Re: [PATCH V5] x86 spinlock: Fix memory corruption on completing completions

Oleg Nesterov
2015-02-15T16:19:53Z

Well, I regret I mentioned the lack of barrier after enter_slowpath ;)

On 02/15, Raghavendra K T wrote:
>
> @@ -46,7 +46,8 @@ static __always_inline bool static_key_false(struct static_key *key);
>
> static inline void __ticket_enter_slowpath(arch_spinlock_t *lock)
> {
> - set_bit(0, (volatile unsigned long *)&lock->tickets.tail);
> + set_bit(0, (volatile unsigned long *)&lock->tickets.head);
> + barrier();
> }

Because this barrier() looks really confusing.

Firsty, it is equally unneeded on x86. At the same time, it can not help.
We need a memory barrier() between set_bit(SLOWPATH) and READ_ONCE(head)
to avoid the race with spin_unlock().

So I think you should replace it with smp_mb__after_atomic() or remove it.

Other than that I believe this version is correct. So I won't insist, this
is cosmetic after all.

Oleg.