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From: Ingo Molnar on 15 Sep 2009 03:50

* Ingo Molnar <mingo(a)elte.hu> wrote:

> > trying to pull in:
> >
> > git://git.kernel.dk/linux-2.6-block.git writeback
> >
> > and see if it reproduces there? That path has been cleaned up
> > considerably there.
>
> I gave it a test-pull - and the bug does not trigger anymore.
>
> Note, that may not mean much: your tree is based on a fresh upstream
> tree so it pulled a lot of new stuff into -tip that i have yet to
> test/validate. It also changed the kernel image size/layout
> considerably and this bug seems to be a very narrow to hit race of
> sorts.

Btw., is there anything in your cleanups that could explain this bug?
Some list handling bug? A double free? Uninitialized memory? Race
between block IRQs and process context?

Ingo
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From: Ingo Molnar on 15 Sep 2009 03:50

* Jens Axboe <jens.axboe(a)oracle.com> wrote:

> On Tue, Sep 15 2009, Ingo Molnar wrote:
> >
> > * Ingo Molnar <mingo(a)elte.hu> wrote:
> >
> > > Hard to tell whether it's BDI, RCU or something else - sadly this is
> > > the only incident i've managed to log so far. (We'd be all much
> > > happier if boxes crashed left and right! ;)
> > >
> > > -tip's been carrying the RCU changes for a long(er) time which would
> > > reduce the chance of this being RCU related. [ It's still possible
> > > though: if it's a bug with a probability of hitting this box on these
> > > workloads with a chance of 1:20,000 or worse. ]
> > >
> > > Plus it triggered shortly after i updated -tip to latest -git which
> > > had the BDI bits - which would indicate the BDI stuff - or just about
> > > anything else in -git for that matter - or something older in -tip.
> > > Every day without having hit this crash once more broadens the range
> > > of plausible possibilities.
> > >
> > > In any case, i'll refrain from trying to fit a line on a single point
> > > of measurement ;-)
> >
> > Ha! I should have checked all logs of today before writing that, not
> > just that box's logs.
> >
> > Another testbox triggered the SLAB corruption yesternight:
> >
> > [ 13.598011] Freeing unused kernel memory: 2820k freed
> > [ 13.602011] Write protecting the kernel read-only data: 13528k
> > [ 13.649011] Not activating Mandatory Access Control now since /sbin/tomoyo-init doesn't exist.
> > [ 14.391012] =============================================================================
> > [ 14.391012] BUG kmalloc-96: Poison overwritten
> > [ 14.391012] -----------------------------------------------------------------------------
> > [ 14.391012]
> > [ 14.391012] INFO: 0xffff88003da4a950-0xffff88003da4a988. First byte 0x0 instead of 0x6b
> > [ 14.391012] INFO: Allocated in bdi_alloc_work+0x20/0x83 age=6 cpu=0 pid=3191
> > [ 14.391012] INFO: Freed in bdi_work_free+0x1b/0x2f age=4 cpu=0 pid=3193
> > [ 14.391012] INFO: Slab 0xffffea000190ae10 objects=24 used=13 fp=0xffff88003da4a930 flags=0x200000000000c3
> > [ 14.391012] INFO: Object 0xffff88003da4a930 @offset=2352 fp=0xffff88003da4a888
> > [ 14.391012]
> > [ 14.391012] Bytes b4 0xffff88003da4a920: 52 a4 fb ff 00 00 00 00 5a 5a 5a 5a 5a 5a 5a 5a R???....ZZZZZZZZ
> > [ 14.391012] Object 0xffff88003da4a930: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
> > [ 14.391012] Object 0xffff88003da4a940: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
> > [ 14.391012] Object 0xffff88003da4a950: 00 0c 47 3d 00 88 ff ff be 22 11 81 ff ff ff ff ..G=..???"..????
> > [ 14.391012] Object 0xffff88003da4a960: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
> > [ 14.391012] Object 0xffff88003da4a970: 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b 6b kkkkkkkkkkkkkkkk
> > [ 14.391012] Object 0xffff88003da4a980: 6b 6b 6b 6b 6b 6b 6b 6b 6a 6b 6b 6b 6b 6b 6b a5 kkkkkkkkjkkkkkk?
> > [ 14.391012] Redzone 0xffff88003da4a990: bb bb bb bb bb bb bb bb ????????
> > [ 14.391012] Padding 0xffff88003da4a9d0: 5a 5a 5a 5a 5a 5a 5a 5a ZZZZZZZZ
> > [ 14.391012] Pid: 3193, comm: mount Not tainted 2.6.31-tip-02377-g78907f0-dirty #88876
> > [ 14.391012] Call Trace:
> > [ 14.391012] [<ffffffff810e6bef>] print_trailer+0x140/0x149
> > [ 14.391012] [<ffffffff810e70db>] check_bytes_and_report+0xb7/0xf7
> > [ 14.391012] [<ffffffff810e71ec>] check_object+0xd1/0x1b4
> > [ 14.391012] [<ffffffff81112078>] ? bdi_alloc_work+0x20/0x83
> > [ 14.391012] [<ffffffff810e7bee>] alloc_debug_processing+0x7b/0xf7
> > [ 14.391012] [<ffffffff810e9842>] __slab_alloc+0x23e/0x282
> > [ 14.391012] [<ffffffff81112078>] ? bdi_alloc_work+0x20/0x83
> > [ 14.391012] [<ffffffff81112078>] ? bdi_alloc_work+0x20/0x83
> > [ 14.391012] [<ffffffff810e9a8f>] kmem_cache_alloc+0xa1/0x13f
> > [ 14.391012] [<ffffffff81112078>] bdi_alloc_work+0x20/0x83
> > [ 14.391012] [<ffffffff81112ae2>] bdi_writeback_all+0x66/0x133
> > [ 14.391012] [<ffffffff810788b8>] ? mark_held_locks+0x4d/0x6b
> > [ 14.391012] [<ffffffff81872268>] ? __mutex_unlock_slowpath+0x12d/0x163
> > [ 14.391012] [<ffffffff81078b45>] ? trace_hardirqs_on_caller+0x11c/0x140
> > [ 14.391012] [<ffffffff815c35e5>] ? usbfs_fill_super+0x0/0xa8
> > [ 14.391012] [<ffffffff81112c87>] writeback_inodes_sb+0x75/0x83
> > [ 14.391012] [<ffffffff8111652d>] __sync_filesystem+0x30/0x6b
> > [ 14.391012] [<ffffffff81116705>] sync_filesystem+0x3a/0x51
> > [ 14.391012] [<ffffffff810f4b23>] do_remount_sb+0x5b/0x11f
> > [ 14.391012] [<ffffffff810f59a3>] get_sb_single+0x92/0xad
> > [ 14.391012] [<ffffffff815c2f01>] usb_get_sb+0x1b/0x1d
> > [ 14.391012] [<ffffffff810f5786>] vfs_kern_mount+0x9e/0x122
> > [ 14.391012] [<ffffffff810f5871>] do_kern_mount+0x4c/0xec
> > [ 14.391012] [<ffffffff8110dd15>] do_mount+0x1e9/0x236
> > [ 14.391012] [<ffffffff8110dde6>] sys_mount+0x84/0xc6
> > [ 14.391012] [<ffffffff8187333a>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> > [ 14.391012] [<ffffffff8100b642>] system_call_fastpath+0x16/0x1b
> > [ 14.391012] FIX kmalloc-96: Restoring 0xffff88003da4a950-0xffff88003da4a988=0x6b
> > [ 14.391012]
> > [ 14.391012] FIX kmalloc-96: Marking all objects used
> > [ 20.597016] usb usb2: uevent
> > [ 20.606016] usb 2-0:1.0: uevent
> > [ 20.615016] usb usb3: uevent
> > [ 20.622016] usb 3-0:1.0: uevent
> > [ 20.631016] usb usb4: uevent
> >
> > Different hardware, different config, but still in bdi_alloc_work().
> >
> > - Which excludes cosmic rays and freak hardware from the list of
> > possibilities.
> >
> > - I'd also say RCU is out too as this incident was 500 iterations after
> > the BDI merge - preceded by a streak of 3000+ successful iterations on
> > that same box with all of -tip (including the RCU changes).
> >
> > - Random memory corruption is probably out as well - the chance of
> > hitting a BDI data structure twice accidentally is low.
> >
> > - It's also two completely different versions of distros - the
> > user-space of the two testboxes affected is 2 years apart or so.
> >
> > - It's a single CPU box - SMP races are out as well.
> >
> > This points towards this being a BDI bug with about ~80% confidence
> > statistically - or [with a lower probability] a SLAB bug. (both failing
> > configs had CONFIG_SLUB=y. But SLUB is the best in detecting corrupted
> > data structures so that alone does not tell much.)
>
> Hmmm, at least this reproduces more consistently. Can I talk you into
> trying to pull in:
>
> git://git.kernel.dk/linux-2.6-block.git writeback
>
> and see if it reproduces there? That path has been cleaned up
> considerably there.

