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From: John Baldwin <jhb@freebsd.org>
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Date: Mon, 8 Dec 2008 15:17:39 -0500
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Cc: Marius =?iso-8859-1?q?N=FCnnerich?= <marius@nuenneri.ch>
Subject: Re: Why safe using msleep with timeout=0 but not tsleep?
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On Sunday 07 December 2008 02:00:30 pm Marius N=FCnnerich wrote:
> See subject.
> Interesting commit:
> http://svn.freebsd.org/viewvc/base?view=3Drevision&revision=3D77059

Lost wakeups.   If you have code like so that doesn't use any locks:

int flag;

void
foo(void)
{

	flag =3D 1;
	wakeup(&flag);
}

void
bar(void)
{

	if (flag =3D=3D 0)
		tsleep(&foo, ..., 0);
}

Then one CPU may run the 'foo' routine to completion after another CPU has=
=20
seen 'flag =3D=3D 0' but before it has put the thread to sleep in tsleep().=
  Even=20
on UP systems with preemption you can still get this race if you get=20
preempted by an interrupt (which runs foo()) in between the 'flag =3D=3D 0'=
 test=20
and calling tsleep().  Using an interlock avoid this:

struct mtx lock;
int flag;

void
foo(void)
{

	mtx_lock(&lock);
	flag =3D 1;
	mtx_unlock(&lock);
	wakeup(&flag);
}

void
bar(void)
{

	mtx_lock(&lock);
	if (flag =3D=3D 0)
		mtx_sleep(&foo, &lock, ..., 0);
	mtx_unlock(&lock);
}

In this case 'lock' closes the SMP/preemption races.

=2D-=20
John Baldwin