linux-kernel - Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] : ima-appraisal patches)

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Message-ID: <CA+55aFxia+u3VAf0qN-2wv7DyAQZK-Z8=n=cqUvo--+C905aFg@mail.gmail.com>
Date:	Fri, 20 Apr 2012 08:56:57 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	linux-fsdevel@...r.kernel.org, James Morris <jmorris@...ei.org>,
	linux-security-module@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	David Safford <safford@...ux.vnet.ibm.com>,
	Dmitry Kasatkin <dmitry.kasatkin@...el.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [RFC] situation with fput() locking (was Re: [PULL REQUEST] :
 ima-appraisal patches)

On Fri, Apr 20, 2012 at 1:09 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Note that we do *not* need to bother with fput_light() - even if it does
> fput(), that fput() won't usually be the final one.

Ack. Most of the time the fput_light()->fput will just decrement the use count.

> We also get something else out of that - AFAICS, the kludge in __scm_destroy()
> can be killed after that.  We did it to prevent recursion on fput(), right?
> Now that recursion will be gone...

Hmm.. That points out that we may have a *lot* of these pending final
fput's, though. So the deferral queueing should be fairly light. What
were your particular plans for it?

This actually sounds like a fairly good usage-case for Oleg's new
task_work_add() thing. That would defer the final fput, but at the
same time guarantee that it gets done before returning to user space -
in case there are any issues with synchronous actions. Have you looked
at Oleg's series? You weren't cc'd because it didn't affect you, but
look at lkml for "task_work_add()" to find it.

NOTE! If pure kernel threads do fput() deferral (and maybe they do -
I'm thinking nfsd etc), then the task-work thing might need some extra
thought.

                     Linus
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