Subject: Re: [PATCH] fanotify: dont destroy mark when ignore mask is cleared
From: Eric Paris <eparis@redhat.com>
To: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
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Date: Mon, 29 Nov 2010 15:45:31 -0500
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On Wed, 2010-11-24 at 13:31 +0100, Lino Sanfilippo wrote:

> 2. I just realized that we cant simply call destroy_mark() if the masks are 0. 
> There may be one or more concurrent processes calling fsnotify_find_inode_mark() 
> (see fanotify_add_inode_mark()) and get the mark we are about to destroy at the 
> same time.
> 
> I will take a closer look at it, but it seems to be difficult to me to safely 
> call destroy_mark() as long as we are not in the context of fanotify_release() (in 
> which we dont have to deal with concurrency like that any more). 

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

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