Re: exclusive sleep mutex pcm0 (sound cdev) r = 0

On Tuesday 24 February 2004 10:52 am, Don Lewis wrote:
> On 24 Feb, John Baldwin wrote:
> > On Tuesday 24 February 2004 10:27 am, Don Lewis wrote:
> >> I currently have some patches for the channel mutexes that are
> >> undergoing testing.  Once these have been committed, I'll take another
> >> look at the "sound cdev" mutex.  My current thought is to convert most
> >> uses of it to a lockmgr lock.
> >
> > Eek, why not a sx lock if you must go to a sleepable lock?
>
> That was my initial thought, but I was scared off by the following
> statement in the sx(9) man page:
>
>      A thread may not own a shared lock and an exclusive lock
> simultaneously; attempting to do so will result in deadlock.
>
> My plan is to hold this lock across large portions of the open() and
> ioctl() methods (and possibly read() and write() as well).  Some of
> these may call code outside of my direct control, such as methods in the
> hardware specific drivers, as well as things like malloc().  I'm
> concerned about causing a deadlock by violating the sx usage rules.

It means on the same lock, i.e. you can't try to do sx_slock(&foo) and then do 
sx_xlock(&foo).  You can mix shared/exclusive of different locks ok.

-- 
John Baldwin <jhb_at_FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org