diff options
| author | Mateusz Guzik <mjguzik@gmail.com> | 2025-03-20 00:58:01 +0300 |
|---|---|---|
| committer | Christian Brauner <brauner@kernel.org> | 2025-03-20 11:45:39 +0300 |
| commit | 5370b43e4bcf60049bfd44db83ba8d2ec43d0152 (patch) | |
| tree | 0d5bfa2b45c0ec532de8f3f2fd3b66c09c850f2d | |
| parent | dba2e3b788f5ac70ebf717523433e1ceae3f0834 (diff) | |
| download | linux-5370b43e4bcf60049bfd44db83ba8d2ec43d0152.tar.xz | |
fs: reduce work in fdget_pos()
1. predict the file was found
2. explicitly compare the ref to "one", ignoring the dead zone
The latter arguably improves the behavior to begin with. Suppose the
count turned bad -- the previously used ref routine is going to check
for it and return 0, indicating the count does not necessitate taking
->f_pos_lock. But there very well may be several users.
i.e. not paying for special-casing the dead zone improves semantics.
While here spell out each condition in a dedicated if statement. This
has no effect on generated code.
Sizes are as follows (in bytes; gcc 13, x86-64):
stock: 321
likely(): 298
likely()+ref: 280
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20250319215801.1870660-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
| -rw-r--r-- | fs/file.c | 11 | ||||
| -rw-r--r-- | include/linux/file_ref.h | 14 |
2 files changed, 22 insertions, 3 deletions
diff --git a/fs/file.c b/fs/file.c index 3fef798b96e5..134274446bf6 100644 --- a/fs/file.c +++ b/fs/file.c @@ -1183,8 +1183,13 @@ struct fd fdget_raw(unsigned int fd) */ static inline bool file_needs_f_pos_lock(struct file *file) { - return (file->f_mode & FMODE_ATOMIC_POS) && - (file_count(file) > 1 || file->f_op->iterate_shared); + if (!(file->f_mode & FMODE_ATOMIC_POS)) + return false; + if (__file_ref_read_raw(&file->f_ref) != FILE_REF_ONEREF) + return true; + if (file->f_op->iterate_shared) + return true; + return false; } struct fd fdget_pos(unsigned int fd) @@ -1192,7 +1197,7 @@ struct fd fdget_pos(unsigned int fd) struct fd f = fdget(fd); struct file *file = fd_file(f); - if (file && file_needs_f_pos_lock(file)) { + if (likely(file) && file_needs_f_pos_lock(file)) { f.word |= FDPUT_POS_UNLOCK; mutex_lock(&file->f_pos_lock); } diff --git a/include/linux/file_ref.h b/include/linux/file_ref.h index 6ef92d765a66..7db62fbc0500 100644 --- a/include/linux/file_ref.h +++ b/include/linux/file_ref.h @@ -208,4 +208,18 @@ static inline unsigned long file_ref_read(file_ref_t *ref) return c >= FILE_REF_RELEASED ? 0 : c + 1; } +/* + * __file_ref_read_raw - Return the value stored in ref->refcnt + * @ref: Pointer to the reference count + * + * Return: The raw value found in the counter + * + * A hack for file_needs_f_pos_lock(), you probably want to use + * file_ref_read() instead. + */ +static inline unsigned long __file_ref_read_raw(file_ref_t *ref) +{ + return atomic_long_read(&ref->refcnt); +} + #endif |