linux-kernel - [PATCH RFC 2/4] backlight: use spin-lock to protect device list

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Message-Id: <1410364463-12692-3-git-send-email-dh.herrmann@gmail.com>
Date:	Wed, 10 Sep 2014 17:54:21 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	dri-devel@...ts.freedesktop.org
Cc:	Dave Airlie <airlied@...il.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Jingoo Han <jg1.han@...sung.com>,
	Bryan Wu <cooloney@...il.com>,
	Lee Jones <lee.jones@...aro.org>,
	Matthew Garrett <matthew.garrett@...ula.com>,
	Adam Jackson <ajax@...hat.com>, linux-kernel@...r.kernel.org,
	David Herrmann <dh.herrmann@...il.com>
Subject: [PATCH RFC 2/4] backlight: use spin-lock to protect device list

There is really no reason to use a mutex to protect a simple list. Convert
the list-lock to a simple spinlock instead.

The spin-locks prepare for a backlight_find() helper, which should
preferably be usable from atomic context. A mutex would prevent that, so
use an irq-save spinlock instead.

Signed-off-by: David Herrmann <dh.herrmann@...il.com>
---
 drivers/video/backlight/backlight.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/video/backlight/backlight.c b/drivers/video/backlight/backlight.c
index 726c6c6..33b64be 100644
--- a/drivers/video/backlight/backlight.c
+++ b/drivers/video/backlight/backlight.c
@@ -16,13 +16,14 @@
 #include <linux/err.h>
 #include <linux/fb.h>
 #include <linux/slab.h>
+#include <linux/spinlock.h>
 
 #ifdef CONFIG_PMAC_BACKLIGHT
 #include <asm/backlight.h>
 #endif
 
 static LIST_HEAD(backlight_dev_list);
-static DEFINE_MUTEX(backlight_dev_list_mutex);
+static DEFINE_SPINLOCK(backlight_dev_list_lock);
 static BLOCKING_NOTIFIER_HEAD(backlight_notifier);
 
 static const char *const backlight_types[] = {
@@ -369,9 +370,9 @@ struct backlight_device *backlight_device_register(const char *name,
 	mutex_unlock(&pmac_backlight_mutex);
 #endif
 
-	mutex_lock(&backlight_dev_list_mutex);
+	spin_lock_irq(&backlight_dev_list_lock);
 	list_add(&new_bd->entry, &backlight_dev_list);
-	mutex_unlock(&backlight_dev_list_mutex);
+	spin_unlock_irq(&backlight_dev_list_lock);
 
 	blocking_notifier_call_chain(&backlight_notifier,
 				     BACKLIGHT_REGISTERED, new_bd);
@@ -384,15 +385,16 @@ bool backlight_device_registered(enum backlight_type type)
 {
 	bool found = false;
 	struct backlight_device *bd;
+	unsigned long flags;
 
-	mutex_lock(&backlight_dev_list_mutex);
+	spin_lock_irqsave(&backlight_dev_list_lock, flags);
 	list_for_each_entry(bd, &backlight_dev_list, entry) {
 		if (bd->props.type == type) {
 			found = true;
 			break;
 		}
 	}
-	mutex_unlock(&backlight_dev_list_mutex);
+	spin_unlock_irqrestore(&backlight_dev_list_lock, flags);
 
 	return found;
 }
@@ -409,9 +411,9 @@ void backlight_device_unregister(struct backlight_device *bd)
 	if (!bd)
 		return;
 
-	mutex_lock(&backlight_dev_list_mutex);
+	spin_lock_irq(&backlight_dev_list_lock);
 	list_del(&bd->entry);
-	mutex_unlock(&backlight_dev_list_mutex);
+	spin_unlock_irq(&backlight_dev_list_lock);
 
 #ifdef CONFIG_PMAC_BACKLIGHT
 	mutex_lock(&pmac_backlight_mutex);
-- 
2.1.0

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