Re: [Xen-devel] [PATCHv2 5/6] xen: use ticket locks for spin locks

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/10/2015 01:29 PM, David Vrabel wrote:
> On 10/04/2015 17:44, Boris Ostrovsky wrote:
> > On 04/10/2015 10:19 AM, David Vrabel wrote:
> >
> > > @@ -138,14 +138,17 @@ void spin_debug_disable(void)
> > >
> > >    void _spin_lock(spinlock_t *lock)
> > >    {
> > > +    spinlock_tickets_t tickets = { .tail = 1, };
> > >        LOCK_PROFILE_VAR;
> > >
> > >        check_lock(&lock->debug);
> > > -    while ( unlikely(!_raw_spin_trylock(&lock->raw)) )
> > > +    tickets.head_tail = xadd(&lock->tickets, tickets.head_tail);
> > > +    if ( tickets.tail != read_atomic(&lock->tickets.head) )
> > >        {
> > >            LOCK_PROFILE_BLOCK;
> > > -        while ( likely(_raw_spin_is_locked(&lock->raw)) )
> > > +        do {
> > >                cpu_relax();
> > > +        } while ( tickets.tail != read_atomic(&lock->tickets.head) );
> > >        }
> > Why do you use both 'if' and 'while"? I.e. why not just
> >
> > while ( tickets.tail != read_atomic(&lock->tickets.head) )
> > {
> >      LOCK_PROFILE_BLOCK;
> >      cpu_relax();
> > }
> We need to only call LOCK_PROFILE_BLOCK once when we start blocking.
LOCK_PROFILE_BLOCK is a "one-shot" assignment. Once 'block' is set to 
NOW() it doesn't change anymore. So it will be updated only on the first 
iteration of the loop.
-boris


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