Messages in this thread Patch in this message | | | Date | Thu, 29 Aug 2013 15:59:08 -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 07:28:02PM +0000, Tomoki Sekiyama wrote: > Hi vivek, > > Thanks for your comments. > > On 8/29/13 14:33 , "Vivek Goyal" <vgoyal@redhat.com> 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 > >> > >> > >>This patch adds acquisition of q->sysfs_lock in > >>blk_init_allocated_queue(). > >> This also adds the lock into elevator_change() to ensure locking from > >>the > >> other path, as it is exposed function (and queue_attr_store will uses > >> __elevator_change() now, the non-locking version of elevator_change()). > > > >I think introducing __elevator_change() is orthogonal to this problem. > >May be keep that in a separate patch. > > OK, I will split it into 2 patches. > > > >> block/blk-core.c | 6 +++++- > >> block/elevator.c | 16 ++++++++++++++-- > >> 2 files changed, 19 insertions(+), 3 deletions(-) > >> > >> diff --git a/block/blk-core.c b/block/blk-core.c > >> index 93a18d1..2323ec3 100644 > >> --- a/block/blk-core.c > >> +++ b/block/blk-core.c > >> @@ -739,9 +739,13 @@ 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)) > >> - return NULL; > >> + q = NULL; > >> + mutex_unlock(&q->sysfs_lock); > >> + > > > >So core of the problem is, what's the locking semantics to make sure > >that we are not trying to switch elevator while it is still initializing. > >IOW, should we allow multiple parallel calls of elevator_init_fn() on a > >queue and is it safe? > > > >I would argue that it is easier to read and maintain the code if we > >provide explicit locking around. So I like the idea of introducing > >some locking around elevator_init(). > > > >Because we are racing against elevator switch path which takes > >q->sysfs_lock, it makes sense to provide mutual exlusion using > >q->sysfs_lock. > > > >What I don't know is that can we take mutex in queue init path. Generally > >drivers call it and do they expect that they can call this function > >while holding a spin lock. > > As elevator_alloc() allocates memory with GFP_KERNEL, elevator_init() might > sleep. So it should be safe to use mutex here.
That's a good point. So it should be safe to add q->sysfs_lock. I would say this patch sounds reasonable to me. Just document around elevator_init_fn() that it should be called with q->sysfs_lock held to provide mutual exclusion between elevator init and elevator switch paths.
Thanks Vivek
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