Re: [PATCH 2/2] rwsem: do optimistic spinning for writer lock acquisition

[Peter Hurley]

On 06/24/2013 05:58 PM, Tim Chen wrote:
> On Sat, 2013-06-22 at 03:57 -0400, Peter Hurley wrote:
> > Will this spin for full scheduler value on a reader-owned lock?
> >
> > > +		/* wait_lock will be acquired if write_lock is obtained */
> > > +		if (rwsem_try_write_lock(sem->count, true, sem)) {
> > > +			ret = 1;
> > > +			goto out;
> > > +		}
> > > +
> > > +		/*
> > > +		 * When there's no owner, we might have preempted between the
> >                                                           ^^^^^^^^
> >
> > Isn't pre-emption disabled?
>
> Peter, on further review, this code is needed.  This code guard against
> the case of this thread preempting another thread in the middle
> of setting the  owner field.  Disabling preemption does not prevent this
> thread from preempting others, even though others cannot preempt
> this thread.

Yep; so the "we" in the quoted comment really refers to another thread
executing down_write_xxxx().

Thanks for the clarification.

Regards,
Peter Hurley
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/