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    SubjectRE: Serious problem with ticket spinlocks on ia64
    > Hedi Berriche sent me a simple test case that can
    > trigger the failure on the siglock.

    Can you post the test case please. How long does it typically take
    to reproduce the problem?

    > Next, CPU 5 releases the spinlock with st2.rel, changing the lock
    > value to 0x0 (correct).
    > Now, CPU 4, CPU 5 and CPU 7 all want to acquire the lock again.
    > Interestingly, CPU 5 and CPU 7 are both granted the same ticket,

    What is the duplicate ticket number that CPUs 5 & 7 get at this point?
    Presumably 0x0, yes? Or do they see a stale 0x7fff?

    > and the spinlock value (as seen from the debug fault handler) is
    > 0x0 after single-stepping over the fetchadd4.acq, in both cases.
    > CPU 4 correctly sets the spinlock value to 0x1.

    Is the fault handler using "ld.acq" to look at the spinlock value?
    If not, then this might be a red herring. [Though clearly something
    bad is going on here].

    > Any ideas?

    What cpu model are you running on?
    What is the topological connection between CPU 4, 5 and 7 - are any of
    them hyper-threaded siblings? Cores on same socket? N.B. topology may
    change from boot to boot, so you may need to capture /proc/cpuinfo from
    the same boot where this problem is detected. But the variation is
    usually limited to which socket gets to own logical cpu 0.

    If this is a memory ordering problem (and that seems quite plausible)
    then a liberal sprinkling of "ia64_mf()" calls throughout the spinlock
    routines would probably make it go away.


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