linux-kernel - Re: [PATCH 3/3] proc: simplify proc_task_readdir/first

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Message-ID: <20130531183419.GA4493@redhat.com>
Date:	Fri, 31 May 2013 20:34:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Michal Hocko <mhocko@...e.cz>,
	Sergey Dyasly <dserrg@...il.com>,
	Sha Zhengju <handai.szj@...bao.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] proc: simplify proc_task_readdir/first_tid paths

On 05/31, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...hat.com> writes:
>
> > OK. But this means that even 1/3 is not 100% right, exactly because
> > leader can be unhashed right before first_tid() takes rcu lock. Easy
> > to fix, we should simply factor out the "nr != 0" check.
> >
> > And this also means that 3/3 is not right by the same reason. I'll
> > make a simpler patch which only avoids the unnecessary get/put in
> > proc_task_readdir().
> >
> > Unless we can tolerate this very unlikely rase when the leader goes
> > away after initial ENOENT check at the start, of course... Or unless
> > we add canceldir() which resets getdents_callback->previous so that
> > we could return ENOENT after filldir() was already called ;)
>
> A small race is fine and is fundamental to the process of readdir.
>
> The guarantee of open+readdir+close is that all directory entries that
> exited before open and after close are returned.  Directory entries that
> are added or removed during the open+readir+close are returned at most
> once.
>
> The important case to handle is when someone has opened the directory a
> very long time ago or has chdir'd to the directory.  With the result
> the directory was removed before we start the readdir process entirely.
>
> If the tasks die in the narrow window while we are inside of readdir
> races are impossible to avoid.

Ah OK. This means that v2 only needs the simple update.

Thanks again Eric.

Oleg.

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