fs: make the i_size_read/write helpers be smp_load_acquire/store_release()

In [Link] Linus mentions that acquire/release makes it clear which
_particular_ memory accesses are the ordered ones, and it's unlikely
to make any performance difference, so it's much better to pair up
the release->acquire ordering than have a "wmb->rmb" ordering.

=========================================================
 update pagecache
 folio_mark_uptodate(folio)
   smp_wmb()
   set_bit PG_uptodate

 === ↑↑↑ STLR ↑↑↑ === smp_store_release(&inode->i_size, i_size)

 folio_test_uptodate(folio)
   test_bit PG_uptodate
   smp_rmb()

 === ↓↓↓ LDAR ↓↓↓ === smp_load_acquire(&inode->i_size)

 copy_page_to_iter()
=========================================================

Calling smp_store_release() in i_size_write() ensures that the data
in the page and the PG_uptodate bit are updated before the isize is
updated, and calling smp_load_acquire() in i_size_read ensures that
it will not read a newer isize than the data in the page. Therefore,
this avoids buffered read-write inconsistencies caused by Load-Load
reordering.

Link: https://lore.kernel.org/r/CAHk-=wifOnmeJq+sn+2s-P46zw0SFEbw9BSCGgp2c5fYPtRPGw@mail.gmail.com/
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Link: https://lore.kernel.org/r/20240124142857.4146716-2-libaokun1@huawei.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

include/linux/fs.h

@@ -907,7 +907,8 @@ static inline loff_t i_size_read(const struct inode *inode)
 	preempt_enable();
 	return i_size;
 #else
-	return inode->i_size;
+	/* Pairs with smp_store_release() in i_size_write() */
+	return smp_load_acquire(&inode->i_size);
 #endif
 }
 
@@ -929,7 +930,12 @@ static inline void i_size_write(struct inode *inode, loff_t i_size)
 	inode->i_size = i_size;
 	preempt_enable();
 #else
-	inode->i_size = i_size;
+	/*
+	 * Pairs with smp_load_acquire() in i_size_read() to ensure
+	 * changes related to inode size (such as page contents) are
+	 * visible before we see the changed inode size.
+	 */
+	smp_store_release(&inode->i_size, i_size);
 #endif
 }