On Fri, Feb 08, 2013 at 04:13:40PM -0700, Ian Lepore wrote: > On Wed, 2013-02-06 at 17:58 +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. > > > > > > -- Ian > > > > > > > > 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) > > > +{ > > > + int err, timo; > > > + pps_seq_t aseq, cseq; > > > + struct timeval tv; > > > + > > > + if (fapi->tsformat && fapi->tsformat != PPS_TSFMT_TSPEC) > > > + return (EINVAL); > > > + > > > + /* > > > + * If no timeout is requested, immediately return whatever values were > > > + * most recently captured. If timeout seconds is -1, that's a request > > > + * to block without a timeout. WITNESS won't let us sleep forever > > > + * without a lock (we really don't need a lock), so just repeatedly > > > + * sleep a long time. > > > + */ > > Regarding no need for the lock, it would just move the implementation into > > the low quality one, for the case when one timestamp capture is lost > > and caller of time_pps_fetch() sleeps until next pps event is generated. > > > > I understand the desire to avoid lock, esp. in the pps_event() called > > from the arbitrary driver context. But the race is also real. > > > > What race? A user of the pps interface understands that there is one > event per second, and understands that if you ask to block until the > next event at approximately the time that event is expected to occur, > then it is ambiguous whether the call completes almost-immediately or in > about 1 second. > > Looking at it another way, if a blocking call is made right around the > time of the PPS, the thread could get preempted before getting to > pps_fetch() function and not get control again until after the PPS has > occurred. In that case it's going to block for about a full second, > even though the call was made before top-of-second. That situation is > exactly the same with or without locking, so what extra functionality is > gained with locking? What guarantee does locking let us make to the > caller that the lockless code doesn't?
No guarantees, but I noted in the original reply that this is about the quality of the implementation and not about correctness. As I said there as well, I am not sure that any locking can be useful for the situation at all.
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