Message-ID: <43DE1D28.1030100@sw.ru>
Date: Mon, 30 Jan 2006 17:05:28 +0300
From: Kirill Korotaev <dev@sw.ru>
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To: Jan Blunck <jblunck@suse.de>
CC: Olaf Hering <olh@suse.de>, linux-kernel@vger.kernel.org,
       Andrew Morton <akpm@osdl.org>
Subject: Re: [PATCH] Busy inodes after unmount, be more verbose in generic_shutdown_super
References: <20060116223431.GA24841@suse.de> <43CC2AF8.4050802@sw.ru> <20060118224953.GA31364@hasse.suse.de> <43CF6170.3050608@sw.ru> <20060119100443.GD10267@hasse.suse.de> <43CF693D.4020104@sw.ru> <20060120190653.GE24401@hasse.suse.de> <43D4907B.4060801@sw.ru> <20060130115435.GA9181@hasse.suse.de>
In-Reply-To: <20060130115435.GA9181@hasse.suse.de>
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Hello Jan,

>>Jan, I still have not heard a single comment about what's wrong with 
>>it... I would really appreciate if you provide me one.
>>
> 
> 
> Sorry for the delay. I had to fix a totally bogus patch (mine ;).
> 
> The problem with your patch is that it hides too early mntput's. Think about
> following situation:
> 
>  mntput(path->mnt);   // too early mntput()
>  dput(path->dentry);
> 
> Assuming that in-between this sequence someone unmounts the file system, your
> patch will wait for this dput() to finish before it proceeds with unmounting
> the file system. I think this isn't what we want.
No, it won't wait for anything, because if umount happened between 
mntput/dput, dentry is not in s_dshrinkers list.
if umount happens in parallell with dput() (where shrinker operations 
are), then it will behave ok - will wait for dput() and then umount. It 
was intended behaviour!

Also, please, note that such early mntput()'s are bugs!!! because such 
dentries can reference freed memory after last mntput(). And I remember 
some patches in 2.4.x/2.6.x which fixed this sequence everywhere.

Kirill

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