Re: [Bugme-new] [Bug 12754] New: inotify doesn't free memory allocated to watches

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 25 Feb 2009 00:28:33 +0000
Al Viro <viro@ZenIV.linux.org.uk> wrote:

> On Tue, Feb 24, 2009 at 03:40:23PM -0800, Andrew Morton wrote:
> > ./Documentation/filesystems/inotify.txt
> > ./kernel/audit.c
> > ./kernel/audit_tree.c
> > ./fs/notify/inotify/inotify_user.c
> > ./fs/notify/inotify/inotify.c
> > ./include/linux/inotify.h
> > 
> > I assume it's inotify_dev_queue_event()?
> > 
> >         if (mask & IN_IGNORED || w->mask & IN_ONESHOT)
> >                 put_inotify_watch(w); /* final put */
> 
> Duh.  Nevermind, I'd misparsed the report.  If we are talking about
> inotify_user.c (and not some new client), then we are back to "let's
> see what gets leaked"...
> 
> Actually, looking at inotify_user.c, we seem to be doing something rather
> fishy.  Look: event gets triggered, we pick the watch, get inotify_device
> (inotify_user-specific stuff) from it, grap mutex on it (dev->ev_mutex)
> and drop reference to inotify_watch.  Which happily triggers ->destroy_watch,
> which does put_inotify_dev().  Which is
>         if (atomic_dec_and_test(&dev->count)) {
>                 atomic_dec(&dev->user->inotify_devs);
>                 free_uid(dev->user);
>                 kfree(dev);
>         }
> What's to stop that from happening when we'd been holding the last reference
> to that sucker?  kfree() while holding a mutex inside the structure being
> freed is not nice...

slab and slub have runtime checking for kfree() of a currently-locked
lock.  I forget which DEBUG_foo option turns it on. 
CONFIG_DEBUG_OBJECTS_FREE?