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Subject: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md

 device initialization

To:	linux-kernel@vger.kernel.org

From:	Tomoki Sekiyama <tomoki.sekiyama@hds.com>

Cc:	axboe@kernel.dk, tj@kernel.org, seiji.aguchi@hds.com,

	vgoyal@redhat.com, majianpeng@gmail.com

Date:	Fri, 30 Aug 2013 18:47:07 -0400

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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@hds.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 93a18d1..2f6275f 100644

--- a/block/blk-core.c

+++ b/block/blk-core.c

@@ -739,9 +739,17 @@ 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;

--

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