linux-kernel - Re: [PATCH] Fix i_mutex handling in nfsd readdir.

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Message-Id: <1240125529.3589.44.camel@macbook.infradead.org>
Date:	Sun, 19 Apr 2009 08:18:49 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	hooanon05@...oo.co.jp
Cc:	Al Viro <viro@...IV.linux.org.uk>, bfields@...i.umich.edu,
	hch@...radead.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Fix i_mutex handling in nfsd readdir.

On Sat, 2009-04-18 at 12:11 +0900, hooanon05@...oo.co.jp wrote:
> David Woodhouse:
> > This patch fixes it by locking the directory's i_mutex again before
> > calling the filldir functions. The original deadlocks which commit
> > 14f7dd63 was designed to avoid are still avoided, because they were due
> > to fs-internal locking, not i_mutex.
> 	:::
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -1248,6 +1248,8 @@ struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
> >  	int err;
> >  	struct qstr this;
> >  
> > +	WARN_ON_ONCE(!mutex_is_locked(&base->d_inode->i_mutex));
> > +
> >  	err = __lookup_one_len(name, &this, base, len);
> 
> I'd suggest this checking is done only when CONFIG_DEBUG_KERNEL (or
> something) is enabled. Because unconditional checking costs high for the
> well-reviewed lookup code.

It's supposed to be locked. It's likely to have been locked quite
recently, so it'll be in the cache. I don't think the mutex_is_locked()
check is going to be that expensive, is it?

-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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