usb: chaoskey: fix locking for O_NONBLOCK

A failure to take a lock with O_NONBLOCK needs to result
in -EAGAIN. Change it.

Fixes: 66e3e59 ("usb: Add driver for Altus Metrum ChaosKey device (v2)")
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Link: https://patch.msgid.link/20251030093918.2248104-1-oneukum@suse.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: oneukum

drivers/usb/misc/chaoskey.c

@@ -444,9 +444,19 @@ static ssize_t chaoskey_read(struct file *file,
 			goto bail;
 		mutex_unlock(&dev->rng_lock);
 
-		result = mutex_lock_interruptible(&dev->lock);
-		if (result)
-			goto bail;
+		if (file->f_flags & O_NONBLOCK) {
+			result = mutex_trylock(&dev->lock);
+			if (result == 0) {
+				result = -EAGAIN;
+				goto bail;
+			} else {
+				result = 0;
+			}
+		} else {
+			result = mutex_lock_interruptible(&dev->lock);
+			if (result)
+				goto bail;
+		}
 		if (dev->valid == dev->used) {
 			result = _chaoskey_fill(dev);
 			if (result < 0) {