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    SubjectRe: [PATCH] race condition in procfs
    2005/11/29, Andrew Morton <akpm@osdl.org>:
    > > I found a race condition in procfs on SMP systems. The result is an
    > > oops in processes like pidof. Apparently ->proc_read() gets passed a
    > > potentially NULL pointer.
    >
    > Do you know what the race is?

    Apparently it's a race between deleting a process and accessing its
    /proc/pid entries. It came out in pidof while it was accessing
    /proc/pid/stat (fs/proc/array.c:do_task_stat crashed on first
    instruction - it was an inline function accessing task->state,
    get_task_state IIRC). oops (with vserver history data - I'm using a
    patch mentioned below) is attached.

    >
    > How does one reproduce it?

    I managed to reproduce it (although not reliably) during high CPU load
    and I/O (parallel kernel compiles) on SMP systems with the vserver
    patch (http://linux-vserver.org, the exact patch is
    http://vserver.13thfloor.at/Experimental/patch-2.6.14.2-vs2.1.0-rc8.diff),
    but the vserver maintainer pointed out that it probably is a mainline
    issue. We're not using 2.6 systems too much except for the vserver
    test beds so I cannot tell if it happens on vanilla kernels.

    >
    > > The following micro-patch seems to fix it.
    >
    > It might be right, or it might be a workaround..
    >

    I'm not a kernel guru so it's just my proposal. Can it break anything?
    An alternative _might_ be somewhat coarser task_struct locking
    (do_task_stat grabs a spinlock but then it's already too late).
    However, if no "right" solution appears, I'll keep using my two-liner
    because it seems to help, at least in my setup.

    Best regards,
    Grzegorz Nosek
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