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Re: [Xen-devel] [PATCH V4] x86 spinlock: Fix memory corruption on completing completions

On 02/13/2015 09:02 PM, Oleg Nesterov wrote:
On 02/13, Raghavendra K T wrote:

@@ -164,7 +161,7 @@ static inline int arch_spin_is_locked(arch_spinlock_t *lock)
        struct __raw_tickets tmp = READ_ONCE(lock->tickets);

-       return tmp.tail != tmp.head;
+       return tmp.tail != (tmp.head & ~TICKET_SLOWPATH_FLAG);

Well, this can probably use __tickets_equal() too. But this is cosmetic.

That looks good. Added.

It seems that arch_spin_is_contended() should be fixed with this change,

        (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC

can be true because of TICKET_SLOWPATH_FLAG in .head, even if it is actually

Hmm! it was because I was still under impression that slowpath bit is
in tail. You are right, situation could lead to positive max and may
report false contention.

And the "(__ticket_t)" typecast looks unnecessary, it only adds more
confusuin, but this is cosmetic too.


@@ -772,7 +773,8 @@ __visible void kvm_lock_spinning(struct arch_spinlock 
*lock, __ticket_t want)
         * check again make sure it didn't become free while
         * we weren't looking.
-       if (ACCESS_ONCE(lock->tickets.head) == want) {
+       head = READ_ONCE(lock->tickets.head);
+       if (__tickets_equal(head, want)) {
                add_stats(TAKEN_SLOW_PICKUP, 1);
                goto out;

This is off-topic, but with or without this change perhaps it makes sense
to add smp_mb__after_atomic(). It is nop on x86, just to make this code
more understandable for those (for me ;) who can never remember even the
x86 rules.

Hope you meant it for add_stat. yes  smp_mb__after_atomic() would be
harmless barrier() in x86. Did not add this V5 as yoiu though  but this
made me look at slowpath_enter code and added an explicit barrier() there :).

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