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    SubjectRe: Bug: fio traps into kernel without exiting because futex has a deadloop
    Peter Zijlstra wrote:
    > On Thu, 2009-06-11 at 07:55 +0200, Peter Zijlstra wrote:
    >> On Thu, 2009-06-11 at 11:08 +0800, Zhang, Yanmin wrote:
    >>> I investigate a fio hang issue. When I run fio multi-process
    >>> testing on many disks, fio traps into kernel and doesn't exit
    >>> (mostly hit once after runing sub test cases for hundreds of times).
    >>> Oprofile data shows kernel consumes time with some futex functions.
    >>> Command kill couldn't kill the process and machine reboot also hangs.
    >>> Eventually, I locate the root cause as a bug of futex. Kernel enters
    >>> a deadloop between 'retry' and 'goto retry' in function futex_wake_op.
    >>> By unknown reason (might be an issue of fio or glibc), parameter uaddr2
    >>> points to an area which is READONLY. So futex_atomic_op_inuser returns
    >>> -EFAULT when trying to changing the data at uaddr2, but later get_user
    >>> still succeeds becasue the area is READONLY. Then go back to retry.
    >>> I create a simple test case to trigger it, which just shmat an READONLY
    >>> area for address uaddr2.
    >>> It could be used as a DOS attack.
    > /me has morning juice and notices he sent the wrong commit...
    > commit 64d1304a64477629cb16b75491a77bafe6f86963
    > Author: Thomas Gleixner <>
    > Date: Mon May 18 21:20:10 2009 +0200
    > futex: setup writeable mapping for futex ops which modify user space data

    Yup, that's the one. I was trying to locate it myself, but you beat me
    to it. Thanks Peter.

    Darren Hart
    IBM Linux Technology Center
    Real-Time Linux Team
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