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@xxxxxxxxxx>
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@xxxxxxxxxxxxx>
Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
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@xxxxxxxxxx
www.netapp.com
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