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    SubjectRe: Serious problem with ticket spinlocks on ia64
    Date
    On Friday 27 of August 2010 16:31:35 Hedi Berriche wrote:
    > On Fri, Aug 27, 2010 at 15:09 Petr Tesarik wrote:
    > | On Friday 27 of August 2010 15:48:02 Hedi Berriche wrote:
    > | > One more fact, the problem was introduced by commit
    > | >
    > | > commit 9d40ee200a527ce08ab8c793ba8ae3e242edbb0e
    > | > Author: Tony Luck <tony.luck@intel.com>
    > | > Date: Wed Oct 7 10:54:19 2009 -0700
    > | >
    > | > [IA64] Squeeze ticket locks back into 4 bytes.
    > | >
    > | > Reverting the patch makes the problem go away.
    > | >
    > | > IOW, and as far as testing shows, the first incarnation of the ticket
    > | > locks implementation on IA64 (commit 2c8696), the one that used 8
    > | > bytes, does not exhibit this problem.
    > |
    > | I wouldn't be so sure about it. Given that I have only observed the
    > | problem when the spinlock value wraps around, then an 8-byte spinlock
    > | might only need much more time to trigger the bug.
    >
    > That's a possibility and that's why I said "as far as testing shows".
    >
    > That said, I'm letting my already over 36 hours run carry on chewing CPU
    > time, and see if it will eventually trip the same problem seen with 4-byte
    > ticket locks.

    Hm, this doesn't sound like a viable approach. Since the siglock gets
    initialized to 0 when a new process is started, it may never actually wrap
    around.

    I would rather attach a SystemTap probe somewhere during process fork and add
    a bias to the siglock. That should work fine. Let me knock up the SystemTap
    script...

    Petr


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