Re: [PATCH] elevator: Fix a race in elevator switching and md device initialization

[Tomoki Sekiyama <tomoki.sekiyama@hds.com>]

On 8/29/13 14:43 , "Vivek Goyal" <vgoyal@redhat.com> wrote:
>On Thu, Aug 29, 2013 at 02:33:10PM -0400, Vivek Goyal wrote:
>> On Mon, Aug 26, 2013 at 09:45:15AM -0400, Tomoki Sekiyama wrote:
>> > The soft lockup below happes at the boot time of the system using dm
>> > multipath and automated elevator switching udev rules.
>> > 
>> > [  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
>> > 
>> 
>> Tokomi, 
>> 
>> As you noticed, there is a fedora bug open with similar signature. May
>> be this patch will fix that issue also.
>> 
>> https://bugzilla.redhat.com/show_bug.cgi?id=902012
>> 
>> 
>> > This is caused by a race between md device initialization and sysfs
>>knob
>> > to switch the scheduler.
>> > 
>> > * 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'
>> >  SyS_write -> vfs_write -> sysfs_write_file -> queue_attr_store
>> >      ( mutex_lock(&q->sysfs_lock) here. )
>> >   -> elv_iosched_store -> elevator_change:
>> > 
>> >   elevator_exit(old); // try to de-init uninitialized elevator and
>>hang up
>> > 
>
>If problem in this case is that we are trying to exit() the elevator
>which has not been properly initialized, then we should not attach
>the elevator to the queue yet.
>
>In cfq_init_queue(), can we move following code towards the end of
>function.
>
>        spin_lock_irq(q->queue_lock);
>        q->elevator = eq;
>        spin_unlock_irq(q->queue_lock);
>
>So till elevator is initialized, we will not attach it to queue and
>elevator_switch() will return as it will not find a valid elevator
>on the queue.
>
>
>elevator_change() {
>	        if (!q->elevator)
>                return -ENXIO;
>}
>
>Thanks
>Vivek

I think it also works, though I prefer introducing explicit locking,
as you said, so that code won't break again in some future.

Thanks,
Tomoki