Messages in this thread | | | Date | Thu, 29 Aug 2013 16:01:57 -0400 | From | Vivek Goyal <> | Subject | Re: [PATCH] elevator: Fix a race in elevator switching and md device initialization |
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On Thu, Aug 29, 2013 at 07:29:07PM +0000, Tomoki Sekiyama wrote: > 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.
I agree. Providing explicit locking and making sure only one elevator can be initializing at a time on a queue and others wait till that operation is complete, will make up the code more readable and less bug suspecible.
Thanks Vivek
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