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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

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.