Re: raid1 with nbd member hangs MD on SLES10 and RHEL5

From: Mike Snitzer
Date: Wed Jun 13 2007 - 14:23:27 EST

On 6/13/07, Mike Snitzer <snitzer@xxxxxxxxx> wrote:
On 6/12/07, Neil Brown <neilb@xxxxxxx> wrote:
> > > On 6/12/07, Neil Brown <neilb@xxxxxxx> wrote:
> > > > On Tuesday June 12, snitzer@xxxxxxxxx wrote:
> > > > >
> > > > > I can provided more detailed information; please just ask.
> > > > >
> > > >
> > > > A complete sysrq trace (all processes) might help.

Bringing this back to a wider audience. I provided the full sysrq
trace of the RHEL5 kernel to Neil; in it we saw that md0_raid1 had the
following trace:

md0_raid1 D ffff810026183ce0 5368 31663 11 3822 29488 (L-TLB)
ffff810026183ce0 ffff810031e9b5f8 0000000000000008 000000000000000a
ffff810037eef040 ffff810037e17100 00043e64d2983c1f 0000000000004c7f
ffff810037eef210 0000000100000001 000000081c506640 00000000ffffffff
Call Trace:
[<ffffffff8003e371>] keventd_create_kthread+0x0/0x61
[<ffffffff801b9364>] md_super_wait+0xa8/0xbc
[<ffffffff8003e711>] autoremove_wake_function+0x0/0x2e
[<ffffffff801b9adb>] md_update_sb+0x1dd/0x23a
[<ffffffff801bed2a>] md_check_recovery+0x15f/0x449
[<ffffffff882a1af3>] :raid1:raid1d+0x27/0xc1e
[<ffffffff80233209>] thread_return+0x0/0xde
[<ffffffff8023279c>] __sched_text_start+0xc/0xa79
[<ffffffff8003e371>] keventd_create_kthread+0x0/0x61
[<ffffffff80233a9f>] schedule_timeout+0x1e/0xad
[<ffffffff8003e371>] keventd_create_kthread+0x0/0x61
[<ffffffff801bd06c>] md_thread+0xf8/0x10e
[<ffffffff8003e711>] autoremove_wake_function+0x0/0x2e
[<ffffffff801bcf74>] md_thread+0x0/0x10e
[<ffffffff8003e5e7>] kthread+0xd4/0x109
[<ffffffff8000a505>] child_rip+0xa/0x11
[<ffffffff8003e371>] keventd_create_kthread+0x0/0x61
[<ffffffff8003e513>] kthread+0x0/0x109
[<ffffffff8000a4fb>] child_rip+0x0/0x11

To which Neil had the following to say:

> md0_raid1 is holding the lock on the array and trying to write out the
> superblocks for some reason, and the write isn't completing.
> As it is holding the locks, mdadm and /proc/mdstat are hanging.
> You seem to have nbd-servers running on this machine. Are they
> serving the device that md is using. (i.e. a loop-back situation). I
> would expect memory deadlocks would be very easy to hit in that
> situation, but I don't know if that is what has happened.
> Nothing else stands out.
> Could you clarify the arrangement of nbd. Where are the servers and
> what are they serving?

We're using MD+NBD for disaster recovery (one local scsi device, one
remote via nbd). The nbd-server is not contributing to md0. The
nbd-server is connected to a remote machine that is running a raid1

To take this further I've now collected a full sysrq trace of this
hang on a SLES10 SP1 RC5 kernel, the relevant
md0_raid1 trace is comparable to the RHEL5 trace from above:

md0_raid1 D ffff810001089780 0 8583 51 8952 8260 (L-TLB)
ffff810812393ca8 0000000000000046 ffff8107b7fbac00 000000000000000a
ffff81081f3c6a18 ffff81081f3c67d0 ffff8104ffe8f100 000044819ddcd5e2
000000000000eb8b 00000007028009c7
Call Trace: <ffffffff801e1f94>{generic_make_request+501}
<ffffffff8026f056>{write_page+128} <ffffffff80269ac7>{md_update_sb+220}
<ffffffff80145965>{kthread+236} <ffffffff8010bdce>{child_rip+8}

Taking a step back, here is what was done to reproduce on SLES10:
1) establish a raid1 mirror (md0) using one local member (sdc1) and
one remote member (nbd0)
2) power off the remote machine, whereby severing nbd0's connection
3) perform IO to the filesystem that is on the md0 device to enduce
the MD layer to mark the nbd device as "faulty"
4) cat /proc/mdstat hangs, sysrq trace was collected and showed the
above md0_raid1 trace.

To be clear, the MD superblock update hangs indefinitely on RHEL5.
But with SLES10 it eventually succeeds (and MD marks the nbd0 member
faulty); and the other tasks that were blocking waiting for the MD
lock (e.g. 'cat /proc/mdstat') then complete immediately.

It should be noted that this MD+NBD configuration has worked
flawlessly using a stock kernel (ontop of a
RHEL4U4 distro). Steps have not been taken to try to reproduce with on SLES10; it may be useful to pursue but I'll defer to
others to suggest I do so. does not have the SMP race fixes that were made in 2.6.16;
yet both SLES10 and RHEL5 kernels do:;a=commitdiff;h=4b2f0260c74324abca76ccaa42d426af163125e7

If not this specific NBD change, something appears to have changed
with how NBD behaves in the face of it's connection to the server
being lost. Almost like the MD superblock update that would be
written to nbd0 is blocking within nbd or the network layer because of
a network timeout issue?

I will try to get a better understanding of what is _really_ happening
with systemtap; but others' suggestions/insight is very welcome.

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