FAILED: patch "[PATCH] NFS: fix mount/umount race in nlmclnt." failed to apply to 4.4-stable tree

The patch below does not apply to the 4.4-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 4a9be28c45bf02fa0436808bb6c0baeba30e120e Mon Sep 17 00:00:00 2001
From: NeilBrown <[email protected]>
Date: Tue, 19 Mar 2019 11:33:24 +1100
Subject: [PATCH] NFS: fix mount/umount race in nlmclnt.

If the last NFSv3 unmount from a given host races with a mount from the same host, we can destroy an nlm_host that is still in use.

Specifically nlmclnt_lookup_host() can increment h_count on an nlm_host that nlmclnt_release_host() has just successfully called refcount_dec_and_test() on.
Once nlmclnt_lookup_host() drops the mutex, nlm_destroy_host_lock() will be called to destroy the nlmclnt which is now in use again.

The cause of the problem is that the dec_and_test happens outside the locked region. This is easily fixed by using refcount_dec_and_mutex_lock().

index 93fb7cf0b92b..f0b5c987d6ae 100644
@@ -290,12 +290,11 @@ void nlmclnt_release_host(struct nlm_host *host)
 
 	WARN_ON_ONCE(host->h_server);
 
-	if (refcount_dec_and_test(&host->h_count)) {
+	if (refcount_dec_and_mutex_lock(&host->h_count, &nlm_host_mutex)) {
 		WARN_ON_ONCE(!list_empty(&host->h_lockowners));
 		WARN_ON_ONCE(!list_empty(&host->h_granted));
 		WARN_ON_ONCE(!list_empty(&host->h_reclaim));
 
-		mutex_lock(&nlm_host_mutex);
 		nlm_destroy_host_locked(host);
 		mutex_unlock(&nlm_host_mutex);
 	}