gpio: shared: fix a deadlock

It's possible that the auxiliary proxy device we add when setting up the
GPIO controller exposing shared pins, will get matched and probed
immediately. The gpio-proxy-driver will then retrieve the shared
descriptor structure. That will cause a recursive mutex locking and
a deadlock because we're already holding the gpio_shared_lock in
gpio_device_setup_shared() and try to take it again in
devm_gpiod_shared_get() like this:

[ 4.298346] gpiolib_shared: GPIO 130 owned by f100000.pinctrl is shared by multiple consumers
[ 4.307157] gpiolib_shared: Setting up a shared GPIO entry for speaker@0,3
[ 4.314604]
[ 4.316146] ============================================
[ 4.321600] WARNING: possible recursive locking detected
[ 4.327054] 6.18.0-rc7-next-20251125-g3f300d0674f6-dirty #3887 Not tainted
[ 4.334115] --------------------------------------------
[ 4.339566] kworker/u32:3/71 is trying to acquire lock:
[ 4.344931] ffffda019ba71850 (gpio_shared_lock){+.+.}-{4:4}, at: devm_gpiod_shared_get+0x34/0x2e0
[ 4.354057]
[ 4.354057] but task is already holding lock:
[ 4.360041] ffffda019ba71850 (gpio_shared_lock){+.+.}-{4:4}, at: gpio_device_setup_shared+0x30/0x268
[ 4.369421]
[ 4.369421] other info that might help us debug this:
[ 4.376126] Possible unsafe locking scenario:
[ 4.376126]
[ 4.382198] CPU0
[ 4.384711] ----
[ 4.387223] lock(gpio_shared_lock);
[ 4.390992] lock(gpio_shared_lock);
[ 4.394761]
[ 4.394761] *** DEADLOCK ***
[ 4.394761]
[ 4.400832] May be due to missing lock nesting notation
[ 4.400832]
[ 4.407802] 5 locks held by kworker/u32:3/71:
[ 4.412279] #0: ffff000080020948 ((wq_completion)events_unbound){+.+.}-{0:0}, at: process_one_work+0x194/0x64c
[ 4.422650] #1: ffff800080963d60 (deferred_probe_work){+.+.}-{0:0}, at: process_one_work+0x1bc/0x64c
[ 4.432117] #2: ffff00008165c8f8 (&dev->mutex){....}-{4:4}, at: __device_attach+0x3c/0x198
[ 4.440700] #3: ffffda019ba71850 (gpio_shared_lock){+.+.}-{4:4}, at: gpio_device_setup_shared+0x30/0x268
[ 4.450523] #4: ffff0000810fe918 (&dev->mutex){....}-{4:4}, at: __device_attach+0x3c/0x198
[ 4.459103]
[ 4.459103] stack backtrace:
[ 4.463581] CPU: 6 UID: 0 PID: 71 Comm: kworker/u32:3 Not tainted 6.18.0-rc7-next-20251125-g3f300d0674f6-dirty #3887 PREEMPT
[ 4.463589] Hardware name: Qualcomm Technologies, Inc. Robotics RB5 (DT)
[ 4.463593] Workqueue: events_unbound deferred_probe_work_func
[ 4.463602] Call trace:
[ 4.463604] show_stack+0x18/0x24 (C)
[ 4.463617] dump_stack_lvl+0x70/0x98
[ 4.463627] dump_stack+0x18/0x24
[ 4.463636] print_deadlock_bug+0x224/0x238
[ 4.463643] __lock_acquire+0xe4c/0x15f0
[ 4.463648] lock_acquire+0x1cc/0x344
[ 4.463653] __mutex_lock+0xb8/0x840
[ 4.463661] mutex_lock_nested+0x24/0x30
[ 4.463667] devm_gpiod_shared_get+0x34/0x2e0
[ 4.463674] gpio_shared_proxy_probe+0x18/0x138
[ 4.463682] auxiliary_bus_probe+0x40/0x78
[ 4.463688] really_probe+0xbc/0x2c0
[ 4.463694] __driver_probe_device+0x78/0x120
[ 4.463701] driver_probe_device+0x3c/0x160
[ 4.463708] __device_attach_driver+0xb8/0x140
[ 4.463716] bus_for_each_drv+0x88/0xe8
[ 4.463723] __device_attach+0xa0/0x198
[ 4.463729] device_initial_probe+0x14/0x20
[ 4.463737] bus_probe_device+0xb4/0xc0
[ 4.463743] device_add+0x578/0x76c
[ 4.463747] __auxiliary_device_add+0x40/0xac
[ 4.463752] gpio_device_setup_shared+0x1f8/0x268
[ 4.463758] gpiochip_add_data_with_key+0xdac/0x10ac
[ 4.463763] devm_gpiochip_add_data_with_key+0x30/0x80
[ 4.463768] msm_pinctrl_probe+0x4b0/0x5e0
[ 4.463779] sm8250_pinctrl_probe+0x18/0x40
[ 4.463784] platform_probe+0x5c/0xa4
[ 4.463793] really_probe+0xbc/0x2c0
[ 4.463800] __driver_probe_device+0x78/0x120
[ 4.463807] driver_probe_device+0x3c/0x160
[ 4.463814] __device_attach_driver+0xb8/0x140
[ 4.463821] bus_for_each_drv+0x88/0xe8
[ 4.463827] __device_attach+0xa0/0x198
[ 4.463834] device_initial_probe+0x14/0x20
[ 4.463841] bus_probe_device+0xb4/0xc0
[ 4.463847] deferred_probe_work_func+0x90/0xcc
[ 4.463854] process_one_work+0x214/0x64c
[ 4.463860] worker_thread+0x1bc/0x360
[ 4.463866] kthread+0x14c/0x220
[ 4.463871] ret_from_fork+0x10/0x20
[ 77.265041] random: crng init done

