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

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Message-ID: <20101130155951.GB4814@lsanfilippo.unix.rd.tt.avira.com>
Date:	Tue, 30 Nov 2010 16:59:51 +0100
From:	Lino Sanfilippo <LinoSanfilippo@....de>
To:	eparis@...hat.com
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared

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().  

If that solution is ok I'll prepare the patches for it. Otherwise i am not sure how to solve this... 


Lino

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