[dm-devel] [PATCH 28/57] libmultipath: use a shared lock to co-operate with udev

[Hannes Reinecke]

udev since v214 is placing a shared lock on the device node
whenever it's processing the event. This introduces a race
condition with multipathd, as multipathd is processing the
event for the block device at the same time as udev is
processing the events for the partitions.
And a lock on the partitions will also be visible on the
block device itself, hence multipathd won't be able to
lock the device.
When multipath manages to take a lock on the device,
udev will fail, and consequently ignore this entire event.
Which in turn might cause the system to malfunction as it
might have been a crucial event like 'remove' or 'link down'.

So we should better use LOCK_SH here; with that the flock
call in multipathd _and_ udev will succeed and the events
can be processed.

Signed-off-by: Hannes Reinecke <hare at suse.de>
---
 libmultipath/configure.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libmultipath/configure.c b/libmultipath/configure.c
index 30c7259..ca20ba5 100644
--- a/libmultipath/configure.c
+++ b/libmultipath/configure.c
@@ -546,7 +546,7 @@ lock_multipath (struct multipath * mpp, int lock)
 		if (!pgp->paths)
 			continue;
 		vector_foreach_slot(pgp->paths, pp, j) {
-			if (lock && flock(pp->fd, LOCK_EX | LOCK_NB) &&
+			if (lock && flock(pp->fd, LOCK_SH | LOCK_NB) &&
 			    errno == EWOULDBLOCK)
 				goto fail;
 			else if (!lock)
-- 
2.6.6