linux-kernel - [PATCH v5 3/5] gpiolib: cdev: reduce locking in gpio_desc_to

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Date: Tue, 19 Dec 2023 08:41:56 +0800
From: Kent Gibson <warthog618@...il.com>
To: linux-kernel@...r.kernel.org,
	linux-gpio@...r.kernel.org,
	brgl@...ev.pl,
	linus.walleij@...aro.org,
	andy@...nel.org
Cc: Kent Gibson <warthog618@...il.com>
Subject: [PATCH v5 3/5] gpiolib: cdev: reduce locking in gpio_desc_to_lineinfo()

Reduce the time holding the gpio_lock by snapshotting the desc flags,
rather than testing them individually while holding the lock.

Accept that the calculation of the used field is inherently racy, and
only check the availability of the line from pinctrl if other checks
pass, so avoiding the check for lines that are otherwise in use.

Signed-off-by: Kent Gibson <warthog618@...il.com>
Reviewed-by: Andy Shevchenko <andy@...nel.org>
---
 drivers/gpio/gpiolib-cdev.c | 74 ++++++++++++++++++-------------------
 1 file changed, 36 insertions(+), 38 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index aecc4241b6c8..9f5104d7532f 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -2399,74 +2399,72 @@ static void gpio_desc_to_lineinfo(struct gpio_desc *desc,
 				  struct gpio_v2_line_info *info)
 {
 	struct gpio_chip *gc = desc->gdev->chip;
-	bool ok_for_pinctrl;
-	unsigned long flags;
+	unsigned long dflags;
 
 	memset(info, 0, sizeof(*info));
 	info->offset = gpio_chip_hwgpio(desc);
 
-	/*
-	 * This function takes a mutex so we must check this before taking
-	 * the spinlock.
-	 *
-	 * FIXME: find a non-racy way to retrieve this information. Maybe a
-	 * lock common to both frameworks?
-	 */
-	ok_for_pinctrl = pinctrl_gpio_can_use_line(gc, info->offset);
+	scoped_guard(spinlock_irqsave, &gpio_lock) {
+		if (desc->name)
+			strscpy(info->name, desc->name, sizeof(info->name));
 
-	spin_lock_irqsave(&gpio_lock, flags);
+		if (desc->label)
+			strscpy(info->consumer, desc->label,
+				sizeof(info->consumer));
 
-	if (desc->name)
-		strscpy(info->name, desc->name, sizeof(info->name));
-
-	if (desc->label)
-		strscpy(info->consumer, desc->label, sizeof(info->consumer));
+		dflags = READ_ONCE(desc->flags);
+	}
 
 	/*
-	 * Userspace only need to know that the kernel is using this GPIO so
-	 * it can't use it.
+	 * Userspace only need know that the kernel is using this GPIO so it
+	 * can't use it.
+	 * The calculation of the used flag is slightly racy, as it may read
+	 * desc, gc and pinctrl state without a lock covering all three at
+	 * once.  Worst case if the line is in transition and the calculation
+	 * is inconsistent then it looks to the user like they performed the
+	 * read on the other side of the transition - but that can always
+	 * happen.
+	 * The definitive test that a line is available to userspace is to
+	 * request it.
 	 */
-	info->flags = 0;
-	if (test_bit(FLAG_REQUESTED, &desc->flags) ||
-	    test_bit(FLAG_IS_HOGGED, &desc->flags) ||
-	    test_bit(FLAG_USED_AS_IRQ, &desc->flags) ||
-	    test_bit(FLAG_EXPORT, &desc->flags) ||
-	    test_bit(FLAG_SYSFS, &desc->flags) ||
+	if (test_bit(FLAG_REQUESTED, &dflags) ||
+	    test_bit(FLAG_IS_HOGGED, &dflags) ||
+	    test_bit(FLAG_USED_AS_IRQ, &dflags) ||
+	    test_bit(FLAG_EXPORT, &dflags) ||
+	    test_bit(FLAG_SYSFS, &dflags) ||
 	    !gpiochip_line_is_valid(gc, info->offset) ||
-	    !ok_for_pinctrl)
+	    !pinctrl_gpio_can_use_line(gc, info->offset))
 		info->flags |= GPIO_V2_LINE_FLAG_USED;
 
-	if (test_bit(FLAG_IS_OUT, &desc->flags))
+	if (test_bit(FLAG_IS_OUT, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_OUTPUT;
 	else
 		info->flags |= GPIO_V2_LINE_FLAG_INPUT;
 
-	if (test_bit(FLAG_ACTIVE_LOW, &desc->flags))
+	if (test_bit(FLAG_ACTIVE_LOW, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_ACTIVE_LOW;
 
-	if (test_bit(FLAG_OPEN_DRAIN, &desc->flags))
+	if (test_bit(FLAG_OPEN_DRAIN, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_OPEN_DRAIN;
-	if (test_bit(FLAG_OPEN_SOURCE, &desc->flags))
+	if (test_bit(FLAG_OPEN_SOURCE, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_OPEN_SOURCE;
 
-	if (test_bit(FLAG_BIAS_DISABLE, &desc->flags))
+	if (test_bit(FLAG_BIAS_DISABLE, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_BIAS_DISABLED;
-	if (test_bit(FLAG_PULL_DOWN, &desc->flags))
+	if (test_bit(FLAG_PULL_DOWN, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_BIAS_PULL_DOWN;
-	if (test_bit(FLAG_PULL_UP, &desc->flags))
+	if (test_bit(FLAG_PULL_UP, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_BIAS_PULL_UP;
 
-	if (test_bit(FLAG_EDGE_RISING, &desc->flags))
+	if (test_bit(FLAG_EDGE_RISING, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EDGE_RISING;
-	if (test_bit(FLAG_EDGE_FALLING, &desc->flags))
+	if (test_bit(FLAG_EDGE_FALLING, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EDGE_FALLING;
 
-	if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &desc->flags))
+	if (test_bit(FLAG_EVENT_CLOCK_REALTIME, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_REALTIME;
-	else if (test_bit(FLAG_EVENT_CLOCK_HTE, &desc->flags))
+	else if (test_bit(FLAG_EVENT_CLOCK_HTE, &dflags))
 		info->flags |= GPIO_V2_LINE_FLAG_EVENT_CLOCK_HTE;
-
-	spin_unlock_irqrestore(&gpio_lock, flags);
 }
 
 struct gpio_chardev_data {
-- 
2.39.2