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

From: Mathieu Desnoyers <[email-masked]>
Date: Sun, 26 Apr 2009 15:31:35 -0400

* Eric Dumazet ([email-masked]) wrote:
> From: Stephen Hemminger <[email-masked]>
> 
> > Epilogue due to master Jarek. Lockdep carest not about the locking
> > doth bestowed. Therefore no keys are needed.
> > 
> > Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> 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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