Re: Infiniate systemd loop when power off the machine with multiple MD RAIDs

From: Hannes Reinecke
Date: Wed Aug 23 2023 - 02:04:23 EST


On 8/23/23 03:28, Yu Kuai wrote:
Hi,

在 2023/08/23 3:13, Carlos Carvalho 写道:
Song Liu (song@xxxxxxxxxx) wrote on Tue, Aug 22, 2023 at 03:56:04PM -03:
>From systemd code, i.e. function delete_md(), this error:

[ 205.957004] systemd-shutdown[1]: Stopping MD /dev/md124p1 (259:6).
[ 205.964177] systemd-shutdown[1]: Could not stop MD /dev/md124p1:
Device or resource busy

is most likely triggered by ioctl(STOP_ARRAY).

And based on the code, I think the ioctl fails here:

         if (cmd == STOP_ARRAY || cmd == STOP_ARRAY_RO) {
                 /* Need to flush page cache, and ensure no-one else opens
                  * and writes
                  */
                 mutex_lock(&mddev->open_mutex);
                 if (mddev->pers && atomic_read(&mddev->openers) > 1) {
                         mutex_unlock(&mddev->open_mutex);
                         err = -EBUSY;
                         goto out;        ////////////////////// HERE

Yes, I suspect here as well, but I do some test with error injection to
gurantee that ioctl(STOP_ARRAY) always return -EBUSY, but I found that
system reboot didn't hang, it'll try a few times but eventually reboot
finished.

                 }
                 if (test_and_set_bit(MD_CLOSING, &mddev->flags)) {
                         mutex_unlock(&mddev->open_mutex);
                         err = -EBUSY;
                         goto out;
                 }
                 did_set_md_closing = true;
                 mutex_unlock(&mddev->open_mutex);
                 sync_blockdev(bdev);
         }

Probably. The problem is why doesn't it manage to flush the page cache? I find
strange that the problem appears only when trying to stop the array, I get it
when trying to umount the filesystem, where it also hangs because of the same
reason. The kworker thread runs continuously using 100% cpu of only 1 core.

The kworker do you mean the daemon thread or the sync thread?  runs
continuously using 100% cpu doen't sounds correct to me.

That sounds more like kworker waiting on a mutex to be released.
Can you trigger a kdump while in that state?
Or do a sysrq-T to get a stack trace of all running processes?
That should tell you where exactly the 100% cpu time is being
wasted.

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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