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

On 02/12/2015 07:20 PM, Oleg Nesterov wrote:
On 02/12, Raghavendra K T wrote:

@@ -191,8 +189,7 @@ static inline void arch_spin_unlock_wait(arch_spinlock_t 
                 * 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)

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

Good catch. othewise we would wrongly break out even when somebody
does halt wait.

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

Yes again :)

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