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

[Raghavendra K T]

On 02/15/2015 09:47 PM, Oleg Nesterov wrote:
> 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.

I resent the patch the above change.

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

Thanks Oleg.

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