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    SubjectRe: Kernel rwlock design, Multicore and IGMP
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    Le jeudi 11 novembre 2010 à 16:23 +0100, Eric Dumazet a écrit :
    > Le jeudi 11 novembre 2010 à 21:49 +0800, Cypher Wu a écrit :
    >
    > Hi
    >
    > CC netdev, since you ask questions about network stuff _and_ rwlock
    >
    >
    > > I'm using TILEPro and its rwlock in kernel is a liitle different than
    > > other platforms. It have a priority for write lock that when tried it
    > > will block the following read lock even if read lock is hold by
    > > others. Its code can be read in Linux Kernel 2.6.36 in
    > > arch/tile/lib/spinlock_32.c.
    >
    > This seems a bug to me.
    >
    > read_lock() can be nested. We used such a schem in the past in iptables
    > (it can re-enter itself),
    > and we used instead a spinlock(), but with many discussions with lkml
    > and Linus himself if I remember well.

    I meant, a percpu spinlock, and extra logic to spin_lock() it one time,
    even if nested.

    static inline void xt_info_rdlock_bh(void)
    {
    struct xt_info_lock *lock;

    local_bh_disable();
    lock = &__get_cpu_var(xt_info_locks);
    if (likely(!lock->readers++))
    spin_lock(&lock->lock);
    }

    static inline void xt_info_rdunlock_bh(void)
    {
    struct xt_info_lock *lock = &__get_cpu_var(xt_info_locks);

    if (likely(!--lock->readers))
    spin_unlock(&lock->lock);
    local_bh_enable();
    }

    The write 'rwlock' side has to lock the percpu spinlock of all possible
    cpus.

    /*
    * The "writer" side needs to get exclusive access to the lock,
    * regardless of readers. This must be called with bottom half
    * processing (and thus also preemption) disabled.
    */
    static inline void xt_info_wrlock(unsigned int cpu)
    {
    spin_lock(&per_cpu(xt_info_locks, cpu).lock);
    }

    static inline void xt_info_wrunlock(unsigned int cpu)
    {
    spin_unlock(&per_cpu(xt_info_locks, cpu).lock);
    }



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