[Question] race between spin_lock and spin_unlock

[Ming Lei]

On Tue, Jun 25, 2013 at 4:48 PM, Will Deacon <will.deacon at arm.com> wrote:
> On Tue, Jun 25, 2013 at 05:31:44AM +0100, Ming Lei wrote:
>> On Tue, Jun 25, 2013 at 12:37 AM, Will Deacon <will.deacon at arm.com> wrote:
>> > You mean, if you had two CPUs spinning on a lock held by a third CPU? In
>> > this case, the third CPU would have to release the lock, then one of the
>> > waiters would have to acquire it, complete its critical section and finally
>> > release it before the other waiter had executed its wfe (about three
>> > instructions)...
>>
>> I mean the below situation:
>>
>> CPU0                                                                          CPU1
>>
>> spin_lock(&A)
>>       StoreExcl(lock->next)
>>                                                                                       spin_unlock(&A)
>>                                                                                               Store(&lock->owner)
>>                                                                                               dsb_sev()
>>       interrupt comes
>>               - wfe() called in the
>>               irq handler
>
> Is this just a vanilla wfe, or as part of a spin_lock? If it's part of a
> spin_lock, then we'll do a sev when we unlock it. An opencoded wfe in an

Suppose it is part of spin_lock(&B), and the sev in spin_unlock(&B) may happen
in CPU2 or CPU3, then two sev() will come in CPU0.

My question is that if the wfe in spin_lock(&B) called from irq
handler may clear
the two sev().

If yes, the wfe() in spin_lock(&A) may still wait for later sev, right?  If not,
it should be fine.

Thanks,
-- 
Ming Lei