linux-kernel - Re: [PATCH 1/2] workqueue: Allow to expose ordered workqueues via sysfs

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Message-ID: <20260205133913.538FuakM@linutronix.de>
Date: Thu, 5 Feb 2026 14:39:13 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-rt-devel@...ts.linux.dev
Cc: "Luis Claudio R. Goncalves" <lgoncalv@...hat.com>,
	Ard Biesheuvel <ardb@...nel.org>,
	John Ogness <john.ogness@...utronix.de>,
	Lai Jiangshan <jiangshanlai@...il.com>, Tejun Heo <tj@...nel.org>,
	Thomas Weißschuh <thomas.weissschuh@...utronix.de>
Subject: Re: [PATCH 1/2] workqueue: Allow to expose ordered workqueues via
 sysfs

On 2026-02-05 12:55:58 [+0100], To linux-efi@...r.kernel.org wrote:
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -7097,6 +7097,13 @@ static ssize_t max_active_store(struct device *dev,
>  	struct workqueue_struct *wq = dev_to_wq(dev);
>  	int val;
>  
> +	/*
> +	 * Adjusting max_active breaks ordering guarantee. Changing it has no
> +	 * effect on BH worker.
> +	 */
> +	if (wq->flags & (WQ_BH | __WQ_ORDERED))
> +		return -EACCES;
> +
>  	if (sscanf(buf, "%d", &val) != 1 || val <= 0)
>  		return -EINVAL;

I have been informed that instead of this -EACCES I could do the
following and exposing only the max_active (and per_cpu) attribute as
RO instead.

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 625ee8cc47f40..793b59ce99edb 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -7097,13 +7097,6 @@ static ssize_t max_active_store(struct device *dev,
 	struct workqueue_struct *wq = dev_to_wq(dev);
 	int val;
 
-	/*
-	 * Adjusting max_active breaks ordering guarantee. Changing it has no
-	 * effect on BH worker.
-	 */
-	if (wq->flags & (WQ_BH | __WQ_ORDERED))
-		return -EACCES;
-
 	if (sscanf(buf, "%d", &val) != 1 || val <= 0)
 		return -EINVAL;
 
@@ -7117,7 +7110,26 @@ static struct attribute *wq_sysfs_attrs[] = {
 	&dev_attr_max_active.attr,
 	NULL,
 };
-ATTRIBUTE_GROUPS(wq_sysfs);
+
+static umode_t wq_sysfs_is_visible(struct kobject *kobj, struct attribute *a, int n)
+{
+	struct device *dev = kobj_to_dev(kobj);
+	struct workqueue_struct *wq = dev_to_wq(dev);
+
+	/*
+	 * Adjusting max_active breaks ordering guarantee. Changing it has no
+	 * effect on BH worker. Limit max_active to RO in such case.
+	 */
+	if (wq->flags & (WQ_BH | __WQ_ORDERED))
+		return 0444;
+	return a->mode;
+}
+
+static const struct attribute_group wq_sysfs_group = {
+	.is_visible = wq_sysfs_is_visible,
+	.attrs = wq_sysfs_attrs,
+};
+__ATTRIBUTE_GROUPS(wq_sysfs);
 
 static ssize_t wq_nice_show(struct device *dev, struct device_attribute *attr,
 			    char *buf)