[PATCH 3.2 021/185] dm mpath: fix race condition between multipath_dtr and pg_init_done

Ben Hutchings
2013-12-29T02:11:41Z

From: Shiva Krishna Merla <shivakrishna.merla@netapp.com>

commit 954a73d5d3073df2231820c718fdd2f18b0fe4c9 upstream.

Whenever multipath_dtr() is happening we must prevent queueing any
further path activation work. Implement this by adding a new
'pg_init_disabled' flag to the multipath structure that denotes future
path activation work should be skipped if it is set. By disabling
pg_init and then re-enabling in flush_multipath_work() we also avoid the
potential for pg_init to be initiated while suspending an mpath device.

Without this patch a race condition exists that may result in a kernel
panic:

1) If after pg_init_done() decrements pg_init_in_progress to 0, a call
to wait_for_pg_init_completion() assumes there are no more pending path
management commands.
2) If pg_init_required is set by pg_init_done(), due to retryable
mode_select errors, then process_queued_ios() will again queue the
path activation work.
3) If free_multipath() completes before activate_path() work is called a
NULL pointer dereference like the following can be seen when
accessing members of the recently destructed multipath:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000090
RIP: 0010:[<ffffffffa003db1b>] [<ffffffffa003db1b>] activate_path+0x1b/0x30 [dm_multipath]
[<ffffffff81090ac0>] worker_thread+0x170/0x2a0
[<ffffffff81096c80>] ? autoremove_wake_function+0x0/0x40

[switch to disabling pg_init in flush_multipath_work & header edits by Mike Snitzer]
Signed-off-by: Shiva Krishna Merla <shivakrishna.merla@netapp.com>
Reviewed-by: Krishnasamy Somasundaram <somasundaram.krishnasamy@netapp.com>
Tested-by: Speagle Andy <Andy.Speagle@netapp.com>
Acked-by: Junichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[bwh: Backported to 3.2:
- Adjust context
- Bump version to 1.3.2 not 1.6.0]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
drivers/md/dm-mpath.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

--- a/drivers/md/dm-mpath.c
+++ b/drivers/md/dm-mpath.c
@@ -84,6 +84,7 @@ struct multipath {
unsigned queue_io; /* Must we queue all I/O? */
unsigned queue_if_no_path; /* Queue I/O if last path fails? */
unsigned saved_queue_if_no_path;/* Saved state during suspension */
+ unsigned pg_init_disabled:1; /* pg_init is not currently allowed */
unsigned pg_init_retries; /* Number of times to retry pg_init */
unsigned pg_init_count; /* Number of times pg_init called */
unsigned pg_init_delay_msecs; /* Number of msecs before pg_init retry */
@@ -473,7 +474,8 @@ static void process_queued_ios(struct wo
(!pgpath && !m->queue_if_no_path))
must_queue = 0;

- if (m->pg_init_required && !m->pg_init_in_progress && pgpath)
+ if (m->pg_init_required && !m->pg_init_in_progress && pgpath &&
+ !m->pg_init_disabled)
__pg_init_all_paths(m);

out:
@@ -887,10 +889,20 @@ static void multipath_wait_for_pg_init_c

static void flush_multipath_work(struct multipath *m)
{
+ unsigned long flags;
+
+ spin_lock_irqsave(&m->lock, flags);
+ m->pg_init_disabled = 1;
+ spin_unlock_irqrestore(&m->lock, flags);
+
flush_workqueue(kmpath_handlerd);
multipath_wait_for_pg_init_completion(m);
flush_workqueue(kmultipathd);
flush_work_sync(&m->trigger_event);
+
+ spin_lock_irqsave(&m->lock, flags);
+ m->pg_init_disabled = 0;
+ spin_unlock_irqrestore(&m->lock, flags);
}

static void multipath_dtr(struct dm_target *ti)
@@ -1111,7 +1123,7 @@ static int pg_init_limit_reached(struct

spin_lock_irqsave(&m->lock, flags);

- if (m->pg_init_count <= m->pg_init_retries)
+ if (m->pg_init_count <= m->pg_init_retries && !m->pg_init_disabled)
m->pg_init_required = 1;
else
limit_reached = 1;
@@ -1621,7 +1633,7 @@ out:
*---------------------------------------------------------------*/
static struct target_type multipath_target = {
.name = "multipath",
- .version = {1, 3, 1},
+ .version = {1, 3, 2},
.module = THIS_MODULE,
.ctr = multipath_ctr,
.dtr = multipath_dtr,