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SubjectRe: cgroup: rmdir() does not complete
On Thu, 26 Aug 2010 16:51:55 +0100 (BST)
Mark Hills <> wrote:

> I am experiencing hung tasks when trying to rmdir() on a cgroup. One task
> spins, others queue up behind it with the following:
> INFO: task soaked-cgroup:27257 blocked for more than 120 seconds.
> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> soaked-cgrou D ffff8800058157c0 0 27257 29411 0x00000000
> ffff88004ffffdd8 0000000000000086 ffff88004ffffda8 ffff88004ffffeb8
> 0000000000000010 ffff880119813780 ffff88004ffffd48 ffff88004fffffd8
> ffff88004fffffd8 000000000000f9b0 00000000000157c0 ffff880137693268
> Call Trace:
> [<ffffffff81115edb>] ? mntput_no_expire+0x24/0xe7
> [<ffffffff81427acd>] __mutex_lock_common+0x14d/0x1b4
> [<ffffffff81108a7c>] ? path_put+0x1d/0x22
> [<ffffffff81427b48>] __mutex_lock_slowpath+0x14/0x16
> [<ffffffff81427c4f>] mutex_lock+0x31/0x4b
> [<ffffffff8110bdf8>] do_rmdir+0x74/0x102
> [<ffffffff8110bebd>] sys_rmdir+0x11/0x13
> [<ffffffff81009b02>] system_call_fastpath+0x16/0x1b
> Kernel is from Fedora, In all cases the cgroup contains no
> tasks.
> Commit ec64f5 ("fix frequent -EBUSY at rmdir") adds a busy wait loop to
> the rmdir. It looks like what I am seeing here and indicates that some
> cgroup subsystem is busy, indefinitely.

Hmm. really spin ? sleeping-forever-no-wake-up ?

> I have not worked out how to reproduce it quickly. My only way is to
> complete a 'dd' command in the cgroup, but then the problem is so rare it
> is slow progress.
please show how-to-reproduce in your way.
And what cgroup is mounted ? memory cgroup only ?

> Documentation/cgroup.memory.txt describes how force_empty can be required
> in some cases.

Ah, maybe that's wrong text. rmdir() calls force-empty automatically.

> Does this mean that with the patch above, these cases will
> now spin on rmdir(), instead of returning -EBUSY? How can produce a
> reliable test case requiring memory.force_empty to be used, to test this?

Hmm. I'm not sure fedora-kernel has other (its own) featrues than stock kernel.
I'm grad if you can check it can happen in stock kernel, 2.6.35.

> Or is it likely to be some other cause, and how best to find it?

At the first look, above mutex is the mutex in do_rmdir(), not kernel/cgroup.c
Then, rmdir doesn't seem to reach cgroup code...
Do you do another operation on the directory while rmdir is called ?


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