linux-kernel - Re: Anonymous inode cleanup?

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Date:	Fri, 13 Nov 2015 06:52:34 +0100
From:	Mateusz Guzik <mguzik@...hat.com>
To:	Rajat Jain <rajatxjain@...il.com>
Cc:	linux-fsdevel@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Davide Libenzi <davidel@...ilserver.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Anonymous inode cleanup?

On Thu, Nov 12, 2015 at 09:43:00PM -0800, Rajat Jain wrote:
> Hello,
> 
> I'm writing a module that wants to get anonymous fd [using
> anon_inode_getfd()] and my code looks like this:
> 
> fd = anon_inode_getfd(...)
> if (fd < 0)
>     return -EINVAL;
> 
> if (foobar_fail()) {
>     /* undo everything */
>     return -EINVAL;
> }
> 
> My question is that in case of a failure after the anon_inode_getfd(),
> I want to cleanup and undo whatever needs to be done w.r.t. anodnymous
> fd I just allocated. (May be put a reference, or return the fd to the
> free pool or whatever). Can some one please let me know what cleanup
> needs to be done?
> 
> However neither I see a cleanup function, nor I see any of the drivers
> attempting
> to free the fd in case of failure.
> 

It is impossible to properly clean up in this case without serious
tinkering. In fact this code cannot realiably work without weird
locking. By the time anon_inode_getfd returns, the file could have been
closed by a different thread.

What you want instead is anon_inode_getfile. See perf_event_open for an
example how to use it.

-- 
Mateusz Guzik
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