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Date: Mon, 06 Nov 2000 22:32:59 +0000
From: "Andrew Morton" <morton@nortelnetworks.com>
Organization: Nortel Networks, Wollongong Australia
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To: kuznet@ms2.inr.ac.ru
CC: Andrew Morton <andrewm@uow.edu.au>, linux-kernel@vger.kernel.org
Subject: Re: [patch] NE2000
In-Reply-To: <3A039E77.5DD87DF0@uow.edu.au> from "Andrew Morton" at Nov 4,
            0 08:45:01 am <200011061846.VAA20608@ms2.inr.ac.ru>
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kuznet@ms2.inr.ac.ru wrote:
> 
> Hello!
> 
> > No, that code is correct, provided (current->state == TASK_RUNNING)
> > on entry.  If it isn't, there's a race window which can cause
> > lost wakeups.   As a check you could add:
> >
> >       if ((current->state & (TASK_INTERRUPTIBLE|TASK_UNINTERRUPTIBLE)) == 0)
> >               BUG();
> 
> Though it really cannot happen and really happens, as we have seen... 8)
> 
> In any case, Andrew, where is the race, when we enter in sleeping state?
> Wakeup is not lost, it is just not required when we are not going
> to schedule and force task to running state.

	set_current_state(TASK_INTERRUPTIBLE);
	add_wait_queue(...);
	/* window here */
	set_current_state(TASK_INTERRUPTIBLE);
	schedule();

If there's a wakeup by another CPU (or this CPU in an interrupt) in
that window, current->state will get switched to TASK_RUNNING.

Then it's immediately overwritten and we go to sleep.  Lost wakeup.

> I still do not see how it is possible that task runs in sleeping state.
> Apparently, set_current_state is forgotten somewhere. Do you see, where? 8)

Nope.  Is Jorge running SMP?
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