Before 2011 there was no meaningful synchronization between
read/readdir/write/seek. Only in commit
ef3d0fd27e90 ("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 9c225f2655e3 ("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@xxxxxxxxxx>
---
fs/file.c | 10 ++++++++++
fs/internal.h | 1 +
fs/read_write.c | 13 +++++++++----
3 files changed, 20 insertions(+), 4 deletions(-)
diff --git a/fs/file.c b/fs/file.c
index d868cdb95d1e..44efdc8c1e27 100644
--- a/fs/file.c
+++ b/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);
diff --git a/fs/internal.h b/fs/internal.h
index 84607e7b05dc..1cb85a62c07f 100644
--- a/fs/internal.h
+++ b/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);
diff --git a/fs/read_write.c b/fs/read_write.c
index a6133241dfb8..bb0ed26a0b3a 100644
--- a/fs/read_write.c
+++ b/fs/read_write.c
@@ -169,11 +169,16 @@ 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.
*/
- guard(spinlock)(&file->f_lock);
+ 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);
}
--
2.47.2
--volnlywmczmjulmh--
