Re: [PATCH] race condition in procfs

From: Grzegorz Nosek (grzegorz.nosek_at_gmail.com)
Date: 11/29/05

  • Next message: Vivek Goyal: "Re: [Fastboot] Re: [PATCH & RFC] kdump and stack overflows" 
    Date:	Tue, 29 Nov 2005 14:25:12 +0100
To: Andrew Morton <akpm@osdl.org>

    > >
    > > 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.
    >

    Oh well, I got another oops in the very same place with the patch
    applied. So now I surrounded the check with
    read_[un]lock(&tasklist_lock) and added a check to do_task_stat (both
    now have a printk). If it builds, boots and doesn't crash, I'll post
    the patch.

    Best regards,
     Grzegorz Nosek
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