I gave it a test-pull - and the bug does not trigger anymore.

Note, that may not mean much: your tree is based on a fresh upstream
tree so it pulled a lot of new stuff into -tip that i have yet to
test/validate. It also changed the kernel image size/layout considerably
and this bug seems to be a very narrow to hit race of sorts.

Ingo
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From: Jens Axboe on 15 Sep 2009 04:00
On Tue, Sep 15 2009, Ingo Molnar wrote:
>
> * Ingo Molnar <mingo(a)elte.hu> wrote:
>
> > > trying to pull in:
> > >
> > > git://git.kernel.dk/linux-2.6-block.git writeback
> > >
> > > and see if it reproduces there? That path has been cleaned up
> > > considerably there.
> >
> > I gave it a test-pull - and the bug does not trigger anymore.
> >
> > Note, that may not mean much: your tree is based on a fresh upstream
> > tree so it pulled a lot of new stuff into -tip that i have yet to
> > test/validate. It also changed the kernel image size/layout
> > considerably and this bug seems to be a very narrow to hit race of
> > sorts.
>
> Btw., is there anything in your cleanups that could explain this bug?
> Some list handling bug? A double free? Uninitialized memory? Race
> between block IRQs and process context?

No that that I noticed, it was just a thought since that path has been
cleaned up and I have been looking at the newer tree only. I'll take a
good look at the current -git situation.

--
Jens Axboe

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