On Fri, Feb 25, 2011 at 1:51 AM, Russ Cox <rsc@swtch.com> wrote:

> > your layout in your first email (i think) assumes that wakeup
> > can be called twice.
>
> it doesn't.  the scenario in my first email has exactly one
> sleep and one wakeup.
>
> the canonical problem you have to avoid when implementing
> sleep and wakeup is that the wakeup might happen before
> the sleep has gotten around to sleeping.
>

There is a pattern in DragonFly BSD for use with tsleep (tsleep is
evolutionarily related to p9's sleep, though the descent is tortuous),
tsleep_interlock.

tsleep_interlock(WAITCHAN);
/* Kick off some operation or release some higher level lock or w/e */
tsleep(WAITCHAN, ...);

tsleep_interlock queues a process but does not actually go to sleep. If a
wakeup happens after the tsleep_interlock but before tsleep has fully slept,
the process does not sleep in tsleep().

I do not know if a similar design would be feasible given Plan 9's
sleep/wakeup; at first glance, it looks to be so - sleep_interlock would
look similar to sleep's front-end, but it would not 'gotolabel(&m->sched)'.

-- vs