From: Philippe Anel <xigh@bouyapop.org>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Subject: Re: [9fans] 9vx, kproc and *double sleep*
Date: Sun, 13 Jun 2010 16:26:24 +0200 [thread overview]
Message-ID: <4C14EA90.8020108@bouyapop.org> (raw)
In-Reply-To: <8ebe5bdc32ec6d9b7efc27b0b0b3697c@hamnavoe.com>
In fact you're right, and this shows us this would only happens to 9vx.
Indeed, the proc is a kproc and thus is not scheduled by the
9vx/a/proc.c scheduler,
but by the one in 9vx/sched.c ... where dequeueproc() is not called and
where p->mach
is not checked.
Thank you !
Phil;
Richard Miller wrote:
> Philippe said:
>
>
>> Again, the change I proposed is not about sleep/wakeup/postnote, but
>> because wakeup() is ready()'ing the awakened process while the mach on
>> which sleep() runs is still holdind a pointer (up) to the awakened
>> process and can later (in schedinit()) assumes it is safe to access
>> (up)->state. Because of this, schedinit() can tries to call ready() on
>> (up), because because (up)->state may have been changed to Running by
>> a third mach entity.
>>
>
> and I tried to summarize:
>
>
>> It's a race between a process in sleep() returning to the scheduler on
>> cpu A, and the same process being readied and rescheduled on cpu B
>> after the wakeup.
>>
>
> But after further study of proc.c, I now believe we were both wrong.
>
> A process on the ready queue can only be taken off the queue and
> scheduled by calling dequeueproc(), which contains this:
> /*
> * p->mach==0 only when process state is saved
> */
> if(p == 0 || p->mach){
> unlock(runq);
> return nil;
> }
> So the process p can only be scheduled (i.e. p->state set to Running)
> if p->mach==nil.
>
> The only place p->mach gets set to nil is in schedinit(), *after*
> the test for p->state==Running.
>
> This seems to mean there isn't a race after all, and Philippe's
> thought experiment is impossible.
>
> Am I missing something?
>
>
>
>
next prev parent reply other threads:[~2010-06-13 14:26 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-06-11 14:06 Philippe Anel
2010-06-11 14:40 ` ron minnich
2010-06-11 14:49 ` erik quanstrom
2010-06-11 14:54 ` Philippe Anel
2010-06-11 15:03 ` erik quanstrom
2010-06-11 15:22 ` Philippe Anel
2010-06-11 15:25 ` Philippe Anel
2010-06-11 14:59 ` Philippe Anel
2010-06-11 17:11 ` Bakul Shah
2010-06-11 17:31 ` Philippe Anel
2010-06-11 18:34 ` Bakul Shah
2010-06-11 18:36 ` erik quanstrom
2010-06-11 18:51 ` Philippe Anel
2010-06-12 7:02 ` Philippe Anel
2010-06-12 9:22 ` Philippe Anel
2010-06-12 11:51 ` erik quanstrom
2010-06-13 13:01 ` Richard Miller
2010-06-13 13:43 ` Philippe Anel
2010-06-13 14:26 ` Philippe Anel [this message]
2010-06-13 16:20 ` ron minnich
2010-06-13 16:34 ` Philippe Anel
2010-06-13 17:23 ` Philippe Anel
2010-06-13 18:03 ` Philippe Anel
2010-06-14 19:15 ` Charles Forsyth
2010-06-14 19:36 ` Philippe Anel
2010-06-15 2:57 ` ron minnich
2010-06-15 3:36 ` ron minnich
2010-06-12 20:15 ` Richard Miller
2010-06-12 20:30 ` ron minnich
2010-06-12 22:15 ` Charles Forsyth
2010-06-13 0:04 ` ron minnich
2010-06-13 13:32 ` erik quanstrom
2010-06-13 22:34 ` Charles Forsyth
2010-06-13 9:00 ` Richard Miller
2010-06-11 14:49 ` Philippe Anel
2010-06-11 14:59 ` ron minnich
2010-06-11 15:02 ` ron minnich
2010-06-11 15:04 ` erik quanstrom
2010-06-11 15:43 ` ron minnich
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