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    SubjectRe: [PATCH -v7][RFC]: mutex: implement adaptive spinning

    On Thu, 8 Jan 2009, Linus Torvalds wrote:
    >
    > And I don't even believe that is the bug. I suspect the bug is simpler.
    >
    > I think the "need_resched()" needs to go in the outer loop, or at least
    > happen in the "!owner" case. Because at least with preemption, what can
    > happen otherwise is
    >
    > - process A gets the lock, but gets preempted before it sets lock->owner.
    >
    > End result: count = 0, owner = NULL.
    >
    > - processes B/C goes into the spin loop, filling up all CPU's (assuming
    > dual-core here), and will now both loop forever if they hold the kernel
    > lock (or have some other preemption disabling thing over their down()).
    >
    > And all the while, process A would _happily_ set ->owner, and eventually
    > release the mutex, but it never gets to run to do either of them so.
    >
    > In fact, you might not even need a process C: all you need is for B to be
    > on the same runqueue as A, and having enough load on the other CPU's that
    > A never gets migrated away. So "C" might be in user space.
    >
    > I dunno. There are probably variations on the above.

    Ouch! I think you are on to something:

    for (;;) {
    struct thread_info *owner;

    old_val = atomic_cmpxchg(&lock->count, 1, 0);
    if (old_val == 1) {
    lock_acquired(&lock->dep_map, ip);
    mutex_set_owner(lock);
    return 0;
    }

    if (old_val < 0 && !list_empty(&lock->wait_list))
    break;

    /* See who owns it, and spin on him if anybody */
    owner = ACCESS_ONCE(lock->owner);

    The owner was preempted before assigning lock->owner (as you stated).

    if (owner && !spin_on_owner(lock, owner))
    break;

    We just spin :-(

    I think adding the:

    + if (need_resched())
    + break;

    would solve the problem.

    Thanks,

    -- Steve


    cpu_relax();
    }



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