Re: [Xen-devel] [PATCH V2] x86 spinlock: Fix memory corruption on completing completions

[Oleg Nesterov <oleg@xxxxxxxxxx>]

On 02/09, Raghavendra K T wrote:
>
> +static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t *lock)
> +{
> +     arch_spinlock_t old, new;
> +     __ticket_t diff;
> +
> +     old.tickets = READ_ONCE(lock->tickets);
> +     diff = (old.tickets.tail & ~TICKET_SLOWPATH_FLAG) - old.tickets.head;
> +
> +     /* try to clear slowpath flag when there are no contenders */
> +     if ((old.tickets.tail & TICKET_SLOWPATH_FLAG) &&
> +             (diff == TICKET_LOCK_INC)) {
> +             new = old;
> +             new.tickets.tail &= ~TICKET_SLOWPATH_FLAG;
> +             cmpxchg(&lock->head_tail, old.head_tail, new.head_tail);
> +     }
> +}

Can't we simplify it? We own .head, and we already know it. We only need
to clear TICKET_SLOWPATH_FLAG in .tail atomically?

IOW,

        static inline void __ticket_check_and_clear_slowpath(arch_spinlock_t 
*lock, __ticket_t head)
        {
                __ticket_t old_tail, new_tail;

                new_tail = head + TICKET_LOCK_INC;
                old_tail = new_tail | TICKET_SLOWPATH_FLAG;

                if (READ_ONCE(lock->tickets.tail) == old_tail)
                        cmpxchg(&lock->tickets.tail, old_tail, new_tail);
        }

Plus

        -       __ticket_check_and_clear_slowpath(lock);
        +       __ticket_check_and_clear_slowpath(lock, inc.tail);

Or I missed something?

And I think it would be better to avoid ifdef(CONFIG_PARAVIRT_SPINLOCKS),
ww can just do

        if (TICKET_SLOWPATH_FLAG)
                __ticket_check_and_clear_slowpath();

Oleg.


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