fs: Split fcntl_rw_hint()

Split fcntl_rw_hint() such that there is one helper function per fcntl.
Use READ_ONCE() and WRITE_ONCE() to access the i_write_hint member
instead of protecting such accesses with the inode lock. READ_ONCE() is
not used in I/O path code that reads i_write_hint. Users who want
F_SET_RW_HINT to affect I/O need to make sure that F_SET_RW_HINT has
completed before I/O is submitted that should use the configured write
hint.

Cc: Christoph Hellwig <hch@lst.de>
Suggested-by: Christoph Hellwig <hch@lst.de>
Cc: Kanchan Joshi <joshi.k@samsung.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Link: https://lore.kernel.org/r/20240202203926.2478590-4-bvanassche@acm.orgSigned-off-by: Christian Brauner <brauner@kernel.org>

fs/fcntl.c

@@ -290,32 +290,33 @@ static bool rw_hint_valid(u64 hint)
 	}
 }
 
-static long fcntl_rw_hint(struct file *file, unsigned int cmd,
+static long fcntl_get_rw_hint(struct file *file, unsigned int cmd,
 			      unsigned long arg)
 {
 	struct inode *inode = file_inode(file);
 	u64 __user *argp = (u64 __user *)arg;
-	u64 hint;
+	u64 hint = READ_ONCE(inode->i_write_hint);
 
-	switch (cmd) {
-	case F_GET_RW_HINT:
-		hint = inode->i_write_hint;
 	if (copy_to_user(argp, &hint, sizeof(*argp)))
 		return -EFAULT;
 	return 0;
-	case F_SET_RW_HINT:
+}
+
+static long fcntl_set_rw_hint(struct file *file, unsigned int cmd,
+			      unsigned long arg)
+{
+	struct inode *inode = file_inode(file);
+	u64 __user *argp = (u64 __user *)arg;
+	u64 hint;
+
 	if (copy_from_user(&hint, argp, sizeof(hint)))
 		return -EFAULT;
 	if (!rw_hint_valid(hint))
 		return -EINVAL;
 
-		inode_lock(inode);
-		inode->i_write_hint = hint;
-		inode_unlock(inode);
+	WRITE_ONCE(inode->i_write_hint, hint);
+
 	return 0;
-	default:
-		return -EINVAL;
-	}
 }
 
 static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
@@ -421,8 +422,10 @@ static long do_fcntl(int fd, unsigned int cmd, unsigned long arg,
 		err = memfd_fcntl(filp, cmd, argi);
 		break;
 	case F_GET_RW_HINT:
+		err = fcntl_get_rw_hint(filp, cmd, arg);
+		break;
 	case F_SET_RW_HINT:
-		err = fcntl_rw_hint(filp, cmd, arg);
+		err = fcntl_set_rw_hint(filp, cmd, arg);
 		break;
 	default:
 		break;