don't take nfsd_mutex twice when setting number of threads

["J. Bruce Fields" <bfields@fieldses.org>]

I'm applying the following patch for 2.6.31.

--b.

commit 82e12fe9244ff653f703722a8937b595e10e71f4
Author: NeilBrown <neilb@suse.de>
Date:   Tue Jun 16 11:03:07 2009 +1000

    nfsd: don't take nfsd_mutex twice when setting number of threads.
    
    Currently when we write a number to 'threads' in nfsdfs,
    we take the nfsd_mutex, update the number of threads, then take the
    mutex again to read the number of threads.
    
    Mostly this isn't a big deal.  However if we are write '0', and
    portmap happens to be dead, then we can get unpredictable behaviour.
    If the nfsd threads all got killed quickly and the last thread is
    waiting for portmap to respond, then the second time we take the mutex
    we will block waiting for the last thread.
    However if the nfsd threads didn't die quite that fast, then there
    will be no contention when we try to take the mutex again.
    
    Unpredictability isn't fun, and waiting for the last thread to exit is
    pointless, so avoid taking the lock twice.
    To achieve this, get nfsd_svc return a non-negative number of active
    threads when not returning a negative error.
    
    Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
index 877e713..1250fb9 100644
--- a/fs/nfsd/nfsctl.c
+++ b/fs/nfsd/nfsctl.c
@@ -207,10 +207,14 @@ static struct file_operations pool_stats_operations = {
 static ssize_t write_svc(struct file *file, char *buf, size_t size)
 {
 	struct nfsctl_svc *data;
+	int err;
 	if (size < sizeof(*data))
 		return -EINVAL;
 	data = (struct nfsctl_svc*) buf;
-	return nfsd_svc(data->svc_port, data->svc_nthreads);
+	err = nfsd_svc(data->svc_port, data->svc_nthreads);
+	if (err < 0)
+		return err;
+	return 0;
 }
 
 /**
@@ -692,12 +696,12 @@ static ssize_t write_threads(struct file *file, char *buf, size_t size)
 		if (newthreads < 0)
 			return -EINVAL;
 		rv = nfsd_svc(NFS_PORT, newthreads);
-		if (rv)
+		if (rv < 0)
 			return rv;
-	}
+	} else
+		rv = nfsd_nrthreads();
 
-	return scnprintf(buf, SIMPLE_TRANSACTION_LIMIT, "%d\n",
-							nfsd_nrthreads());
+	return scnprintf(buf, SIMPLE_TRANSACTION_LIMIT, "%d\n", rv);
 }
 
 /**
diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
index cbba4a9..209eaa0 100644
--- a/fs/nfsd/nfssvc.c
+++ b/fs/nfsd/nfssvc.c
@@ -413,6 +413,12 @@ nfsd_svc(unsigned short port, int nrservs)
 		goto failure;
 
 	error = svc_set_num_threads(nfsd_serv, NULL, nrservs);
+	if (error == 0)
+		/* We are holding a reference to nfsd_serv which
+		 * we don't want to count in the return value,
+		 * so subtract 1
+		 */
+		error = nfsd_serv->sv_nrthreads - 1;
  failure:
 	svc_destroy(nfsd_serv);		/* Release server */
  out: