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SubjectRe: [PATCH][noop-iosched] don't reuse a freed request
Hi, guys.

Jens Axboe wrote:
> On Mon, Oct 31 2005, Arnaldo Carvalho de Melo wrote:
>
>>Hi,
>>
>> I'm getting the oops below when trying to use qemu with a kernel
>>built with just the noop iosched, I'm never had looked at this code before,
>>so I did a quick hack that seems enough for my case.
>>
>> Ah, this is with a fairly recent git tree (today), haven't checked
>>if it is present in 2.6.14.
>>
>>Best Regards,
>>
>>- Arnaldo
>>
>>Unable to handle kernel paging request at virtual address c5f20f60
>> printing eip:
>>c01b0ecd
>>*pde = 00017067
>>*pte = 05f20000
>>Oops: 0000 [#1]
>>DEBUG_PAGEALLOC
>>Modules linked in:
>>CPU: 0
>>EIP: 0060:[<c01b0ecd>] Not tainted VLI
>>EFLAGS: 00000046 (2.6.14acme)
>>EIP is at elv_rq_merge_ok+0x15/0x7b
>>eax: 00000014 ebx: c5f20f58 ecx: 000003f8 edx: 00000046
>>esi: c12a5a90 edi: c5f20f58 ebp: c11658d0 esp: c11658c4
>>ds: 007b es: 007b ss: 0068
>>Process swapper (pid: 1, threadinfo=c1165000 task=c1164af0)
>>Stack: c0251883 c5ecfe4c c5d688c0 c1165904 c01b0f48 c5f20f58 c12a5a90 00000000
>> c5874000 c018c5e1 c5f15f24 0000002b 00000000 c5ecfe4c c5d688c0 c12a5a90
>> c1165920 c01b128d c5f20f58 c12a5a90 000a568a 00000000 00000002 c1165960
>>Call Trace:
>> [<c0102a63>] show_stack+0x78/0x83
>> [<c0102b88>] show_registers+0x100/0x167
>> [<c0102d35>] die+0xcb/0x140
>> [<c0234308>] do_page_fault+0x393/0x53a
>> [<c0102777>] error_code+0x4f/0x54
>> [<c01b0f48>] elv_try_merge+0x15/0x84
>> [<c01b128d>] elv_merge+0x1d/0x4f
>> [<c01b41d9>] __make_request+0xb2/0x425
>> [<c01b46f9>] generic_make_request+0x125/0x137
>
>
> Hrmpf, this looks really bad. Tejun, clearly there are still paths where
> ->last_rq isn't being cleared.
>

I'm currently debugging this. The problem is that we are using generic
dispatch queue directly in the noop and merging is NOT allowed on
dispatch queues but generic handling of last_merge tries to merge
requests. I'm still trying to verify this, so I'll be back with results
soon.

>
>>--- a/drivers/block/ll_rw_blk.c
>>+++ b/drivers/block/ll_rw_blk.c
>>@@ -1787,6 +1787,9 @@ static inline void blk_free_request(requ
>> if (rq->flags & REQ_ELVPRIV)
>> elv_put_request(q, rq);
>> mempool_free(rq, q->rq.rq_pool);
>>+
>>+ if (rq == q->last_merge)
>>+ q->last_merge = NULL;
>> }
>>
>> static inline struct request *
>
>
> It's most likely a bug getting this far in the first place, but does it
> fix things for you? I'll get on this asap.
>

If the bug is where I think it is, I think the proper thing to do is to
use separate list_head in noop instead of using generic dispatch queue
directly thus making noop consistent with other ioscheds.

I'm more worried about oops w/ cfq Arnaldo reported in this thread.
I'll track that down as soon as I'm done with this one.

Many bugs. Sorry. :-)

--
tejun
-
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