule(n->cpu);
> - else if (swait_active(&n->wq))
> + else if (swq_has_sleeper(&n->wq))
> swake_up(&n->wq);
> }

After Nick's patch, swake_up starts with:

smp_mb();
if (!swait_active(q))
return;

so we can just remove the test here (and in patch 2).

The other patches could also use a better swait API, for example:

1) add a public __swake_up routine that omits the memory barrier, and
which can be used in patch 3. Perhaps better: omit the out-of-lock
check in __swake_up: then the caller can use it if it knows there is a
waiter. In those cases the memory barrier is expensive.

2) change swake_up and __swake_up to return true if they woke up a
process (or alternatively 0/-EAGAIN). Patches 5 and 6 now need not call
anymore either swq_has_sleepers or swait_active, and that saves a memory
barrier too.

What do you think?

Thanks,

Paolo

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