Date:	Thu, 22 Mar 2007 17:33:27 -0700
From:	"Dan Williams" <dan.j.williams@...il.com>
To:	"Neil Brown" <neilb@...e.de>
Cc:	"Jens Axboe" <jens.axboe@...cle.com>, linux@...izon.com,
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
	linux-kernel@...e.us, cebbert@...hat.com
Subject: Re: 2.6.20.3 AMD64 oops in CFQ code

On 3/22/07, Dan Williams <dan.j.williams@...el.com> wrote:
> On 3/22/07, Neil Brown <neilb@...e.de> wrote:
> > On Thursday March 22, jens.axboe@...cle.com wrote:
> > > On Thu, Mar 22 2007, linux@...izon.com wrote:
> > > > > 3 (I think) seperate instances of this, each involving raid5. Is your
> > > > > array degraded or fully operational?
> > > >
> > > > Ding! A drive fell out the other day, which is why the problems only
> > > > appeared recently.
> > > >
> > > > md5 : active raid5 sdf4[5] sdd4[3] sdc4[2] sdb4[1] sda4[0]
> > > >       1719155200 blocks level 5, 64k chunk, algorithm 2 [6/5] [UUUU_U]
> > > >       bitmap: 149/164 pages [596KB], 1024KB chunk
> > > >
> > > > H'm... this means that my alarm scripts aren't working.  Well, that's
> > > > good to know.  The drive is being re-integrated now.
> > >
> > > Heh, at least something good came out of this bug then :-)
> > > But that's reaffirming. Neil, are you following this? It smells somewhat
> > > fishy wrt raid5.
> >
> > Yes, I've been trying to pay attention....
> >
> > The evidence does seem to point to raid5 and degraded arrays being
> > implicated.  However I'm having trouble finding how the fact that an array
> > is degraded would be visible down in the elevator except for having a
> > slightly different distribution of reads and writes.
> >
> > One possible way is that if an array is degraded, then some read
> > requests will go through the stripe cache rather than direct to the
> > device.  However I would more expect the direct-to-device path to have
> > problems as it is much newer code.  Going through the cache for reads
> > is very well tested code - and reads come from the cache for most
> > writes anyway, so the elevator will still see lots of single-page.
> > reads.  It only ever sees single-page write.
> >
> > There might be more pressure on the stripe cache when running
> > degraded, so we might call the ->unplug_fn a little more often, but I
> > doubt that would be noticeable.
> >
> > As you seem to suggest by the patch, it does look like some sort of
> > unlocked access to the cfq_queue structure.  However apart from the
> > comment before cfq_exit_single_io_context being in the wrong place
> > (should be before __cfq_exit_single_io_context) I cannot see anything
> > obviously wrong with the locking around that structure.
> >
> > So I'm afraid I'm stumped too.
> >
> > NeilBrown
>
> Not a cfq failure, but I have been able to reproduce a different oops
> at array stop time while i/o's were pending.  I have not dug into it
> enough to suggest a patch, but I wonder if it is somehow related to
> the cfq failure since it involves congestion and drives going away:
>
> md: md0: recovery done.
> Unable to handle kernel NULL pointer dereference at virtual address 000000bc
> pgd = 40004000
> [000000bc] *pgd=00000000
> Internal error: Oops: 17 [#1]
> Modules linked in:
> CPU: 0
> PC is at raid5_congested+0x14/0x5c
> LR is at sync_sb_inodes+0x278/0x2ec
> pc : [<402801cc>]    lr : [<400a39e8>]    Not tainted
> sp : 8a3e3ec4  ip : 8a3e3ed4  fp : 8a3e3ed0
> r10: 40474878  r9 : 40474870  r8 : 40439710
> r7 : 8a3e3f30  r6 : bfa76b78  r5 : 4161dc08  r4 : 40474800
> r3 : 402801b8  r2 : 00000004  r1 : 00000001  r0 : 00000000
> Flags: nzCv  IRQs on  FIQs on  Mode SVC_32  Segment kernel
> Control: 400397F
> Table: 7B7D4018  DAC: 00000035
> Process pdflush (pid: 1371, stack limit = 0x8a3e2250)
> Stack: (0x8a3e3ec4 to 0x8a3e4000)
> 3ec0:          8a3e3f04 8a3e3ed4 400a39e8 402801c4 8a3e3f24 000129f9 40474800
> 3ee0: 4047483c 40439a44 8a3e3f30 40439710 40438a48 4045ae68 8a3e3f24 8a3e3f08
> 3f00: 400a3ca0 400a377c 8a3e3f30 00001162 00012bed 40438a48 8a3e3f78 8a3e3f28
> 3f20: 40069b58 400a3bfc 00011e41 8a3e3f38 00000000 00000000 8a3e3f28 00000400
> 3f40: 00000000 00000000 00000000 00000000 00000000 00000025 8a3e3f80 8a3e3f8c
> 3f60: 40439750 8a3e2000 40438a48 8a3e3fc0 8a3e3f7c 4006ab68 40069a8c 00000001
> 3f80: bfae2ac0 40069a80 00000000 8a3e3f8c 8a3e3f8c 00012805 00000000 8a3e2000
> 3fa0: 9e7e1f1c 4006aa40 00000001 00000000 fffffffc 8a3e3ff4 8a3e3fc4 4005461c
> 3fc0: 4006aa4c 00000001 ffffffff ffffffff 00000000 00000000 00000000 00000000
> 3fe0: 00000000 00000000 00000000 8a3e3ff8 40042320 40054520 00000000 00000000
> Backtrace:
> [<402801b8>] (raid5_congested+0x0/0x5c) from [<400a39e8>]
> (sync_sb_inodes+0x278/0x2ec)
> [<400a3770>] (sync_sb_inodes+0x0/0x2ec) from [<400a3ca0>]
> (writeback_inodes+0xb0/0xb8)
> [<400a3bf0>] (writeback_inodes+0x0/0xb8) from [<40069b58>]
> (wb_kupdate+0xd8/0x160)
>  r7 = 40438A48  r6 = 00012BED  r5 = 00001162  r4 = 8A3E3F30
> [<40069a80>] (wb_kupdate+0x0/0x160) from [<4006ab68>] (pdflush+0x128/0x204)
>  r8 = 40438A48  r7 = 8A3E2000  r6 = 40439750  r5 = 8A3E3F8C
>  r4 = 8A3E3F80
> [<4006aa40>] (pdflush+0x0/0x204) from [<4005461c>] (kthread+0x108/0x134)
> [<40054514>] (kthread+0x0/0x134) from [<40042320>] (do_exit+0x0/0x844)
> Code: e92dd800 e24cb004 e5900000 e3a01001 (e59030bc)
> md: md0 stopped.
> md: unbind<sda>
> md: export_rdev(sda)
> md: unbind<sdd>
> md: export_rdev(sdd)
> md: unbind<sdc>
> md: export_rdev(sdc)
> md: unbind<sdb>
> md: export_rdev(sdb)
>
> 2.6.20-rc3-iop1 on an iop348 platform.  SATA controller is sata_vsc.
Sorry, that's 2.6.21-rc3-iop1.
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