Andrew Doran wrote:
> 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_enter(b)
> 	...
> 	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..
>   

Wrong.  This works fine in Solaris.  The spin mutex doesn't
automatically release the spl -- instead it releases a _reference_ on
the spl.  The spl is only dropped when the reference reaches zero.

    -- Garrett
> Thanks,
> Andrew
>   


-- 
Garrett D'Amore, Principal Software Engineer
Tadpole Computer / Computing Technologies Division,
General Dynamics C4 Systems
http://www.tadpolecomputer.com/
Phone: 951 325-2134  Fax: 951 325-2191