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Date: Tue, 9 Dec 2008 00:45:56 +0100
From: "=?ISO-8859-1?Q?Marius_N=FCnnerich?=" <marius@nuenneri.ch>
To: "John Baldwin" <jhb@freebsd.org>
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References: <b649e5e0812071100v2bf7b377n63e85b3f2b123c24@mail.gmail.com>
	<200812081517.39375.jhb@freebsd.org>
Cc: freebsd-hackers@freebsd.org
Subject: Re: Why safe using msleep with timeout=0 but not tsleep?
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On Mon, Dec 8, 2008 at 9:17 PM, John Baldwin <jhb@freebsd.org> wrote:
> 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 ha=
s
> seen 'flag =3D=3D 0' but before it has put the thread to sleep in tsleep(=
).  Even
> on UP systems with preemption you can still get this race if you get
> preempted by an interrupt (which runs foo()) in between the 'flag =3D=3D =
0' test
> 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.
>
> --
> John Baldwin
>

Thank you for the explanation, John!