Re: [PATCH] netfilter: use per-CPU recursive lock {XV}

Mathieu Desnoyers
2009-04-26T19:31:54Z

* 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
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