Forced dismounts of NFS mounts can fail when thread(s) are stuck

waiting for an RPC reply from the server while holding the mount
point busy (mnt_lockref incremented). This happens because dounmount()
msleep()s waiting for mnt_lockref to become 0, before calling
VFS_UNMOUNT(). This patch adds a new VFS operation called VFS_PURGE(),
which the NFS client implements as purging RPCs in progress. Making
this call before checking mnt_lockref fixes the problem, by ensuring
that the VOP_xxx() calls will fail and unbusy the mount point.

Reported by:	sbruno
Reviewed by:	kib
MFC after:	2 weeks

1 files changed, 15 insertions, 0 deletions

diff --git a/sys/fs/nfsclient/nfs_clvfsops.c b/sys/fs/nfsclient/nfs_clvfsops.c
index 863c418..4a180c5 100644
--- a/sys/fs/nfsclient/nfs_clvfsops.c
+++ b/sys/fs/nfsclient/nfs_clvfsops.c
@@ -120,6 +120,7 @@ static vfs_root_t nfs_root;
 static vfs_statfs_t nfs_statfs;
 static vfs_sync_t nfs_sync;
 static vfs_sysctl_t nfs_sysctl;
+static vfs_purge_t nfs_purge;
 
 /*
  * nfs vfs operations.
@@ -134,6 +135,7 @@ static struct vfsops nfs_vfsops = {
 	.vfs_uninit =		ncl_uninit,
 	.vfs_unmount =		nfs_unmount,
 	.vfs_sysctl =		nfs_sysctl,
+	.vfs_purge =		nfs_purge,
 };
 VFS_SET(nfs_vfsops, nfs, VFCF_NETWORK | VFCF_SBDRY);
 
@@ -1676,6 +1678,19 @@ nfs_sysctl(struct mount *mp, fsctlop_t op, struct sysctl_req *req)
 }
 
 /*
+ * Purge any RPCs in progress, so that they will all return errors.
+ * This allows dounmount() to continue as far as VFS_UNMOUNT() for a
+ * forced dismount.
+ */
+static void
+nfs_purge(struct mount *mp)
+{
+	struct nfsmount *nmp = VFSTONFS(mp);
+
+	newnfs_nmcancelreqs(nmp);
+}
+
+/*
  * Extract the information needed by the nlm from the nfs vnode.
  */
 static void