Whoops. They need to repeat the read after obtaining the write lock and only update the file if the contents are still bad in that case. On Aug 21, 2011 4:46 AM, "David Henningsson" < david.henningsson at canonical.com> wrote: > On 08/19/2011 08:14 PM, Thomas Bushnell, BSG wrote: >> Sorry for breaking the threading, but I only just subscribed to the list >> so I can't reply properly. >> >> I'm the origin of the patch recently posted by David Henningsson which >> alters the way locking works. Maarten Bosmans had some questions I'd >> like to address. >> >> The confusing formatting of the diff in core-util.c is just unidiff >> being clever. Basically I created a new function to wrap around fcntl to >> share the common code between pa_lock_fd and pa_read_lock_fd. >> >> I have no objection of course to simply defining it unconditionally and >> using it always. I do not know Windows, so I was trying to make the >> minimally disruptive change. I didn't know that Windows has read locks. >> >> In Unix, promoting a read lock to a write lock converts the lock--it >> does not add another lock--without releasing the readlock in the middle. >> >> I am not wedded at all to the specific details of what the generic >> functions in core-util.c do. >> >> The root issue is as David Henningsson explained. By using an exclusive >> lock, pulseaudio creates an unnecessary contention in reading the >> .pulse-cookie file, and because of the less-than-ideal (but quite >> unchangeable) behavior of NFSv3, this forces a thirty-second delay >> anytime two pulse clients try to read the cookie at the same time. >> Switching to a read lock for the read, and only using an exclusive lock >> when the cookie needs to be written (a much rarer operation), avoids >> this problem entirely. > > Hi Thomas and thanks for coming here! > > I have a question about the proposed handling. Assume that the cookie is > wrong, and that two clients both find that out in parallel. Then they > both want to promote their read lock to a write lock. What will happen > in that case? > > -- > David Henningsson, Canonical Ltd. > http://launchpad.net/~diwic -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.freedesktop.org/archives/pulseaudio-discuss/attachments/20110821/2d90762b/attachment.html>
