Re: [PATCH/RFC] Futex mmap_sem deadlock

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2005-02-22 at 14:10 -0800, Linus Torvalds wrote:

> Oh, well. The reason I hate the rwsem behaviour is exactly because it
> results in this very subtle class of deadlocks. This one case is certainly
> solvable several ways, but do we have other issues somewhere else? Things
> like kobject might be ripe with things like this. The mm semaphore tends
> to be pretty well-behaved - and I'm not sure the same is true of the
> kobject one.

We could detect those tho. When the appropriate DEBUG option is set, by
storing a cpumask in the semaphore we could detect if it's already taken
on this cpu...

> Normal recursive deadlocks are wonderful - most of them show up
> immediately, so assuming you just have enough coverage, you're fine. This
> fairness-related deadlock requires a race to happen.

Unless you consider that taking the read semaphore twice on the same CPU
is always a bug, thus the above stuff would work for catching them at
least more often...
 
> Maybe it would be sufficient to have a debugging version of rwsems that
> just notice recursion?
> 
> 		Linus
-- 
Benjamin Herrenschmidt <benh@kernel.crashing.org>