Messages in this thread | | | From | Tomoki Sekiyama <> | Subject | Re: [PATCH] elevator: Fix a race in elevator switching and md device initialization | Date | Thu, 29 Aug 2013 19:28:02 +0000 |
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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.
>I am CCing Tejun also to the thread. He also might have some ideas here. > >Thanks >Vivek
Thanks, Tomoki Sekiyama
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