linux-kernel - Re: [PATCH v3 6/7] fs: struct file move from call_rcu() to SLAB_DESTROY_BY

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Date:	Wed, 17 Dec 2008 21:25:20 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Christoph Hellwig <hch@...radead.org>,
	David Miller <davem@...emloft.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	"kernel-testers@...r.kernel.org >> Kernel Testers List" 
	<kernel-testers@...r.kernel.org>, Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Netdev List <netdev@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v3 6/7] fs: struct file move from call_rcu() to SLAB_DESTROY_BY_RCU

Christoph Lameter a écrit :
> On Fri, 12 Dec 2008, Eric Dumazet wrote:
> 
>>> a truly allocated file. At this point the file is
>>> a truly allocated file but not anymore ours.
> 
> Its a valid file. Does ownership matter here?
> 
>> Reading again this mail I realise we call put_filp(file), while this should
>> be fput(file) or put_filp(file), we dont know.
>>
>> Damned, this patch is wrong as is.
>>
>> Christoph, Paul, do you see the problem ?
> 
> Yes.
> 
>> In fget()/fget_light() we dont know if the other thread (the one who re-allocated the file,
>> and tried to close it while we got a reference on file) had to call put_filp() or fput()
>> to release its own reference. So we call atomic_long_dec_and_test() and cannot
>> take the appropriate action (calling the full __fput() version or the small one,
>> that some systems use to 'close' an not really opened file.
> 
> The difference is mainly that fput() does full processing whereas
> put_filp() is used when we know that the file was not fully operational.
> If the checks in __fput are able to handle the put_filp() situation by not
> releasing resources that were not allocated then we should be fine.
> 
>> I believe put_filp() is only called on slowpath (error cases).
> 
> Looks like it. It seems to assume that no dentry is associated.
> 
>> Should we just zap it and always call fput() ?
> 
> Only if fput() can handle partially setup files.

It can do that if we add a check for NULL dentry in __fput(), so put_filp() can disappear.

But there is a remaining point where we do an atomic_long_dec_and_test(&...->f_count),
in fs/aio.c, function __aio_put_req(). This one is tricky :(


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