Fortunately, at the time of creating of the auxiliary device, we already
know the correct entry so let's store it as the device's platform data.
We don't need to hold gpio_shared_lock in devm_gpiod_shared_get() as
we're not removing the entry or traversing the list anymore but we still
need to protect it from concurrent modification of its fields so add a
more fine-grained mutex.

Fixes: a060b8c511ab ("gpiolib: implement low-level, shared GPIO support")
Reported-by: Dmitry Baryshkov <[e-mail]>
Closes: https://lore.kernel.org/all/fimuvblfy2cmn7o4wzcxjzrux5mwhvlvyxfsgeqs6ore2xg75i@ax46d3sfmdux/
Reviewed-by: Dmitry Baryshkov <[e-mail]>
Tested-by: Dmitry Baryshkov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Bartosz Golaszewski <[e-mail]>

drivers/gpio/gpiolib-shared.c

···
 	unsigned int offset;
 	/* Index in the property value array. */
 	size_t index;
+	struct mutex lock;
 	struct gpio_shared_desc *shared_desc;
 	struct kref ref;
 	struct list_head refs;
···
 				entry->offset = offset;
 				entry->index = count;
 				INIT_LIST_HEAD(&entry->refs);
+				mutex_init(&entry->lock);
 
 				list_add_tail(&entry->list, &gpio_shared_list);
 			}
···
 }
 
 static int gpio_shared_make_adev(struct gpio_device *gdev,
+				 struct gpio_shared_entry *entry,
 				 struct gpio_shared_ref *ref)
 {
 	struct auxiliary_device *adev = &ref->adev;
···
 	adev->id = ref->dev_id;
 	adev->name = "proxy";
 	adev->dev.parent = gdev->dev.parent;
+	adev->dev.platform_data = entry;
 	adev->dev.release = gpio_shared_adev_release;
 
 	ret = auxiliary_device_init(adev);
···
 			pr_debug("Setting up a shared GPIO entry for %s\n",
 				 fwnode_get_name(ref->fwnode));
 
