Re: kern/46121: audiomp: locking protocol error

[David Laight <david%l8s.co.uk@localhost>]

The following reply was made to PR kern/46121; it has been noted by GNATS.

From: David Laight <david%l8s.co.uk@localhost>
To: matthew green <mrg%eterna.com.au@localhost>
Cc: ad%netbsd.org@localhost, gnats-bugs%NetBSD.org@localhost, 
kern-bug-people%netbsd.org@localhost,
        gnats-admin%netbsd.org@localhost, netbsd-bugs%netbsd.org@localhost
Subject: Re: kern/46121: audiomp: locking protocol error
Date: Mon, 26 Mar 2012 08:33:01 +0100

 On Mon, Mar 26, 2012 at 06:07:02PM +1100, matthew green wrote:
 > 
 > this looks like it should do the trick.
 ...
 >      mutex_enter(sc->sc_lock);
 > !    can_playback = audio_can_playback(sc);
 > !    can_capture = audio_can_capture(sc);
 > !    mutex_exit(sc->sc_lock);
 > ! 
 > !    if (can_playback) {
 
 Hmmm... that sort of code tends to look dubious [1].
 While acquiring a mutex across a function call can protect the underlying
 data from immediate corruption, it isn't necessarily obvious that
 the checked condition can't change state after the mutex is released.
 
        David
 
 [1] As does releaseing a mutex across a function call!
 
 -- 
 David Laight: david%l8s.co.uk@localhost