Re: [PATCH v6 1/2] percpu: Add percpu_counter_add_local and percpu_counter_sub_local

Sun, Jiebin
2022-09-20T06:01:33Z

On 9/18/2022 7:08 PM, Manfred Spraul wrote:
> Hi Jiebin,
>
> On 9/13/22 21:25, Jiebin Sun wrote:
>>   +/*
>> + * With percpu_counter_add_local() and percpu_counter_sub_local(),
>> counts
>> + * are accumulated in local per cpu counter and not in fbc->count until
>> + * local count overflows PERCPU_COUNTER_LOCAL_BATCH. This makes counter
>> + * write efficient.
>> + * But percpu_counter_sum(), instead of percpu_counter_read(), needs
>> to be
>> + * used to add up the counts from each CPU to account for all the local
>> + * counts. So percpu_counter_add_local() and percpu_counter_sub_local()
>> + * should be used when a counter is updated frequently and read rarely.
>> + */
>> +static inline void
>> +percpu_counter_add_local(struct percpu_counter *fbc, s64 amount)
>> +{
>> +    percpu_counter_add_batch(fbc, amount, PERCPU_COUNTER_LOCAL_BATCH);
>> +}
>> +
>
> Unrelated to your patch, and not relevant for ipc/msg as the functions
> are not called from interrupts, but:
> Aren't there races with interrupts?
>
>> *
>> * This function is both preempt and irq safe. The former is due to
>> explicit
>> * preemption disable. The latter is guaranteed by the fact that the
>> slow path
>> * is explicitly protected by an irq-safe spinlock whereas the fast
>> patch uses
>> * this_cpu_add which is irq-safe by definition. Hence there is no
>> need muck
>> * with irq state before calling this one
>> */
>> void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount,
>> s32 batch)
>> {
>>        s64 count;
>>
>>        preempt_disable();
>>        count = __this_cpu_read(*fbc->counters) + amount;
>>        if (abs(count) >= batch) {
>>                unsigned long flags;
>>                raw_spin_lock_irqsave(&fbc->lock, flags);
>>                fbc->count += count;
>>                __this_cpu_sub(*fbc->counters, count - amount);
>>                raw_spin_unlock_irqrestore(&fbc->lock, flags);
>>        } else {
>>                this_cpu_add(*fbc->counters, amount);
>>        }
>>        preempt_enable();
>> }
>> EXPORT_SYMBOL(percpu_counter_add_batch);
>>
>>
> Race 1:
>
> start: __this_cpu_read(*fbc->counters) = INT_MAX-1.
>
> Call: per_cpu_counter_add_batch(fbc, 1, INT_MAX);
>
> Result:
>
> count=INT_MAX;
>
> if (abs(count) >= batch) { // branch taken
>
> before the raw_spin_lock_irqsave():
>
> Interrupt
>
> Within interrupt:
>
>    per_cpu_counter_add_batch(fbc, -2*(INT_MAX-1), INT_MAX)
>
>    count=-(INT_MAX-1);
>
>    branch not taken
>
>    this_cpu_add() updates fbc->counters, new value is -(INT_MAX-1)
>
>    exit interrupt
>
> raw_spin_lock_irqsave()
>
> __this_cpu_sub(*fbc->counters, count - amount)
>
> will substract INT_MAX-1 from *fbc->counters. But the value is already
> -(INT_MAX-1) -> underflow.
>
>
> Race 2: (much simpler)
>
> start: __this_cpu_read(*fbc->counters) = 0.
>
> Call: per_cpu_counter_add_batch(fbc, INT_MAX-1, INT_MAX);
>
> amont=INT_MAX-1;
>
> - branch not taken.
>
> before this_cpu_add(): interrupt
>
> within the interrupt: call per_cpu_counter_add_batch(fbc, INT_MAX-1,
> INT_MAX)
>
>    new value of *fbc->counters: INT_MAX-1.
>
>    exit interrupt
>
> outside interrupt:
>
> this_cpu_add(*fbc->counters, amount);
>
> <<< overflow.
>
> Attached is an incomplete patch (untested).
> If needed, I could check the whole file and add/move the required
> local_irq_save() calls.
>
>
> --
>
>     Manfred

The interrupt protect patch in the real case looks good to me. Thanks.