-			ret = gpio_shared_make_adev(gdev, ref);
+			ret = gpio_shared_make_adev(gdev, entry, ref);
 			if (ret)
 				return ret;
 		}
···
 {
 	struct gpio_shared_entry *entry =
 		container_of(kref, struct gpio_shared_entry, ref);
-	struct gpio_shared_desc *shared_desc = entry->shared_desc;
+	struct gpio_shared_desc *shared_desc;
 
-	guard(mutex)(&gpio_shared_lock);
+	guard(mutex)(&entry->lock);
 
+	shared_desc = entry->shared_desc;
 	gpio_device_put(shared_desc->desc->gdev);
 	if (shared_desc->can_sleep)
 		mutex_destroy(&shared_desc->mutex);
···
 	struct gpio_shared_desc *shared_desc;
 	struct gpio_device *gdev;
 
+	lockdep_assert_held(&entry->lock);
+
 	shared_desc = kzalloc(sizeof(*shared_desc), GFP_KERNEL);
 	if (!shared_desc)
 		return ERR_PTR(-ENOMEM);
···
 	return shared_desc;
 }
 
-static struct gpio_shared_entry *gpiod_shared_find(struct auxiliary_device *adev)
+struct gpio_shared_desc *devm_gpiod_shared_get(struct device *dev)
 {
 	struct gpio_shared_desc *shared_desc;
 	struct gpio_shared_entry *entry;
-	struct gpio_shared_ref *ref;
+	int ret;
 
-	guard(mutex)(&gpio_shared_lock);
+	lockdep_assert_not_held(&gpio_shared_lock);
 
-	list_for_each_entry(entry, &gpio_shared_list, list) {
-		list_for_each_entry(ref, &entry->refs, list) {
-			if (adev != &ref->adev)
-				continue;
+	entry = dev_get_platdata(dev);
+	if (WARN_ON(!entry))
+		/* Programmer bug */
+		return ERR_PTR(-ENOENT);
 
-			if (entry->shared_desc) {
-				kref_get(&entry->ref);
-				return entry;
-			}
-
+	scoped_guard(mutex, &entry->lock) {
+		if (entry->shared_desc) {
+			kref_get(&entry->ref);
+			shared_desc = entry->shared_desc;
+		} else {
 			shared_desc = gpiod_shared_desc_create(entry);
 			if (IS_ERR(shared_desc))
 				return ERR_CAST(shared_desc);
 
 			kref_init(&entry->ref);
 			entry->shared_desc = shared_desc;
-
-			pr_debug("Device %s acquired a reference to the shared GPIO %u owned by %s\n",
-				 dev_name(&adev->dev), gpiod_hwgpio(shared_desc->desc),
-				 gpio_device_get_label(shared_desc->desc->gdev));
-
-
-			return entry;
 		}
+
+		pr_debug("Device %s acquired a reference to the shared GPIO %u owned by %s\n",
+			 dev_name(dev), gpiod_hwgpio(shared_desc->desc),
+			 gpio_device_get_label(shared_desc->desc->gdev));
 	}
-
-	return ERR_PTR(-ENOENT);
-}
-
-struct gpio_shared_desc *devm_gpiod_shared_get(struct device *dev)
-{
-	struct gpio_shared_entry *entry;
-	int ret;
-
-	entry = gpiod_shared_find(to_auxiliary_dev(dev));
-	if (IS_ERR(entry))
-		return ERR_CAST(entry);
 
 	ret = devm_add_action_or_reset(dev, gpiod_shared_put, entry);
 	if (ret)
 		return ERR_PTR(ret);
 
-	return entry->shared_desc;
+	return shared_desc;
 }
 EXPORT_SYMBOL_GPL(devm_gpiod_shared_get);
 
···
 static void gpio_shared_drop_entry(struct gpio_shared_entry *entry)
 {
 	list_del(&entry->list);
+	mutex_destroy(&entry->lock);
 	fwnode_handle_put(entry->fwnode);
 	kfree(entry);
 }