The patch titled
Subject: lib/percpu_counter: percpu_counter_add_batch() overflow/underflow
has been added to the -mm mm-nonmm-unstable branch. Its filename is
lib-percpu_counter-percpu_counter_add_batch-overflow-underflow.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-percpu_counter-percpu_counter_add_batch-overflow-underflow.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Before you just go and hit "reply", please:
a) Consider who else should be cc'ed
b) Prefer to cc a suitable mailing list as well
c) Ideally: find the original patch on the mailing list and do a
reply-to-all to that, adding suitable additional cc's
*** Remember to use Documentation/process/submit-checklist.rst when testing your code ***
The -mm tree is included into linux-next via the mm-everything
branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
and is updated there every 2-3 working days
------------------------------------------------------
From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Subject: lib/percpu_counter: percpu_counter_add_batch() overflow/underflow
Date: Fri, 16 Dec 2022 16:04:39 +0100
Patch series "various irq handling fixes/docu updates".
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.
Link: https://lkml.kernel.org/r/20221216150155.200389-1-manfred@xxxxxxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20221216150441.200533-1-manfred@xxxxxxxxxxxxxxxx
Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
Cc: "Sun, Jiebin" <jiebin.sun@xxxxxxxxx>
Cc: <1vier1@xxxxxx>
Cc: Alexander Sverdlin <alexander.sverdlin@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
lib/percpu_counter.c | 25 +++++++++++++++----------
1 file changed, 15 insertions(+), 10 deletions(-)
--- a/lib/percpu_counter.c~lib-percpu_counter-percpu_counter_add_batch-overflow-underflow
+++ a/lib/percpu_counter.c
@@ -73,28 +73,33 @@ void percpu_counter_set(struct percpu_co
EXPORT_SYMBOL(percpu_counter_set);
/*
- * 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
+ * local_irq_save() is needed to make the function irq safe:
+ * - The slow path would be ok as protected by an irq-safe spinlock.
+ * - this_cpu_add would be ok as it is irq-safe by definition.
+ * But:
+ * The decision slow path/fast path and the actual update must be atomic, too.
+ * Otherwise a call in process context could check the current values and
+ * decide that the fast path can be used. If now an interrupt occurs before
+ * the this_cpu_add(), and the interrupt updates this_cpu(*fbc->counters),
+ * then the this_cpu_add() that is executed after the interrupt has completed
+ * can produce values larger than "batch" or even overflows.
*/
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);
}
- preempt_enable();
+ local_irq_restore(flags);
}
EXPORT_SYMBOL(percpu_counter_add_batch);
_
Patches currently in -mm which might be from manfred@xxxxxxxxxxxxxxxx are
lib-percpu_counter-percpu_counter_add_batch-overflow-underflow.patch
include-linux-percpu_counterh-race-in-uniprocessor-percpu_counter_add.patch
kernel-irq-managec-disable_irq-might-sleep.patch
