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

[Oleg Nesterov <oleg@xxxxxxxxxx>]

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.

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
unlocked. 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.

Oleg.


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