pinctrl: renesas: rzv2m: Fix NULL pointer dereference in rzv2m_dt_subnode_to_map()

Fix the below random NULL pointer crash during boot by serializing
pinctrl group and function creation/remove calls in
rzv2m_dt_subnode_to_map() with mutex lock.

Crash logs:
    pc : __pi_strcmp+0x20/0x140
    lr : pinmux_func_name_to_selector+0x68/0xa4
    Call trace:
    __pi_strcmp+0x20/0x140
    pinmux_generic_add_function+0x34/0xcc
    rzv2m_dt_subnode_to_map+0x2e4/0x418
    rzv2m_dt_node_to_map+0x15c/0x18c
    pinctrl_dt_to_map+0x218/0x37c
    create_pinctrl+0x70/0x3d8

While at it, add a comment for lock.

Fixes: 92a9b82 ("pinctrl: renesas: Add RZ/V2M pin and gpio controller driver")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20230815131558.33787-3-biju.das.jz@bp.renesas.com
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>

Author: bijudas

drivers/pinctrl/renesas/pinctrl-rzv2m.c

@@ -14,6 +14,7 @@
 #include <linux/gpio/driver.h>
 #include <linux/io.h>
 #include <linux/module.h>
+#include <linux/mutex.h>
 #include <linux/of_device.h>
 #include <linux/spinlock.h>
 
@@ -123,7 +124,8 @@ struct rzv2m_pinctrl {
 	struct gpio_chip		gpio_chip;
 	struct pinctrl_gpio_range	gpio_range;
 
-	spinlock_t			lock;
+	spinlock_t			lock; /* lock read/write registers */
+	struct mutex			mutex; /* serialize adding groups and functions */
 };
 
 static const unsigned int drv_1_8V_group2_uA[] = { 1800, 3800, 7800, 11000 };
@@ -322,11 +324,13 @@ static int rzv2m_dt_subnode_to_map(struct pinctrl_dev *pctldev,
 		name = np->name;
 	}
 
+	mutex_lock(&pctrl->mutex);
+
 	/* Register a single pin group listing all the pins we read from DT */
 	gsel = pinctrl_generic_add_group(pctldev, name, pins, num_pinmux, NULL);
 	if (gsel < 0) {
 		ret = gsel;
-		goto done;
+		goto unlock;
 	}
 
 	/*
@@ -340,6 +344,8 @@ static int rzv2m_dt_subnode_to_map(struct pinctrl_dev *pctldev,
 		goto remove_group;
 	}
 
+	mutex_unlock(&pctrl->mutex);
+
 	maps[idx].type = PIN_MAP_TYPE_MUX_GROUP;
 	maps[idx].data.mux.group = name;
 	maps[idx].data.mux.function = name;
@@ -351,6 +357,8 @@ static int rzv2m_dt_subnode_to_map(struct pinctrl_dev *pctldev,
 
 remove_group:
 	pinctrl_generic_remove_group(pctldev, gsel);
+unlock:
+	mutex_unlock(&pctrl->mutex);
 done:
 	*index = idx;
 	kfree(configs);
@@ -1071,6 +1079,7 @@ static int rzv2m_pinctrl_probe(struct platform_device *pdev)
 	}
 
 	spin_lock_init(&pctrl->lock);
+	mutex_init(&pctrl->mutex);
 
 	platform_set_drvdata(pdev, pctrl);