Mingming Cao <cmm@xxxxxxxxxx> wrote:
>
> [PATCH 1/3] - Currently the"long" type counter maintained in percpu
> counter could have issue when handling a counter that is a unsigned long
> type. Most cases this could be easily fixed by casting the returned
> value to "unsigned long". But for the "overflow" issue, i.e. because of
> the percpu global counter is a approsimate value, there is a
> possibility that at some point the global counter is close to the max
> limit (oxffff_feee) but after updating from a local counter a positive
> value, the global counter becomes a small value (i.e.0x 00000012).
>
> This patch tries to avoid this overflow happen. When updating from a
> local counter to the global counter, add a check to see if the updated
> value is less than before if we are doing an positive add, or if the
> updated value is greater than before if we are doing an negative add.
> Either way we should postpone the updating from this local counter to
> the global counter.
>
>
> -static void __percpu_counter_mod(struct percpu_counter *fbc, long amount)
> +static void __percpu_counter_mod(struct percpu_counter *fbc, long amount,
> + int llcheck)
Confused. What does "ll" stand for throughout this patch?
Whatever it is, I suspect we need to choose something better ;)
> {
> long count;
> long *pcount;
> int cpu = smp_processor_id();
> + unsigned long before, after;
> + int update = 1;
>
> pcount = per_cpu_ptr(fbc->counters, cpu);
> count = *pcount + amount;
> if (count >= FBC_BATCH || count <= -FBC_BATCH) {
> spin_lock(&fbc->lock);
> - fbc->count += count;
> - *pcount = 0;
> + before = fbc->count;
> + after = before + count;
> + if (llcheck && ((count > 0 && after < before) ||
> + ( count < 0 && after > before)))
> + update = 0;
> +
> + if (update) {
> + fbc->count = after;
> + *pcount = 0;
> + }
The above bit of magic deserves an explanatory comment.
> spin_unlock(&fbc->lock);
> } else {
> *pcount = count;
> }
> }
>
> +void percpu_counter_mod_ll(struct percpu_counter *fbc, long amount)
> +{
> + preempt_disable();
> + __percpu_counter_mod(fbc, amount, 1);
> + preempt_enable();
> +}
> +EXPORT_SYMBOL(percpu_counter_mod_ll);
An introductory comment which describes the difference between this and
percpu_counter_mod() would be helpful, please.
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