livelock in __writeback_inodes_wb ?

[Dave Jones]

We had a user report the soft lockup detector kicked after 22
seconds of no progress, with this trace..

:BUG: soft lockup - CPU#1 stuck for 22s! [flush-8:16:3137]
:Pid: 3137, comm: flush-8:16 Not tainted 3.6.7-4.fc17.x86_64 #1
:RIP: 0010:[<ffffffff812eeb8c>]  [<ffffffff812eeb8c>] __list_del_entry+0x2c/0xd0
:Call Trace:
: [<ffffffff811b783e>] redirty_tail+0x5e/0x80
: [<ffffffff811b8212>] __writeback_inodes_wb+0x72/0xd0
: [<ffffffff811b980b>] wb_writeback+0x23b/0x2d0
: [<ffffffff811b9b5c>] wb_do_writeback+0xac/0x1f0
: [<ffffffff8106c0e0>] ? __internal_add_timer+0x130/0x130
: [<ffffffff811b9d2b>] bdi_writeback_thread+0x8b/0x230
: [<ffffffff811b9ca0>] ? wb_do_writeback+0x1f0/0x1f0
: [<ffffffff8107fde3>] kthread+0x93/0xa0
: [<ffffffff81627e04>] kernel_thread_helper+0x4/0x10
: [<ffffffff8107fd50>] ? kthread_freezable_should_stop+0x70/0x70
: [<ffffffff81627e00>] ? gs_change+0x13/0x13

Looking over the code, is it possible that something could be
dirtying pages faster than writeback can get them written out,
keeping us in this loop indefitely ?

Should there be something in this loop periodically poking
the watchdog perhaps ?

	Dave

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