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

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

On 02/16/2015 10:17 PM, David Vrabel wrote:
> 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.
Thanks.
Yes, will send out a separate patch for -stable without READ_ONCE 
changes once this patches goes in.




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