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    SubjectRE: Could "bio_vec" be referenced any time?
    Date
    Hi NeilBrown,
    Thank you for your help and introducing "ksymoops" to me.
    I think you are right.
    The BIO is passed by MD put BIO into a share "kfifo".
    THX : )
    -----Original Message-----
    From: linux-raid-owner@vger.kernel.org
    [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Neil Brown
    Sent: Wednesday, February 07, 2007 3:46 AM
    To: Yu-Chen Wu
    Cc: linux-kernel@vger.kernel.org; linux-raid@vger.kernel.org
    Subject: Re: Could "bio_vec" be referenced any time?

    On Tuesday February 6, g944370@oz.nthu.edu.tw wrote:
    > Hi all,
    > I write a module that creates a kernel thread to show the BIOs from
    > MD modules.
    > The kernel thread will call show_bio() when md passing a BIO to my
    > module,else sleep.
    > Sometimes, show_bio() continues working successfully ,but it
    > somtimes makes "general protection fault".
    > The show_bio() always works when I comment the
    > "bio_for_each_segment" loop.
    > Is the zone I comment the cause of the fault?
    > As above, I consider it's the main problem.Also, I strongly want to
    > know your opinions.Thank you for help.
    >
    > THX

    Without seeing how the bio gets to show_bio it is hard to be certain,
    but my guess would be that by the time show_bio tries to inspect the
    bio, the IO request involving it has already completed and the bio has
    been freed, so you are accessing freed memory.

    > Feb 6 22:00:28 RAID-SUSE kernel: Code: 8b 00 f6 c4 08 74 0e 48 c7 c7 14
    9c
    > 45 88 31 c0 e8 b5 bf e2

    If you feed this line into ksymoops you get:

    Code; 0000000000000000 Before first symbol
    0: 8b 00 mov (%rax),%eaxC
    ...

    so it is trying to dereference $rax.

    > Feb 6 22:00:28 RAID-SUSE kernel: RAX: 6b6b6b6b6b6b6b6b RBX:
    > ffff810037f52668 RCX: 0000000000040000

    Which contains 6b6b6b6b6b6b6b6b.
    which is lots of copies of 'POISON_FREE' (defined in
    include/linux/poison.h) which makes it really look like that memory
    has already been freed.

    NeilBrown
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