linux-kernel - Re: [PATCH v4 1/1] dcache: Translating dentry into pathname without taking rename

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Date:	Mon, 09 Sep 2013 13:56:50 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Al Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	George Spelvin <linux@...izon.com>,
	John Stoffel <john@...ffel.org>
Subject: Re: [PATCH v4 1/1] dcache: Translating dentry into pathname without
 taking rename_lock

On 09/09/2013 01:45 PM, Linus Torvalds wrote:
> On Mon, Sep 9, 2013 at 10:29 AM, Al Viro<viro@...iv.linux.org.uk>  wrote:
>> I'm not sure I like mixing rcu_read_lock() into that - d_path() and friends
>> can do that themselves just fine (it needs to be taken when seq is even),
>> and e.g. d_walk() doesn't need it at all.  Other than that, I'm OK with
>> this variant.
> Hmm.. I think you need the RCU read lock even when you get the write_seqlock().
>
> Yes, getting the seqlock for write implies that you get a spinlock and
> in many normal circumstances that basically is equvalent to being
> rcu-locked, but afaik in some configurations that is *not* sufficient
> protection against an RCU grace period on another CPU. You need to do
> a real rcu_read_lock that increments that whole rcu_read_lock_nesting
> level, which a spinlock won't do.
>
> And while the rename sequence lock protects against _renames_, it does
> not protect against just plain dentries getting free'd under memory
> pressure.
>
> So I think the RCU-readlockness really needs to be independent of the
> sequence lock.
>
>              Linus

Yes, you are right. It will be safer to take rcu_read_lock() even if we 
are taking the rename_lock. It wasn't needed before as d_lock was taken. 
Will update the patch to take rcu_read_lock() out to reflect that.

Regards,
Longman
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