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

Raghavendra K T
2015-02-17T10:03:23Z

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@citrix.com>
>
> 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.