linux-kernel - Re: [PATCH] counter: interrupt-cnt: Protect enable/disable OPs with mutex

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Message-ID: <91e5c0dd0b71e5fbf441b7a6f2a8937a7bfc366f.camel@siemens.com>
Date: Tue, 1 Apr 2025 08:38:18 +0000
From: "Sverdlin, Alexander" <alexander.sverdlin@...mens.com>
To: "a.fatoum@...gutronix.de" <a.fatoum@...gutronix.de>,
	"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>
CC: "o.rempel@...gutronix.de" <o.rempel@...gutronix.de>,
	"kernel@...gutronix.de" <kernel@...gutronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"wbg@...nel.org" <wbg@...nel.org>
Subject: Re: [PATCH] counter: interrupt-cnt: Protect enable/disable OPs with
 mutex

Hi Ahmad,

On Tue, 2025-04-01 at 06:50 +0200, Ahmad Fatoum wrote:
> > Enable/disable seems to be racy on SMP, consider the following scenario:
> > 
> > CPU0					CPU1
> > 
> > interrupt_cnt_enable_write(true)
> > {
> >  	if (priv->enabled == enable)
> >  		return 0;
> > 
> >  	if (enable) {
> >  		priv->enabled = true;
> >  					interrupt_cnt_enable_write(false)
> >  					{
> >  						if (priv->enabled == enable)
> >  							return 0;
> > 
> >  						if (enable) {
> >  							priv->enabled = true;
> >  							enable_irq(priv->irq);
> >  						} else {
> >  							disable_irq(priv->irq)
> >  							priv->enabled = false;
> >  						}
> >  		enable_irq(priv->irq);
> >  	} else {
> >  		disable_irq(priv->irq);
> >  		priv->enabled = false;
> >  	}
> > 
> > The above would result in priv->enabled == false, but IRQ left enabled.
> > Protect both write (above race) and read (to propagate the value on SMP)
> > callbacks with a mutex.
> 
> Doesn't sysfs/kernfs already ensure that the ops may not be called concurrently
> on the same open file?

as I understand the code, the operations on the same FD will be serialized, i.e.
if you open an entry and access it concurrently within the same process.

If you apply the following patch on top of the proposed patch:

--- a/drivers/counter/interrupt-cnt.c
+++ b/drivers/counter/interrupt-cnt.c
@@ -3,6 +3,7 @@
  * Copyright (c) 2021 Pengutronix, Oleksij Rempel <kernel@...gutronix.de>
  */
 
+#include <linux/delay.h>
 #include <linux/cleanup.h>
 #include <linux/counter.h>
 #include <linux/gpio/consumer.h>
@@ -56,6 +57,9 @@ static int interrupt_cnt_enable_write(struct counter_device *counter,
 {
 	struct interrupt_cnt_priv *priv = counter_priv(counter);
 
+	WARN_ON(!mutex_trylock(&priv->lock));
+	mutex_unlock(&priv->lock);
+
 	guard(mutex)(&priv->lock);
 
 	if (priv->enabled == enable)
@@ -69,6 +73,8 @@ static int interrupt_cnt_enable_write(struct counter_device *counter,
 		priv->enabled = false;
 	}
 
+	msleep(1000);
+
 	return 0;
 }
 

And would run `while true; do echo 0 > /sys/.../enable; echo 1 > /sys/.../enable; done &`
twice, you'd see the following quickly:

WARNING: CPU: 1 PID: 754 at /drivers/counter/interrupt-cnt.c:60 interrupt_cnt_enable_write+0xa0/0xb0 [interrupt_cnt]
CPU: 1 UID: 0 PID: 754 Comm: sh
pc : interrupt_cnt_enable_write+0xa0/0xb0 [interrupt_cnt]
lr : interrupt_cnt_enable_write+0x34/0xb0 [interrupt_cnt]
Call trace:
 interrupt_cnt_enable_write+0xa0/0xb0 [interrupt_cnt] (P)
 counter_comp_u8_store+0xcc/0x118 [counter]
 dev_attr_store+0x20/0x40
 sysfs_kf_write+0x84/0xa8
 kernfs_fop_write_iter+0x128/0x1e0
 vfs_write+0x248/0x388
 ksys_write+0x78/0x118
 __arm64_sys_write+0x24/0x38
 invoke_syscall+0x50/0x120

-- 
Alexander Sverdlin
Siemens AG
www.siemens.com