Subject: Re: newlock
To: Garrett D'Amore <>
From: Andrew Doran <>
List: tech-kern
Date: 09/04/2006 03:03:18
Hi Garrett,

On Sun, Sep 03, 2006 at 05:28:22PM -0700, Garrett D'Amore wrote:

> What's wrong with the standard solaris way of doing this?
>     mutex_enter(a);
>        find b...
>     mutex_enter(b);
>        /* possibly unlock a now *
>     mutex_exit(b);
>     mutex_exit(a);   /* unless you did it earlier */
> It gives full flexibility, and keeps everything explicit.

If releasing spin mutexes out of order, then we restore the saved SPL too
early and there's a short window where an interrupt can fire and we deadlock
on the mutex:

	mutex_enter(a)		<- original interrupt mask saved
	mutex_exit(a)		<- original interrupt mask restored
	mutex_exit(b)		<- deadlock

The question is: how to allow that cleanly? I don't know how Solaris handles
that with spin mutexes. From what I know, I doubt it does..