[PATCH v4 4/7] gpiolib: cdev: don't sleep in the SRCU read section in read()

[Adrian Ambrozewicz <aambrozewicz@nvidia.com>]

linereq_read(), lineevent_read() and lineinfo_watch_read() stay in the
gdev->srcu read section while they sleep waiting for an event, so
gpiochip_remove() blocks in synchronize_srcu() until an event arrives
or the reader is signalled. The device lock is held meanwhile, which
also blocks device_shutdown():

  task:init state:D stack:0 pid:1 tgid:1 ppid:0
  Call Trace:
   __synchronize_srcu
   gpiochip_remove
   devres_release_all
   device_release_driver_internal
   platform_device_unregister
   gpio_sim_device_deactivate

Check the chip in a scoped SRCU section only; nothing after the check
dereferences it. End the wait once the chip is gone, and return -ENODEV
if there is no event to report.

Reproduced with gpio-sim: setting "live" to 0 hangs while a reader is
blocked in read() on a line request; with this change, the write
returns at once and read() fails with -ENODEV.

Fixes: bdbbae241a04 ("gpiolib: protect the GPIO device against being dropped while in use by user-space")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Adrian Ambrozewicz <aambrozewicz@nvidia.com>
---
 drivers/gpio/gpiolib-cdev.c | 50 ++++++++++++++++++++++++++-----------
 1 file changed, 35 insertions(+), 15 deletions(-)

diff --git a/drivers/gpio/gpiolib-cdev.c b/drivers/gpio/gpiolib-cdev.c
index 6350b2fca8a7..c5869e3f7767 100644
--- a/drivers/gpio/gpiolib-cdev.c
+++ b/drivers/gpio/gpiolib-cdev.c
@@ -1507,6 +1507,20 @@ static __poll_t linereq_poll(struct file *file,
 	return events;
 }
 
+/*
+ * For the read() paths, which must not stay in the SRCU read section while
+ * sleeping: that would stall gpiochip_remove() until an event arrives.
+ */
+static int gpiodev_check_chip(struct gpio_device *gdev)
+{
+	guard(srcu)(&gdev->srcu);
+
+	if (!rcu_access_pointer(gdev->chip))
+		return -ENODEV;
+
+	return 0;
+}
+
 static ssize_t linereq_read(struct file *file, char __user *buf,
 			    size_t count, loff_t *f_ps)
 {
@@ -1515,10 +1529,9 @@ static ssize_t linereq_read(struct file *file, char __user *buf,
 	ssize_t bytes_read = 0;
 	int ret;
 
-	guard(srcu)(&lr->gdev->srcu);
-
-	if (!rcu_access_pointer(lr->gdev->chip))
-		return -ENODEV;
+	ret = gpiodev_check_chip(lr->gdev);
+	if (ret)
+		return ret;
 
 	if (count < sizeof(le))
 		return -EINVAL;
@@ -1533,9 +1546,12 @@ static ssize_t linereq_read(struct file *file, char __user *buf,
 					return -EAGAIN;
 
 				ret = wait_event_interruptible_locked(lr->wait,
-						!kfifo_is_empty(&lr->events));
+						!kfifo_is_empty(&lr->events) ||
+						!rcu_access_pointer(lr->gdev->chip));
 				if (ret)
 					return ret;
+				if (kfifo_is_empty(&lr->events))
+					return -ENODEV;
 			}
 
 			if (kfifo_out(&lr->events, &le, 1) != 1) {
@@ -1830,10 +1846,9 @@ static ssize_t lineevent_read(struct file *file, char __user *buf,
 	ssize_t ge_size;
 	int ret;
 
-	guard(srcu)(&le->gdev->srcu);
-
-	if (!rcu_access_pointer(le->gdev->chip))
-		return -ENODEV;
+	ret = gpiodev_check_chip(le->gdev);
+	if (ret)
+		return ret;
 
 	/*
 	 * When compatible system call is being used the struct gpioevent_data,
@@ -1861,9 +1876,12 @@ static ssize_t lineevent_read(struct file *file, char __user *buf,
 					return -EAGAIN;
 
 				ret = wait_event_interruptible_locked(le->wait,
-						!kfifo_is_empty(&le->events));
+						!kfifo_is_empty(&le->events) ||
+						!rcu_access_pointer(le->gdev->chip));
 				if (ret)
 					return ret;
+				if (kfifo_is_empty(&le->events))
+					return -ENODEV;
 			}
 
 			if (kfifo_out(&le->events, &ge, 1) != 1) {
@@ -2607,10 +2625,9 @@ static ssize_t lineinfo_watch_read(struct file *file, char __user *buf,
 	int ret;
 	size_t event_size;
 
-	guard(srcu)(&cdev->gdev->srcu);
-
-	if (!rcu_access_pointer(cdev->gdev->chip))
-		return -ENODEV;
+	ret = gpiodev_check_chip(cdev->gdev);
+	if (ret)
+		return ret;
 
 #ifndef CONFIG_GPIO_CDEV_V1
 	event_size = sizeof(struct gpio_v2_line_info_changed);
@@ -2628,9 +2645,12 @@ static ssize_t lineinfo_watch_read(struct file *file, char __user *buf,
 					return -EAGAIN;
 
 				ret = wait_event_interruptible_locked(cdev->wait,
-						!kfifo_is_empty(&cdev->events));
+						!kfifo_is_empty(&cdev->events) ||
+						!rcu_access_pointer(cdev->gdev->chip));
 				if (ret)
 					return ret;
+				if (kfifo_is_empty(&cdev->events))
+					return -ENODEV;
 			}
 #ifdef CONFIG_GPIO_CDEV_V1
 			/* must be after kfifo check so watch_abi_version is set */
-- 
2.43.0