linux-kernel - Re: v5.10.1 xfs deadlock

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Date:   Sun, 27 Dec 2020 18:34:24 +0100
From:   Donald Buczek <buczek@...gen.mpg.de>
To:     Brian Foster <bfoster@...hat.com>
Cc:     linux-xfs@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        it+linux-xfs@...gen.mpg.de
Subject: Re: v5.10.1 xfs deadlock

On 18.12.20 19:35, Donald Buczek wrote:
> On 18.12.20 16:35, Brian Foster wrote:
>> On Thu, Dec 17, 2020 at 10:30:37PM +0100, Donald Buczek wrote:
>>> On 17.12.20 20:43, Brian Foster wrote:
>>>> On Thu, Dec 17, 2020 at 06:44:51PM +0100, Donald Buczek wrote:
>>>>> Dear xfs developer,
>>>>>
>>>>> I was doing some testing on a Linux 5.10.1 system with two 100 TB xfs filesystems on md raid6 raids.
>>>>>
>>>>> The stress test was essentially `cp -a`ing a Linux source repository with two threads in parallel on each filesystem.
>>>>>
>>>>> After about on hour, the processes to one filesystem (md1) blocked, 30 minutes later the process to the other filesystem (md0) did.
>>>>>
>>>>>       root      7322  2167  0 Dec16 pts/1    00:00:06 cp -a /jbod/M8068/scratch/linux /jbod/M8068/scratch/1/linux.018.TMP
>>>>>       root      7329  2169  0 Dec16 pts/1    00:00:05 cp -a /jbod/M8068/scratch/linux /jbod/M8068/scratch/2/linux.019.TMP
>>>>>       root     13856  2170  0 Dec16 pts/1    00:00:08 cp -a /jbod/M8067/scratch/linux /jbod/M8067/scratch/2/linux.028.TMP
>>>>>       root     13899  2168  0 Dec16 pts/1    00:00:05 cp -a /jbod/M8067/scratch/linux /jbod/M8067/scratch/1/linux.027.TMP
>>>>>
>>
>> Do you have any indication of whether these workloads actually hung or
>> just became incredibly slow?
> 
> There is zero progress. iostat doesn't show any I/O on any of the block devices (md or member)
> 
>>>>> Some info from the system (all stack traces, slabinfo) is available here: https://owww.molgen.mpg.de/~buczek/2020-12-16.info.txt
>>>>>
>>>>> It stands out, that there are many (549 for md0, but only 10 for md1)  "xfs-conv" threads all with stacks like this
>>>>>
>>>>>       [<0>] xfs_log_commit_cil+0x6cc/0x7c0
>>>>>       [<0>] __xfs_trans_commit+0xab/0x320
>>>>>       [<0>] xfs_iomap_write_unwritten+0xcb/0x2e0
>>>>>       [<0>] xfs_end_ioend+0xc6/0x110
>>>>>       [<0>] xfs_end_io+0xad/0xe0
>>>>>       [<0>] process_one_work+0x1dd/0x3e0
>>>>>       [<0>] worker_thread+0x2d/0x3b0
>>>>>       [<0>] kthread+0x118/0x130
>>>>>       [<0>] ret_from_fork+0x22/0x30
>>>>>
>>>>> xfs_log_commit_cil+0x6cc is
>>>>>
>>>>>     xfs_log_commit_cil()
>>>>>       xlog_cil_push_background(log)
>>>>>         xlog_wait(&cil->xc_push_wait, &cil->xc_push_lock);
>>>>>
>>
>> This looks like the transaction commit throttling code. That was
>> introduced earlier this year in v5.7 via commit 0e7ab7efe7745 ("xfs:
>> Throttle commits on delayed background CIL push"). The purpose of that
>> change was to prevent the CIL from growing too large. FWIW, I don't
>> recall that being a functional problem so it should be possible to
>> simply remove that blocking point and see if that avoids the problem or
>> if we simply stall out somewhere else, if you wanted to give that a
>> test.
> 
> Will do. Before trying with this commit reverted, I will repeat the test without any change to see if the problem is reproducible at all.

I'm now able to reliably reproduce the deadlock with a little less complex setup (e.g. with only one filesystem involved). One key to that was to run the test against a freshly created filesystem (mkfs).

And, yes, you are right: When I revert ef565ab8cc2e ("xfs: Throttle commits on delayed background CIL push") and 7ee6dfa2a245 ("xfs: fix use-after-free on CIL context on shutdown") the deadlock seems to be gone.

Best
   Donald