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Date: Tue, 23 Nov 2010 16:51:11 +0100
From: Spelic <spelic@shiftmail.org>
Subject: Bug: sync's hangup forever in call_rwsem_down_read_failed
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Hi there
on 2.6.37-rc2 I was doing some speed tests. I had just unmounted various 
arrays, recreated them differently with mdadm assume-clean, and then I 
was formatting all of them:

# for i in /dev/md10* ; do echo $i ; time mkfs.ext4 -m0 $i ; time sync ; 
done
         /dev/md101
         mke2fs 1.41.9 (22-Aug-2009)
         /dev/md101 is apparently in use by the system; will not make a 
filesystem here!

         real    0m0.038s
         user    0m0.000s
         sys     0m0.000s

Here I was pretty sure the error message was wrong: md101 was not in use 
by the system anyhow, and was not mounted. I had just created it.
Anyway the subsequent sync hanged!

# cat /proc/7686/stack
         [<ffffffff8129e894>] call_rwsem_down_read_failed+0x14/0x30
         [<ffffffff81137ff1>] iterate_supers+0x71/0xf0
         [<ffffffff8115d92b>] sync_filesystems+0x1b/0x20
         [<ffffffff8115da32>] sys_sync+0x12/0x40
         [<ffffffff8100af42>] system_call_fastpath+0x16/0x1b
         [<ffffffffffffffff>] 0xffffffffffffffff

After a while I tried to launch another sync, from another shell; that 
hanged identically.

/proc# grep call_rwsem */stack
         7686/stack:[<ffffffff8129e894>] 
call_rwsem_down_read_failed+0x14/0x30
         7714/stack:[<ffffffff8129e894>] 
call_rwsem_down_read_failed+0x14/0x30
(this shows only the two syncs)

Then I tried to shut it down with poweroff -f (which does sync) and that 
hanged again

/proc# grep rwsem */stack
         7686/stack:[<ffffffff8129e894>] 
call_rwsem_down_read_failed+0x14/0x30
         7714/stack:[<ffffffff8129e894>] 
call_rwsem_down_read_failed+0x14/0x30
         7815/stack:[<ffffffff8129e894>] 
call_rwsem_down_read_failed+0x14/0x30

so I shut it down with poweroff -n -f ; that worked.

It seems to me some process did not reset the semaphore after having 
finished (= resource leak)

Sorry for not being able to help you much more than this as I have reset 
the computer now, but this is another guy reporting apparently the same 
problem on Ubuntu 10.10 kernel (I was on vanilla 2.6.37-rc2)
http://www.mail-archive.com/ubuntu-bugs@lists.ubuntu.com/msg2579131.html

My dmesg never notified about the problem, in something like 5minutes, btw.

If you reply, please reply also to me as I am not subsribed to 
linuk-kernel .
Thanks for your work
--
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