linux-kernel - Re: [PATCH v2] Fix i_mutex handling in nfsd readdir

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Date:	Mon, 11 May 2009 19:16:34 -0400
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Al Viro <viro@...IV.linux.org.uk>, hooanon05@...oo.co.jp,
	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 v2] Fix i_mutex handling in nfsd readdir

On Mon, Apr 20, 2009 at 03:50:08PM -0400, bfields wrote:
> On Sun, Apr 19, 2009 at 04:51:54PM -0400, bfields wrote:
> > On Sun, Apr 19, 2009 at 01:27:49PM +0100, David Woodhouse wrote:
> > > Commit 14f7dd63 ("Copy XFS readdir hack into nfsd code") introduced a
> > > bug to generic code which had been extant for a long time in the XFS
> > > version -- it started to call through into lookup_one_len() and hence
> > > into the file systems' ->lookup() methods without i_mutex held on the
> > > directory.
> > > 
> > > 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.
> > > 
> > > Commit 05f4f678 ("nfsd4: don't do lookup within readdir in recovery
> > > code") introduced a similar problem there, which this also addresses.
> > > 
> > > While we're at it, fix the return type of nfsd_buffered_readdir() which
> > > should be a __be32 not an int -- it's an NFS errno, not a Linux errno.
> > > And return nfserrno(-ENOMEM) when allocation fails, not just -ENOMEM.
> > > Sparse would have caught both of those if it wasn't so busy bitching
> > > about __cold__.
> > > 
> > > Commit 05f4f678 ("nfsd4: don't do lookup within readdir in recovery
> > > code") introduced a similar problem with calling lookup_one_len()
> > > without i_mutex, which this patch also addresses.
> > > 
> > > Reported-by: J. R. Okajima <hooanon05@...oo.co.jp>
> > > Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
> > > Umm-I-can-live-with-that-by: Al Viro <viro@...iv.linux.org.uk>
> > > ---
> > > Still haven't tested the NFSv4 bit -- Bruce?
> > 
> > Thanks, there's an iterator in there that calls a passed-in function,
> > some examples of which were taking the i_mutex--so some fixing up is
> > needed.  I'll follow up with a patch once I've got one tested.
> 
> Sorry for the delay.  Simpler might be just to drop and reacquire the
> mutex each time through nfsd4_list_rec_dir()'s loop, but I'd just as
> soon rework the called functions to expect the mutex be held (and get
> rid of the unused, probably fragile, clear_clid_dir() in the process).
> 
> So the following could be folded in to your patch.
> 
> I tested the combined patch over 2.6.30-rc2.  I also tested 2.6.29 +
> 05f4f678 + the combined patch.  Both look  OK.  Feel free to add a
> tested-by or acked-by for "J. Bruce Fields" <bfields@...i.umich.edu> as
> appropriate.  Or happy to add a s-o-b and shepherd it along myself if
> it's easier....

Unfortunately, I wasn't watching my logs carefully enough, and missed a
lockdep warning.

(Stupid policy question: is this for stable, current, or next (.29, .30,
or .31?)  On the one hand, it's just a warning.  On the other hand,
people freak out when they see backtraces in their logs.  But I don't
know how common it is to have lockdep on.)

--b.

commit 8daed1e549b55827758b3af7b8132a73fc51526f
Author: J. Bruce Fields <bfields@...i.umich.edu>
Date:   Mon May 11 16:10:19 2009 -0400

    nfsd: silence lockdep warning
    
    Signed-off-by: J. Bruce Fields <bfields@...i.umich.edu>

diff --git a/fs/nfsd/nfs4recover.c b/fs/nfsd/nfs4recover.c
index 5275097..b534840 100644
--- a/fs/nfsd/nfs4recover.c
+++ b/fs/nfsd/nfs4recover.c
@@ -229,7 +229,7 @@ nfsd4_list_rec_dir(struct dentry *dir, recdir_func *f)
 		goto out;
 	status = vfs_readdir(filp, nfsd4_build_namelist, &names);
 	fput(filp);
-	mutex_lock(&dir->d_inode->i_mutex);
+	mutex_lock_nested(&dir->d_inode->i_mutex, I_MUTEX_PARENT);
 	while (!list_empty(&names)) {
 		entry = list_entry(names.next, struct name_list, list);
 
@@ -264,7 +264,7 @@ nfsd4_unlink_clid_dir(char *name, int namlen)
 
 	dprintk("NFSD: nfsd4_unlink_clid_dir. name %.*s\n", namlen, name);
 
-	mutex_lock(&rec_dir.dentry->d_inode->i_mutex);
+	mutex_lock_nested(&rec_dir.dentry->d_inode->i_mutex, I_MUTEX_PARENT);
 	dentry = lookup_one_len(name, rec_dir.dentry, namlen);
 	if (IS_ERR(dentry)) {
 		status = PTR_ERR(dentry);
--
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