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    SubjectRe: [rfc][patch] queued spinlocks (i386)
    On Fri, 23 Mar 2007 09:59:11 +0100
    Nick Piggin <> wrote:

    > Implement queued spinlocks for i386. This shouldn't increase the size of
    > the spinlock structure, while still able to handle 2^16 CPUs.
    > Not completely implemented with assembly yet, to make the algorithm a bit
    > clearer.
    > The queued spinlock has 2 fields, a head and a tail, which are indexes
    > into a FIFO of waiting CPUs. To take a spinlock, a CPU performs an
    > "atomic_inc_return" on the head index, and keeps the returned value as
    > a ticket. The CPU then spins until the tail index is equal to that
    > ticket.
    > To unlock a spinlock, the tail index is incremented (this can be non
    > atomic, because only the lock owner will modify tail).
    > Implementation inefficiencies aside, this change should have little
    > effect on performance for uncontended locks, but will have quite a
    > large cost for highly contended locks [O(N) cacheline transfers vs
    > O(1) per lock aquisition, where N is the number of CPUs contending].
    > The benefit is is that contended locks will not cause any starvation.
    > Just an idea. Big NUMA hardware seems to have fairness logic that
    > prevents starvation for the regular spinlock logic. But it might be
    > interesting for -rt kernel or systems with starvation issues.

    It's a very nice idea Nick.

    You also have for free the number or cpus that are before you.

    On big SMP/NUMA, we could use this information to call a special lock_cpu_relax() function to avoid cacheline transferts.

    asm volatile(LOCK_PREFIX "xaddw %0, %1\n\t"
    : "+r" (pos), "+m" (lock->qhead) : : "memory");
    for (;;) {
    unsigned short nwait = pos - lock->qtail;
    if (likely(nwait == 0))
    lock_cpu_relax(lock, nwait);

    lock_cpu_relax(raw_spinlock_t *lock, unsigned int nwait)
    unsigned int cycles = nwait * lock->min_cycles_per_round;
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