Unverified Commit da06e3c5 authored by Christian Brauner's avatar Christian Brauner
Browse files

fs: don't needlessly acquire f_lock 

Before 2011 there was no meaningful synchronization between
read/readdir/write/seek. Only in commit
ef3d0fd2 ("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 9c225f26 ("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: default avatarChristian Brauner <brauner@kernel.org>
parent 1bb77256

fs/file.c

+10 −0
Original line number Diff line number Diff line
@@ -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

+1 −0
Original line number Diff line number Diff line
@@ -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

+9 −4
Original line number Diff line number Diff line
@@ -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

		 * other SEEK_CURs. Note that parallel writes and reads

		 * behave like SEEK_SET.

		 * If the file requires locking via f_pos_lock we know

		 * that mutual exclusion for SEEK_CUR on the same file

		 * 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);

}