Re: [PATCH 6/6] procfs: Kill the bkl in ioctl

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Mar 31, 2010 at 11:04:30PM +0200, Arnd Bergmann wrote:
> On Wednesday 31 March 2010 22:21:23 Arnd Bergmann wrote:
> > Another crazy idea I had was to simply turn the BKL into a regular mutex
> > as soon as we can show that all remaining users are of the non-recursive
> > kind and don't rely on the autorelease-on-sleep. Doing that would be
> > much easier without the pushdown into .unlocked_ioctl than it would be
> > with it.
> 
> I just looked at all the users of lock_kernel remaining with my patch
> series. For 90% of them, it is completely obvious that they don't rely
> on nested locking, and they very much look like they don't need the
> autorelease either, because the BKL was simply pushed down into the
> open, ioctl and llseek functions.
> 
> There are a few file systems (udf, ncpfs, autofs, coda, ...) and some
> network protocols (appletalk, ipx, irnet and x25) for which it is not
> obviously, though still quite likely, the case.
> 
> So we could actually remove the BKL recursion code soon, or even turn
> all of it into a regular mutex, at least as an experimental option.
> 
> The recursive users that I've removed in my series are the block, tty,
> input and sound subsystems, as well as the init code.


This is a solution that has been tried more than once already. But Linus
has told he wouldn't pull something that turns the bkl into a mutex or a
semaphore.

Plus it's quite hard to tell that it does or not auto-release somewhere
This is often something you can really spot on runtime or on small path
only.

The simple fact the bkl is not always a leaf lock makes it need the
auto-release, otherwise you experience very bad unexpected lock
dependencies.