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From: David Howells on 26 May 2010 11:10

The following commit may be a problem for emergency_remount() [Alt+SysRq+U]:

commit fa4b9074cd8428958c2adf9dc0c831f46e27c193
Author: Tejun Heo <tj(a)kernel.org>
Date: Sat May 15 20:09:27 2010 +0200

buffer: make invalidate_bdev() drain all percpu LRU add caches

invalidate_bdev() should release all page cache pages which are clean
and not being used; however, if some pages are still in the percpu LRU
add caches on other cpus, those pages are considered in used and don't
get released. Fix it by calling lru_add_drain_all() before trying to
invalidate pages.

This problem was discovered while testing block automatic native
capacity unlocking. Null pages which were read before automatic
unlocking didn't get released by invalidate_bdev() and ended up
interfering with partition scan after unlocking.

Signed-off-by: Tejun Heo <tj(a)kernel.org>
Acked-by: David S. Miller <davem(a)davemloft.net>
Signed-off-by: Jens Axboe <jens.axboe(a)oracle.com>

The symptoms are a lockdep warning:

SysRq : Emergency Remount R/O

=============================================
[ INFO: possible recursive locking detected ]
2.6.34-cachefs #101
---------------------------------------------
events/0/9 is trying to acquire lock:
(events){+.+.+.}, at: [<ffffffff81042cf0>] flush_work+0x34/0xec

but task is already holding lock:
(events){+.+.+.}, at: [<ffffffff81042264>] worker_thread+0x19a/0x2e2

other info that might help us debug this:
3 locks held by events/0/9:
#0: (events){+.+.+.}, at: [<ffffffff81042264>] worker_thread+0x19a/0x2e2
#1: ((work)#3){+.+...}, at: [<ffffffff81042264>] worker_thread+0x19a/0x2e2
#2: (&type->s_umount_key#30){++++..}, at: [<ffffffff810b3fc8>] do_emergency_remount+0x54/0xda

stack backtrace:
Pid: 9, comm: events/0 Not tainted 2.6.34-cachefs #101
Call Trace:
[<ffffffff81054e80>] validate_chain+0x584/0xd23
[<ffffffff81052ad4>] ? trace_hardirqs_off+0xd/0xf
[<ffffffff8101c264>] ? flat_send_IPI_mask+0x74/0x86
[<ffffffff81055ea8>] __lock_acquire+0x889/0x8fa
[<ffffffff8102bcc9>] ? try_to_wake_up+0x23b/0x24d
[<ffffffff8108fe45>] ? lru_add_drain_per_cpu+0x0/0xb
[<ffffffff81055f70>] lock_acquire+0x57/0x6d
[<ffffffff81042cf0>] ? flush_work+0x34/0xec
[<ffffffff81042d1c>] flush_work+0x60/0xec
[<ffffffff81042cf0>] ? flush_work+0x34/0xec
[<ffffffff8108fbc1>] ? ____pagevec_lru_add+0x140/0x156
[<ffffffff8108fe45>] ? lru_add_drain_per_cpu+0x0/0xb
[<ffffffff8108fdc5>] ? lru_add_drain+0x3b/0x8f
[<ffffffff81042eba>] schedule_on_each_cpu+0x112/0x152
[<ffffffff8108fc5a>] lru_add_drain_all+0x10/0x12
[<ffffffff810d509e>] invalidate_bdev+0x28/0x3a
[<ffffffff810b3ef3>] do_remount_sb+0x129/0x14e
[<ffffffff810b3ff3>] do_emergency_remount+0x7f/0xda
[<ffffffff810422b9>] worker_thread+0x1ef/0x2e2
[<ffffffff81042264>] ? worker_thread+0x19a/0x2e2
[<ffffffff810b3f74>] ? do_emergency_remount+0x0/0xda
[<ffffffff81045fcd>] ? autoremove_wake_function+0x0/0x34
[<ffffffff810420ca>] ? worker_thread+0x0/0x2e2
[<ffffffff81045be7>] kthread+0x7a/0x82
[<ffffffff81002cd4>] kernel_thread_helper+0x4/0x10
[<ffffffff813e2c3c>] ? restore_args+0x0/0x30
[<ffffffff81045b6d>] ? kthread+0x0/0x82
[<ffffffff81002cd0>] ? kernel_thread_helper+0x0/0x10
Emergency Remount complete

David
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