vfs: only read fops once in fops_get/put

In do_dentry_open() the usage is:
	f->f_op = fops_get(inode->i_fop);

In generated asm the compiler emits 2 reads from inode->i_fop instead of
just one.

This popped up due to false-sharing where loads from that offset end up
bouncing a cacheline during parallel open. While this is going to be fixed,
the spurious load does not need to be there.

This makes do_dentry_open() go down from 1177 to 1154 bytes.

fops_put() is patched to maintain some consistency.

No functional changes.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Link: https://lore.kernel.org/r/20240810064753.1211441-1-mjguzik@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c0286d479c9d43393a257321fe9d0683c6e7812c..696c620f90e0bd0605f86b5896d84d6ca38b1624 100644 (file)
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2553,10 +2553,17 @@ struct super_block *sget(struct file_system_type *type,
 struct super_block *sget_dev(struct fs_context *fc, dev_t dev);
 
 /* Alas, no aliases. Too much hassle with bringing module.h everywhere */
-#define fops_get(fops) \
-       (((fops) && try_module_get((fops)->owner) ? (fops) : NULL))
-#define fops_put(fops) \
-       do { if (fops) module_put((fops)->owner); } while(0)
+#define fops_get(fops) ({                                              \
+       const struct file_operations *_fops = (fops);                   \
+       (((_fops) && try_module_get((_fops)->owner) ? (_fops) : NULL)); \
+})
+
+#define fops_put(fops) ({                                              \
+       const struct file_operations *_fops = (fops);                   \
+       if (_fops)                                                      \
+               module_put((_fops)->owner);                             \
+})
+
 /*
  * This one is to be used *ONLY* from ->open() instances.
  * fops must be non-NULL, pinned down *and* module dependencies