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

From: Raghavendra K T
Date: Sun Feb 15 2015 - 12:32:20 EST

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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