Re: [RFC][PATCH] md: avoid fullsync if a faulty member missed a dirty transition

From: Mike Snitzer
Date: Tue May 20 2008 - 11:33:20 EST

On Mon, May 19, 2008 at 1:27 AM, Neil Brown <neilb@xxxxxxx> wrote:
> On Monday May 19, snitzer@xxxxxxxxx wrote:
> >
> > Hi Neil,
> >
> > Sorry about not getting back with you sooner. Thanks for putting
> > significant time to chasing this problem.
> >
> > I tested your most recent patch and unfortunately still hit the case
> > where the nbd member becomes degraded yet the array continues to clear
> > bits (events_cleared of the non-degraded member is higher than the
> > degraded member). Is this behavior somehow expected/correct?
> It shouldn't be..... ahhh.
> There is a delay between noting that the bit can be cleared, and
> actually writing the zero to disk. This is obviously intentional
> in case the bit gets set again quickly.
> I'm sampling the event count at the latter point instead of the
> former, and there is time for it to change.
> Maybe this patch on top of what I recently sent out?

Hi Neil,

We're much closer. The events_cleared is symmetric on both the failed
and active member of the raid1. But there have been some instances
where the md thread hits a deadlock during my testing. What follows
is the backtrace and live crash info:

md0_raid1 D 000002c4b6483a7f 0 11249 2 (L-TLB)
ffff81005747dce0 0000000000000046 0000000000000000 ffff8100454c53c0
000000000000000a ffff810048fbd0c0 000000000000000a ffff810048fbd0c0
ffff81007f853840 000000000000148e ffff810048fbd2b0 0000000362c10780
Call Trace:
[<ffffffff88ba8503>] :md_mod:bitmap_daemon_work+0x249/0x4d3
[<ffffffff802457a5>] autoremove_wake_function+0x0/0x2e
[<ffffffff88ba53b3>] :md_mod:md_check_recovery+0x20/0x4a5
[<ffffffff8044cb5c>] thread_return+0x0/0xf1
[<ffffffff88bbe0eb>] :raid1:raid1d+0x25/0xd09
[<ffffffff8023bcd7>] lock_timer_base+0x26/0x4b
[<ffffffff8023bd4d>] try_to_del_timer_sync+0x51/0x5a
[<ffffffff8023bd62>] del_timer_sync+0xc/0x16
[<ffffffff8044d38a>] schedule_timeout+0x92/0xad
[<ffffffff88ba6c6c>] :md_mod:md_thread+0xeb/0x101
[<ffffffff802457a5>] autoremove_wake_function+0x0/0x2e
[<ffffffff88ba6b81>] :md_mod:md_thread+0x0/0x101
[<ffffffff8024564d>] kthread+0x47/0x76
[<ffffffff8020aa38>] child_rip+0xa/0x12
[<ffffffff80245606>] kthread+0x0/0x76
[<ffffffff8020aa2e>] child_rip+0x0/0x12

crash> bt 11249
PID: 11249 TASK: ffff810048fbd0c0 CPU: 3 COMMAND: "md0_raid1"
#0 [ffff81005747dbf0] schedule at ffffffff8044cb5c
#1 [ffff81005747dce8] bitmap_daemon_work at ffffffff88ba8503
#2 [ffff81005747dd68] md_check_recovery at ffffffff88ba53b3
#3 [ffff81005747ddb8] raid1d at ffffffff88bbe0eb
#4 [ffff81005747ded8] md_thread at ffffffff88ba6c6c
#5 [ffff81005747df28] kthread at ffffffff8024564d
#6 [ffff81005747df48] kernel_thread at ffffffff8020aa38

0xffffffff88ba84ee <bitmap_daemon_work+0x234>: callq
0xffffffff802458ec <prepare_to_wait>
0xffffffff88ba84f3 <bitmap_daemon_work+0x239>: mov 0x18(%rbx),%rax
0xffffffff88ba84f7 <bitmap_daemon_work+0x23d>: mov 0x28(%rax),%eax
0xffffffff88ba84fa <bitmap_daemon_work+0x240>: test $0x2,%al
0xffffffff88ba84fc <bitmap_daemon_work+0x242>: je
0xffffffff88ba8505 <bitmap_daemon_work+0x24b>
0xffffffff88ba84fe <bitmap_daemon_work+0x244>: callq
0xffffffff8044c200 <__sched_text_start>
0xffffffff88ba8503 <bitmap_daemon_work+0x249>: jmp
0xffffffff88ba84d6 <bitmap_daemon_work+0x21c>
0xffffffff88ba8505 <bitmap_daemon_work+0x24b>: mov 0x18(%rbx),%rdi
0xffffffff88ba8509 <bitmap_daemon_work+0x24f>: mov %rbp,%rsi
0xffffffff88ba850c <bitmap_daemon_work+0x252>: add $0x200,%rdi
0xffffffff88ba8513 <bitmap_daemon_work+0x259>: callq
0xffffffff802457f6 <finish_wait>

So running with your latest patches seems to introduce a race in
bitmap_daemon_work's if (unlikely((*bmc & COUNTER_MAX) ==
COUNTER_MAX)) { } block.

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