NFS: fix reference counting for NFSv4 callback thread

The reference counting for the NFSv4 callback thread stays artificially
high. When this thread comes down, it doesn't properly tear down the
svc_serv, causing a memory leak. In my testing on an older kernel on
x86_64, memory would leak out of the 8k kmalloc slab. So, we're leaking
at least a page of memory every time the thread comes down.

svc_create() creates the svc_serv with a sv_nrthreads count of 1, and
then svc_create_thread() increments that count. Whenever the callback
thread is started it has a sv_nrthreads count of 2. When coming down, it
calls svc_exit_thread() which decrements that count and if it hits 0, it
tears everything down. That never happens here since the count is always
at 2 when the thread exits.

The problem is that nfs_callback_up() should be calling svc_destroy() on
the svc_serv on both success and failure. This is how lockd_up_proto()
handles the reference counting, and doing that here fixes the leak.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

diff --git a/fs/nfs/callback.c b/fs/nfs/callback.c
index bd185a572a23aeb0d015b701d0786dab359f260a..ecc06c619494f6cbbe07942c4c72b3002dd290da 100644 (file)
--- a/fs/nfs/callback.c
+++ b/fs/nfs/callback.c
@@ -105,7 +105,7 @@ static void nfs_callback_svc(struct svc_rqst *rqstp)
  */
 int nfs_callback_up(void)
 {
-       struct svc_serv *serv;
+       struct svc_serv *serv = NULL;
        int ret = 0;
 
        lock_kernel();
@@ -122,24 +122,30 @@ int nfs_callback_up(void)
        ret = svc_create_xprt(serv, "tcp", nfs_callback_set_tcpport,
                              SVC_SOCK_ANONYMOUS);
        if (ret <= 0)
-               goto out_destroy;
+               goto out_err;
        nfs_callback_tcpport = ret;
        dprintk("Callback port = 0x%x\n", nfs_callback_tcpport);
 
        ret = svc_create_thread(nfs_callback_svc, serv);
        if (ret < 0)
-               goto out_destroy;
+               goto out_err;
        nfs_callback_info.serv = serv;
        wait_for_completion(&nfs_callback_info.started);
 out:
+       /*
+        * svc_create creates the svc_serv with sv_nrthreads == 1, and then
+        * svc_create_thread increments that. So we need to call svc_destroy
+        * on both success and failure so that the refcount is 1 when the
+        * thread exits.
+        */
+       if (serv)
+               svc_destroy(serv);
        mutex_unlock(&nfs_callback_mutex);
        unlock_kernel();
        return ret;
-out_destroy:
+out_err:
        dprintk("Couldn't create callback socket or server thread; err = %d\n",
                ret);
-       svc_destroy(serv);
-out_err:
        nfs_callback_info.users--;
        goto out;
 }