linux-kernel - Re: [PATCH v2] vfs: Don't exchange "short" filenames unconditionally.

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Message-ID: <CA+55aFyveyOABqssxLinsmHkBg9=95TwEGC1ZT1POe+RmTbj+A@mail.gmail.com>
Date:	Mon, 29 Sep 2014 09:07:00 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mikhail Efremov <sem@...linux.org>
Subject: Re: [PATCH v2] vfs: Don't exchange "short" filenames unconditionally.

On Mon, Sep 29, 2014 at 8:59 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> Now RCU lookup starts.  And on another CPU we move the first dentry over
> the third one.  copy_name() is called, it decrements the refcount down
> to 1 (__d_free() hasn't happened yet) and doesn't schedule any freeing.

Ahh. If we were to do *all* of the copy-name name freeing under RCU,
we'd be ok. But we don't. We do the refcount decrement immediately
(and have to, if we want to have the rcu/refcount union). Yeah, good
call, although I find that doubvle rcu grace period for the common
case to be very annoying.

Ugh. I'm starting to be sorry I ever suggested this cleanup. Maybe
we're better off with the horrid hack (which is equivalent to what we
did before) after all.

               Linus
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