Re: v5.10.1 xfs deadlock

[Donald Buczek]

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