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

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Message-ID: <Pine.LNX.4.64.0812121958470.15781@quilx.com>
Date:	Fri, 12 Dec 2008 20:07:56 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Eric Dumazet <dada1@...mosbay.com>
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

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.

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