linux-kernel - Re: [PATCH v6 1/2] percpu: Add percpu_counter_add_local and percpu_counter_sub

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Date:   Sun, 18 Sep 2022 13:08:16 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     Jiebin Sun <jiebin.sun@...el.com>, akpm@...ux-foundation.org,
        vasily.averin@...ux.dev, shakeelb@...gle.com, dennis@...nel.org,
        tj@...nel.org, cl@...ux.com, ebiederm@...ssion.com,
        legion@...nel.org, alexander.mikhalitsyn@...tuozzo.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Cc:     tim.c.chen@...el.com, feng.tang@...el.com, ying.huang@...el.com,
        tianyou.li@...el.com, wangyang.guo@...el.com,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        kernel test robot <lkp@...el.com>, 1vier1@....de
Subject: Re: [PATCH v6 1/2] percpu: Add percpu_counter_add_local and
 percpu_counter_sub_local

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

View attachment "0001-lib-percpu_counter-RFC-potential-overflow-underflow.patch" of type "text/x-patch" (1893 bytes)