Request for review, time_pps_fetch() enhancement

[Jilles Tjoelker]

On Wed, Feb 06, 2013 at 05:58:30PM +0200, Konstantin Belousov wrote:
> On Tue, Feb 05, 2013 at 09:41:38PM -0700, Ian Lepore wrote:
> > I'd like feedback on the attached patch, which adds support to our
> > time_pps_fetch() implementation for the blocking behaviors described in
> > section 3.4.3 of RFC 2783.  The existing implementation can only return
> > the most recently captured data without blocking.  These changes add the
> > ability to block (forever or with timeout) until a new event occurs.

> > Index: sys/kern/kern_tc.c
> > ===================================================================
> > --- sys/kern/kern_tc.c	(revision 246337)
> > +++ sys/kern/kern_tc.c	(working copy)
> > @@ -1446,6 +1446,50 @@
> >   * RFC 2783 PPS-API implementation.
> >   */
> >  
> > +static int
> > +pps_fetch(struct pps_fetch_args *fapi, struct pps_state *pps)
> > +{
> > [snip]
> > +		aseq = pps->ppsinfo.assert_sequence;
> > +		cseq = pps->ppsinfo.clear_sequence;
> > +		while (aseq == pps->ppsinfo.assert_sequence &&
> > +		    cseq == pps->ppsinfo.clear_sequence) {
> Note that compilers are allowed to optimize these accesses even over
> the sequential point, which is the tsleep() call. Only accesses to
> volatile objects are forbidden to be rearranged.

> I suggest to add volatile casts to pps in the loop condition.

The memory pointed to by pps is global (other code may have a pointer to
it); therefore, the compiler must assume that the tsleep() call (which
invokes code in a different compilation unit) may modify it.

Because volatile does not make concurrent access by multiple threads
defined either, adding it here only seems to slow down the code
(potentially).

> > +			err = tsleep(pps, PCATCH, "ppsfch", timo);
> > +			if (err == EWOULDBLOCK && fapi->timeout.tv_sec == -1) {
> > +				continue;
> > +			} else if (err != 0) {
> > +				return (err);
> > +			}
> > +		}
> > +	}
-- 
Jilles Tjoelker