linux-kernel - Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression? (also an unrelated ext4 data loss bug)

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Date:	Tue, 23 Oct 2012 18:23:48 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Nix <nix@...eri.org.uk>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Ted Ts'o" <tytso@....edu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Schumaker, Bryan" <Bryan.Schumaker@...app.com>,
	Peng Tao <bergwolf@...il.com>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	Stanislav Kinsbursky <skinsbursky@...allels.com>
Subject: Re: Heads-up: 3.6.2 / 3.6.3 NFS server oops: 3.6.2+ regression?
 (also an unrelated ext4 data loss bug)

On Tue, 2012-10-23 at 13:57 -0400, Trond Myklebust wrote:
> On Tue, 2012-10-23 at 17:44 +0000, Myklebust, Trond wrote:
> > You can't hold a spinlock while sleeping. Both mutex_lock() and nsm_create() can definitely sleep.
> > 
> > The correct way to do this is to grab the spinlock and recheck the value of ln->nsm_users inside the 'if (!IS_ERR())' condition. If it is still zero, bump it and set ln->nsm_clnt, otherwise bump it, get the existing ln->nsm_clnt and call rpc_shutdown_clnt() on the redundant nsm client after dropping the spinlock.
> > 
> > Cheers
> >   Trond
> 
> Can you please check if the following patch fixes the issue?
> 
> Cheers
>   Trond
> 
Meh... This one gets rid of the 100% redundant mutex...

8<-----------------------------------------------------------
From 4187c816a15df12544ebcfa6b961fce96458e244 Mon Sep 17 00:00:00 2001
From: Trond Myklebust <Trond.Myklebust@...app.com>
Date: Tue, 23 Oct 2012 13:51:58 -0400
Subject: [PATCH] LOCKD: fix races in nsm_client_get

Commit e9406db20fecbfcab646bad157b4cfdc7cadddfb (lockd: per-net
NSM client creation and destruction helpers introduced) contains
a nasty race on initialisation of the per-net NSM client because
it doesn't check whether or not the client is set after grabbing
the nsm_create_mutex.

Reported-by: Nix <nix@...eri.org.uk>
Signed-off-by: Trond Myklebust <Trond.Myklebust@...app.com>
Cc: stable@...r.kernel.org
---
 fs/lockd/mon.c | 43 ++++++++++++++++++++++++++-----------------
 1 file changed, 26 insertions(+), 17 deletions(-)

diff --git a/fs/lockd/mon.c b/fs/lockd/mon.c
index e4fb3ba..fe69560 100644
--- a/fs/lockd/mon.c
+++ b/fs/lockd/mon.c
@@ -85,29 +85,38 @@ static struct rpc_clnt *nsm_create(struct net *net)
 	return rpc_create(&args);
 }
 
+static struct rpc_clnt *nsm_client_set(struct lockd_net *ln,
+		struct rpc_clnt *clnt)
+{
+	spin_lock(&ln->nsm_clnt_lock);
+	if (ln->nsm_users == 0) {
+		if (clnt == NULL)
+			goto out;
+		ln->nsm_clnt = clnt;
+	}
+	clnt = ln->nsm_clnt;
+	ln->nsm_users++;
+out:
+	spin_unlock(&ln->nsm_clnt_lock);
+	return clnt;
+}
+
 static struct rpc_clnt *nsm_client_get(struct net *net)
 {
-	static DEFINE_MUTEX(nsm_create_mutex);
-	struct rpc_clnt	*clnt;
+	struct rpc_clnt	*clnt, *new;
 	struct lockd_net *ln = net_generic(net, lockd_net_id);
 
-	spin_lock(&ln->nsm_clnt_lock);
-	if (ln->nsm_users) {
-		ln->nsm_users++;
-		clnt = ln->nsm_clnt;
-		spin_unlock(&ln->nsm_clnt_lock);
+	clnt = nsm_client_set(ln, NULL);
+	if (clnt != NULL)
 		goto out;
-	}
-	spin_unlock(&ln->nsm_clnt_lock);
 
-	mutex_lock(&nsm_create_mutex);
-	clnt = nsm_create(net);
-	if (!IS_ERR(clnt)) {
-		ln->nsm_clnt = clnt;
-		smp_wmb();
-		ln->nsm_users = 1;
-	}
-	mutex_unlock(&nsm_create_mutex);
+	clnt = new = nsm_create(net);
+	if (IS_ERR(clnt))
+		goto out;
+
+	clnt = nsm_client_set(ln, new);
+	if (clnt != new)
+		rpc_shutdown_client(new);
 out:
 	return clnt;
 }
-- 
1.7.11.7


-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com