[3.11.y.z extended stable] Patch "elevator: Fix a race in elevator switching and md device" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    elevator: Fix a race in elevator switching and md device

to the linux-3.11.y-queue branch of the 3.11.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.11.y-queue

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.11.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 15d107d0eee1208c4ff55699401b9434fd97a7bd Mon Sep 17 00:00:00 2001
From: Tomoki Sekiyama <tomoki.sekiyama at hds.com>
Date: Tue, 15 Oct 2013 16:42:16 -0600
Subject: elevator: Fix a race in elevator switching and md device
 initialization

commit eb1c160b22655fd4ec44be732d6594fd1b1e44f4 upstream.

The soft lockup below happens at the boot time of the system using dm
multipath and the udev rules to switch scheduler.

[  356.127001] BUG: soft lockup - CPU#3 stuck for 22s! [sh:483]
[  356.127001] RIP: 0010:[<ffffffff81072a7d>]  [<ffffffff81072a7d>] lock_timer_base.isra.35+0x1d/0x50
...
[  356.127001] Call Trace:
[  356.127001]  [<ffffffff81073810>] try_to_del_timer_sync+0x20/0x70
[  356.127001]  [<ffffffff8118b08a>] ? kmem_cache_alloc_node_trace+0x20a/0x230
[  356.127001]  [<ffffffff810738b2>] del_timer_sync+0x52/0x60
[  356.127001]  [<ffffffff812ece22>] cfq_exit_queue+0x32/0xf0
[  356.127001]  [<ffffffff812c98df>] elevator_exit+0x2f/0x50
[  356.127001]  [<ffffffff812c9f21>] elevator_change+0xf1/0x1c0
[  356.127001]  [<ffffffff812caa50>] elv_iosched_store+0x20/0x50
[  356.127001]  [<ffffffff812d1d09>] queue_attr_store+0x59/0xb0
[  356.127001]  [<ffffffff812143f6>] sysfs_write_file+0xc6/0x140
[  356.127001]  [<ffffffff811a326d>] vfs_write+0xbd/0x1e0
[  356.127001]  [<ffffffff811a3ca9>] SyS_write+0x49/0xa0
[  356.127001]  [<ffffffff8164e899>] system_call_fastpath+0x16/0x1b

This is caused by a race between md device initialization by multipathd and
shell script to switch the scheduler using sysfs.

 - multipathd:
   SyS_ioctl -> do_vfs_ioctl -> dm_ctl_ioctl -> ctl_ioctl -> table_load
   -> dm_setup_md_queue -> blk_init_allocated_queue -> elevator_init
    q->elevator = elevator_alloc(q, e); // not yet initialized

 - sh -c 'echo deadline > /sys/$DEVPATH/queue/scheduler':
   elevator_switch (in the call trace above)
    struct elevator_queue *old = q->elevator;
    q->elevator = elevator_alloc(q, new_e);
    elevator_exit(old);                 // lockup! (*)

 - multipathd: (cont.)
    err = e->ops.elevator_init_fn(q);   // init fails; q->elevator is modified

(*) When del_timer_sync() is called, lock_timer_base() will loop infinitely
while timer->base == NULL. In this case, as timer will never initialized,
it results in lockup.

This patch introduces acquisition of q->sysfs_lock around elevator_init()
into blk_init_allocated_queue(), to provide mutual exclusion between
initialization of the q->scheduler and switching of the scheduler.

This should fix this bugzilla:
https://bugzilla.redhat.com/show_bug.cgi?id=902012

Signed-off-by: Tomoki Sekiyama <tomoki.sekiyama at hds.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
Cc: Josh Boyer <jwboyer at fedoraproject.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 block/blk-core.c | 10 +++++++++-
 block/elevator.c |  6 ++++++
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 365f472..dc9f903 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -741,9 +741,17 @@ blk_init_allocated_queue(struct request_queue *q, request_fn_proc *rfn,

 	q->sg_reserved_size = INT_MAX;

+	/* Protect q->elevator from elevator_change */
+	mutex_lock(&q->sysfs_lock);
+
 	/* init elevator */
-	if (elevator_init(q, NULL))
+	if (elevator_init(q, NULL)) {
+		mutex_unlock(&q->sysfs_lock);
 		return NULL;
+	}
+
+	mutex_unlock(&q->sysfs_lock);
+
 	return q;
 }
 EXPORT_SYMBOL(blk_init_allocated_queue);
diff --git a/block/elevator.c b/block/elevator.c
index 668394d..02d4390 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -186,6 +186,12 @@ int elevator_init(struct request_queue *q, char *name)
 	struct elevator_type *e = NULL;
 	int err;

+	/*
+	 * q->sysfs_lock must be held to provide mutual exclusion between
+	 * elevator_switch() and here.
+	 */
+	lockdep_assert_held(&q->sysfs_lock);
+
 	if (unlikely(q->elevator))
 		return 0;

--
1.8.3.2