Re: [9fans] devmnt crash, fix.

[Bakul Shah <bakul@bitblocks.com>]

On Thu, 23 Aug 2012 12:47:10 EDT Charles Forsyth <charles.forsyth@gmail.com>  wrote:
> No. It's not true that all sleeps might return spuriously (it might have
> been true in Unix, I can't remember).
> It's not true in Plan 9.

It is not that sleep might return spuriously (that is, the
wakeup condition is still false) but that it may have *become*
false once again due to an action by another process before
the sleeper has had a chance to run.  To handle this one would
use a pattern like this

	while (!wakeup_condition(&foo))
		sleep(&foo);

I haven't looked at this in ages but this used to be the case
in unix.  So here awakening everyone *spuriously* is /safe/
(but not efficient).

doc/sleep.ps says
    We assume that a particular call to sleep corresponds to a
    particular call to wakeup, that is, at most one process is
    asleep waiting for a particular event.

But it is not clear to me that another process can't get in to
change the condition between the time wakeup() is called and
the sleeper actually runs -- the above condition is not
violated as the new process won't be sleeping. But if yet
another process tries to access the same condition, it will be
blocked on getting the rendezvous structure lock?