Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared

[Eric Paris <eparis@redhat.com>]

On Tue, 2010-11-30 at 16:59 +0100, Lino Sanfilippo wrote:
> On Tue, Nov 30, 2010 at 01:16:35PM +0100, Lino Sanfilippo wrote:
> 
> > I guess it is a question of safe vs racy.  Yes it is safe, nothing will
> > explode or panic.  But we might have a race between one task removing an
> > event type causing the mask to go to 0 and we should destroy the mark
> > and another task adding an event type.  If it raced just right we might
> > destroy the mark after the second task added to it.  I guess we really
> > need to serialize fsnotify_mark() per group to solve the race...
> > 
> > Do you want to take a stab at fixing these things or should I?
> > 
> > -Eric
> 
> IMHO the right thing to serialize this would be to do 
> 
> LOCK(groups->mark_lock)
> - get the inode mark
> - set the marks mask
> - possibly destroy the mask
> UNLOCK(groups->mark_lock)
> 
> But we cant do this since setting the marks mask requires the lock of the mark 
> - which would mean an incorrect lock order according to fsnotify_add_mark():
> 
> mark->lock
> group->mark_lock
> inode->i_lock
> 
> What we could do very easily is use another mutex instead (use an existing one like the 
> groups notification_mutex, or a completely new one) which is responsible for synchronising
> add_mark()/remove_mark().

I'd think a new per group mutex would be the right way to go.  I'm not
sure how I feel about notification_mutex.  I guess you can go ahead and
overload it and we can split it off later if someone finds it to be a
performance blocker.

-Eric