Re: [Xen-devel] [PATCH V3] x86 spinlock: Fix memory corruption on completing completions

[Oleg Nesterov <oleg@xxxxxxxxxx>]

On 02/12, Raghavendra K T wrote:
>
> @@ -191,8 +189,7 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t 
> *lock)
>                * We need to check "unlocked" in a loop, tmp.head == head
>                * can be false positive because of overflow.
>                */
> -             if (tmp.head == (tmp.tail & ~TICKET_SLOWPATH_FLAG) ||
> -                 tmp.head != head)
> +             if (__tickets_equal(tmp.head, tmp.tail) || tmp.head != head)
>                       break;

Ah, it seems that "tmp.head != head" should be turned into
!__tickets_equal(), no?

Suppose that TICKET_SLOWPATH_FLAG is set after the first ACCESS_ONCE(head),
then tmp.head != head will be true before the first unlock we are waiting
for.

And perhaps you can turn these ACCESS_ONCE into READ_ONCE as well.

Oleg.


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