• Tomoki Sekiyama's avatar
    elevator: Fix a race in elevator switching and md device initialization · 6d53d392
    Tomoki Sekiyama authored
    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=902012Signed-off-by: default avatarTomoki Sekiyama <tomoki.sekiyama@hds.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    6d53d392
blk-core.c 86.5 KB