autofs-5.0.6 - fix recursive mount deadlock

From: Ian Kent <raven@themaw.net>

Prior to the vfs-automount changes that went into 2.6.38

and were finalized in 3.1 it was not possible to block

path walks into multi-mounts whose root was covered by

another mount. To deal with that a write lock was used

to ensure the mount tree construction was completed. This

restricts the types of recursively defined mount maps that

can be used and can lead to a deadlock during lookup.

Now that we can prevent processes walking into multi-mounts

that are under construction we no longer need to use a

write lock.

Also, in the patch below, a cache writelock is changed to

a read lock because a write lock isn't needed since the

map cache entry isn't being updated.

---

 CHANGELOG       |    1 +

 daemon/direct.c |   14 ++++++++++++--

 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/CHANGELOG b/CHANGELOG

index 936c9ab..9cdad6e 100644

--- a/CHANGELOG

+++ b/CHANGELOG

@@ -12,6 +12,7 @@

 - configure.in: allow cross compilation.

 - README: update mailing list subscription info.

 - allow non root user to check status.

+- fix recursive mount deadlock.

 25/07/2012 autofs-5.0.7

 =======================

diff --git a/daemon/direct.c b/daemon/direct.c

index 7e2f0d7..3e09c5d 100644

--- a/daemon/direct.c

+++ b/daemon/direct.c

@@ -1285,6 +1285,8 @@ int handle_packet_missing_direct(struct autofs_point *ap, autofs_packet_missing_

 	struct timespec wait;

 	struct timeval now;

 	int ioctlfd, len, state;

+	unsigned int kver_major = get_kver_major();

+	unsigned int kver_minor = get_kver_minor();

 	pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &state);

@@ -1297,8 +1299,16 @@ int handle_packet_missing_direct(struct autofs_point *ap, autofs_packet_missing_

 	 * cache entry we will not be able to find the mapent. So

 	 * we must take the source writelock to ensure the parent

 	 * has mount is complete before we look for the entry.

+	 *

+	 * Since the vfs-automount kernel changes we can now block

+	 * on covered mounts during mount tree construction so a

+	 * write lock is no longer needed. So we now can handle a

+	 * wider class of recursively define mount lookups.

 	 */

-	master_source_writelock(ap->entry);

+	if (kver_major > 5 || (kver_major == 5 && kver_minor > 1))

+		master_source_readlock(ap->entry);

+	else

+		master_source_writelock(ap->entry);

 	map = ap->entry->maps;

 	while (map) {

 		/*

@@ -1311,7 +1321,7 @@ int handle_packet_missing_direct(struct autofs_point *ap, autofs_packet_missing_

 		}

 		mc = map->mc;

-		cache_writelock(mc);

+		cache_readlock(mc);

 		me = cache_lookup_ino(mc, pkt->dev, pkt->ino);

 		if (me)

 			break;