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 21:09:24 +0000 |
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On 8/29/13 16:29 , "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 >> >> This is caused by a race between md device initialization and sysfs knob >> to switch the scheduler. > >I think we can also improve changelog a bit. So IIUC, softlockup >happens because one thread called del_timer_sync() on a timer which >was not even initilized. Timer initialization should have happened >in cfq_init_queue() using init_timer(). But before init_timer() >could be called, elevator switch path called del_timer_sync(). > >del_timer_sync() in turn calls lock_timer_base() which will loop >infinitely if timer->base == NULL. And because we have not called >init_timer() yet, I am assuming timer->base is null? > >Is this right analysis? If yes, then this patch should most likely >fix following bz. > >https://bugzilla.redhat.com/show_bug.cgi?id=902012
I think your analysis is correct. If del_timer_sync() is called right after cfqd is allocated (with __GFP_ZERO), timer->base == NULL. Otherwise it may hit NULL pointer.
The other reason of NULL timer->base is that the timer is migrating in __mod_timer(), but then the it must be set to non-NULL in a short time.
Maybe __mod_timer should use some illegal pointer value (like LIST_POISON1) instead of NULL to represent the timer is migrating.... Actually, when I changed __mod_timer to timer_set_base(timer, 0xdeadbeaf), made lock_timer_base wait while base == 0xdeadbeaf, and added BUG_ON(!timer->base) to lock_timer_base, my system hits the BUG.
>I had concluded that some how timer->base is NULL but could not understand >how come timer base is NULL when we have called init_timer() on it. > >Thanks >Vivek
Thanks, Tomoki
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