fs: don't needlessly acquire f_lock

Before 2011 there was no meaningful synchronization between
read/readdir/write/seek. Only in commit
ef3d0fd27e ("vfs: do (nearly) lockless generic_file_llseek")
synchronization was added for SEEK_CUR by taking f_lock around
vfs_setpos().

Then in 2014 full synchronization between read/readdir/write/seek was
added in commit 9c225f2655 ("vfs: atomic f_pos accesses as per POSIX")
by introducing f_pos_lock for regular files with FMODE_ATOMIC_POS and
for directories. At that point taking f_lock became unnecessary for such
files.

So only acquire f_lock for SEEK_CUR if this isn't a file that would have
acquired f_pos_lock if necessary.

Link: https://lore.kernel.org/r/20250207-daten-mahlzeit-99d2079864fb@brauner
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/file.c

@@ -1182,6 +1182,16 @@ static inline bool file_needs_f_pos_lock(struct file *file)
 		(file_count(file) > 1 || file->f_op->iterate_shared);
 }
 
+bool file_seek_cur_needs_f_lock(struct file *file)
+{
+	if (!(file->f_mode & FMODE_ATOMIC_POS) && !file->f_op->iterate_shared)
+		return false;
+
+	VFS_WARN_ON_ONCE((file_count(file) > 1) &&
+			 !mutex_is_locked(&file->f_pos_lock));
+	return true;
+}
+
 struct fd fdget_pos(unsigned int fd)
 {
 	struct fd f = fdget(fd);

fs/internal.h

@@ -338,3 +338,4 @@ static inline bool path_mounted(const struct path *path)
 	return path->mnt->mnt_root == path->dentry;
 }
 void file_f_owner_release(struct file *file);
+bool file_seek_cur_needs_f_lock(struct file *file);

fs/read_write.c

@@ -169,13 +169,18 @@ generic_file_llseek_size(struct file *file, loff_t offset, int whence,
 
 	if (whence == SEEK_CUR) {
 		/*
-		 * f_lock protects against read/modify/write race with
+		 * If the file requires locking via f_pos_lock we know
-		 * other SEEK_CURs. Note that parallel writes and reads
+		 * that mutual exclusion for SEEK_CUR on the same file
-		 * behave like SEEK_SET.
+		 * is guaranteed. If the file isn't locked, we take
+		 * f_lock to protect against f_pos races with other
+		 * SEEK_CURs.
 		 */
+		if (file_seek_cur_needs_f_lock(file)) {
 			guard(spinlock)(&file->f_lock);
 			return vfs_setpos(file, file->f_pos + offset, maxsize);
 		}
+		return vfs_setpos(file, file->f_pos + offset, maxsize);
+	}
 
 	return vfs_setpos(file, offset, maxsize);
 }