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

[David Vrabel <dvrabel@xxxxxxxxxx>]

On 15/02/15 17:30, Raghavendra K T wrote:
> --- a/arch/x86/xen/spinlock.c
> +++ b/arch/x86/xen/spinlock.c
> @@ -41,7 +41,7 @@ static u8 zero_stats;
>  static inline void check_zero(void)
>  {
>       u8 ret;
> -     u8 old = ACCESS_ONCE(zero_stats);
> +     u8 old = READ_ONCE(zero_stats);
>       if (unlikely(old)) {
>               ret = cmpxchg(&zero_stats, old, 0);
>               /* This ensures only one fellow resets the stat */
> @@ -112,6 +112,7 @@ __visible void xen_lock_spinning(struct arch_spinlock 
> *lock, __ticket_t want)
>       struct xen_lock_waiting *w = this_cpu_ptr(&lock_waiting);
>       int cpu = smp_processor_id();
>       u64 start;
> +     __ticket_t head;
>       unsigned long flags;
>  
>       /* If kicker interrupts not initialized yet, just spin */
> @@ -159,11 +160,15 @@ __visible void xen_lock_spinning(struct arch_spinlock 
> *lock, __ticket_t want)
>        */
>       __ticket_enter_slowpath(lock);
>  
> +     /* make sure enter_slowpath, which is atomic does not cross the read */
> +     smp_mb__after_atomic();
> +
>       /*
>        * 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;
>       }
> @@ -204,8 +209,8 @@ static void xen_unlock_kick(struct arch_spinlock *lock, 
> __ticket_t next)
>               const struct xen_lock_waiting *w = &per_cpu(lock_waiting, cpu);
>  
>               /* Make sure we read lock before want */
> -             if (ACCESS_ONCE(w->lock) == lock &&
> -                 ACCESS_ONCE(w->want) == next) {
> +             if (READ_ONCE(w->lock) == lock &&
> +                 READ_ONCE(w->want) == next) {
>                       add_stats(RELEASED_SLOW_KICKED, 1);
>                       xen_send_IPI_one(cpu, XEN_SPIN_UNLOCK_VECTOR);
>                       break;

Acked-by: David Vrabel <david.vrabel@xxxxxxxxxx>

Although some of the ACCESS_ONCE to READ_ONCE changes are cosmetic and
are perhaps best left out of a patch destined for stable.

David


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