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

Manfred Spraul
2022-09-18T11:08:25Z

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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

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From 6a1d2a4beb180241b63f9bf57454bbe031915dd1 Mon Sep 17 00:00:00 2001
From: Manfred Spraul <manfred@colorfullife.com>
Date: Sun, 18 Sep 2022 12:17:27 +0200
Subject: [PATCH] lib/percpu_counter: [RFC] potential overflow/underflow

If an interrupt happens between __this_cpu_read(*fbc->counters) and
this_cpu_add(*fbc->counters, amount), and that interrupt modifies
the per_cpu_counter, then the this_cpu_add() after the interrupt
returns may under/overflow.

Thus: Disable interrupts.

Note: The patch is incomplete, if the race is real, then
more functions than just percpu_counter_add_batch() needs to be
updated.

Especially, the !CONFIG_SMP code looks wrong to me as well:
> static inline void
> percpu_counter_add(struct percpu_counter *fbc, s64 amount)
> {
> preempt_disable();
> fbc->count += amount;
> preempt_enable();
> }
The update of fbc->count is not IRQ safe.

Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
---
lib/percpu_counter.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index ed610b75dc32..39de94d59b4f 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -82,18 +82,20 @@ EXPORT_SYMBOL(percpu_counter_set);
void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
{
s64 count;
+ unsigned long flags;

preempt_disable();
+ local_irq_save(flags);
count = __this_cpu_read(*fbc->counters) + amount;
if (abs(count) >= batch) {
- unsigned long flags;
- raw_spin_lock_irqsave(&fbc->lock, flags);
+ raw_spin_lock(&fbc->lock);
fbc->count += count;
__this_cpu_sub(*fbc->counters, count - amount);
- raw_spin_unlock_irqrestore(&fbc->lock, flags);
+ raw_spin_unlock(&fbc->lock);
} else {
this_cpu_add(*fbc->counters, amount);
}
+ local_irq_restore(flags);
preempt_enable();
}
EXPORT_SYMBOL(percpu_counter_add_batch);
--
2.37.2