Commits
Tomoki Sekiyama committed eb1c160b226
elevator: Fix a race in elevator switching and md device initialization
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>
Signed-off-by: Jens Axboe <axboe@kernel.dk>