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    SubjectRe: [PATCH] netfilter: use per-CPU recursive lock {XV}
    * Eric Dumazet (dada1@cosmosbay.com) wrote:
    > From: Stephen Hemminger <shemminger@vyatta.com>
    >
    > > Epilogue due to master Jarek. Lockdep carest not about the locking
    > > doth bestowed. Therefore no keys are needed.
    > >
    > > Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
    >
    > So far, so good, should be ready for inclusion now, nobody complained :)
    >
    > I include the final patch, merge of your last two patches.
    >
    > David, could you please review it once again and apply it if it's OK ?
    >
    [...]
    > +/*
    > + * Per-CPU read/write lock associated with per-cpu table entries.
    > + * This is not a general solution but makes reader locking fast since
    > + * there is no shared variable to cause cache ping-pong; but adds an
    > + * additional write-side penalty since update must lock all
    > + * possible CPU's.
    > + *
    > + * Read lock is used by ip/arp/ip6 tables rule processing which runs per-cpu.
    > + * It needs to ensure that the rules are not being changed while packet
    > + * is being processed. In some cases, the read lock will be acquired
    > + * twice on the same CPU; this is okay because read locks handle nesting.
    > + *
    > + * Write lock is used in two cases:
    > + * 1. reading counter values
    > + * all readers need to be stopped and the per-CPU values are summed.
    > + *
    > + * 2. replacing tables
    > + * any readers that are using the old tables have to complete
    > + * before freeing the old table. This is handled by reading
    > + * as a side effect of reading counters
    > + */
    > +DECLARE_PER_CPU(rwlock_t, xt_info_locks);
    > +
    > +static inline void xt_info_rdlock_bh(void)
    > +{
    > + /*
    > + * Note: can not use read_lock_bh(&__get_cpu_var(xt_info_locks))
    > + * because need to ensure that preemption is disable before
    > + * acquiring per-cpu-variable, so do it as a two step process
    > + */
    > + local_bh_disable();

    Why do you need to disable bottom halves on the read-side ? You could
    probably just disable preemption, given this lock is nestable on the
    read-side anyway. Or I'm missing something obvious ?

    > + read_lock(&__get_cpu_var(xt_info_locks));
    > +}
    > +
    > +static inline void xt_info_rdunlock_bh(void)
    > +{
    > + read_unlock_bh(&__get_cpu_var(xt_info_locks));
    > +}
    > +
    > +static inline void xt_info_wrlock(unsigned int cpu)
    > +{
    > + write_lock(&per_cpu(xt_info_locks, cpu));
    > +}
    > +
    > +static inline void xt_info_wrunlock(unsigned int cpu)
    > +{
    > +
    > + write_unlock(&per_cpu(xt_info_locks, cpu));
    > +}
    >

    [...]

    Mathieu

    --
    Mathieu Desnoyers
    OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68


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