linux-kernel - [PATCH v2] fs: reduce work in fdget

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Message-ID: <20250319215801.1870660-1-mjguzik@gmail.com>
Date: Wed, 19 Mar 2025 22:58:01 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: brauner@...nel.org
Cc: viro@...iv.linux.org.uk,
	jack@...e.cz,
	linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org,
	Mateusz Guzik <mjguzik@...il.com>
Subject: [PATCH v2] 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@...il.com>
---

v2:
- split up conditions in file_needs_f_pos_lock
- fix sizes in the commit message, i read the offset of the last
  instruction

 fs/file.c                | 11 ++++++++---
 include/linux/file_ref.h | 14 ++++++++++++++
 2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/fs/file.c b/fs/file.c
index ddefb5c80398..0e919bed6058 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -1192,8 +1192,13 @@ 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;
 }
 
 bool file_seek_cur_needs_f_lock(struct file *file)
@@ -1211,7 +1216,7 @@ 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
-- 
2.43.0