Re: [announce] vfs-scale git tree update

[Chris Ball <cjb@laptop.org>]

On Thu, Jan 06, 2011 at 05:41:39PM -0800, Linus Torvalds wrote:
> I'm wondering if this is a previously existing race condition leading
> to a deadlock. One that previously would have been serialized enough
> by the dcache lock that you'd never have that happen.
> 
> It might be interesting to re-run it with mutex debugging and lockdep
> enabled, to see if that reports anything. Although it probably won't,
> because it's not about a plain lock dependency, but ends up being
> deadlocked on the uevent being finished (but you have the modprobe and
> the request_firmware ones waiting on each other).
> 
> I dunno. I haven't really though that fully through. But we've had
> cases roughly like that before, and yes, they can be exposed by some
> independent serialization going away - long-standing potential bugs,
> that simply never happened in practice before.

Yeah, that sounds right:

arch/x86/kernel/microcode_core.c:
        mutex_lock(&microcode_mutex);
        ...
        error = sysdev_driver_register(&cpu_sysdev_class, &mc_sysdev_driver);

(which calls request_microcode_fw() indirectly, and goes out to userspace.)

I'm willing to have a try at making this lockless if no-one beats me to it,
although it would need to wait until I've got the MMC merge out of the way.

-- 
Chris Ball   <cjb@laptop.org>   <http://printf.net/>
One Laptop Per Child