Received-SPF: pass (google.com: domain of linux-kernel+bounces-37110-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
From: Baokun Li <libaokun1@huawei.com>
To: <linux-fsdevel@vger.kernel.org>
CC: <torvalds@linux-foundation.org>, <viro@zeniv.linux.org.uk>,
	<brauner@kernel.org>, <jack@suse.cz>, <willy@infradead.org>,
	<akpm@linux-foundation.org>, <arnd@arndb.de>, <linux-arch@vger.kernel.org>,
	<linux-kernel@vger.kernel.org>, <yi.zhang@huawei.com>,
	<yangerkun@huawei.com>, <yukuai3@huawei.com>, <libaokun1@huawei.com>
Subject: [PATCH v2 1/3] fs: make the i_size_read/write helpers be smp_load_acquire/store_release()
Date: Wed, 24 Jan 2024 22:28:55 +0800
Message-ID: <20240124142857.4146716-2-libaokun1@huawei.com>
In-Reply-To: <20240124142857.4146716-1-libaokun1@huawei.com>
References: <20240124142857.4146716-1-libaokun1@huawei.com>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: 8bit

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>
---
 include/linux/fs.h | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 6bb10bbd7035..1cc1f3f08107 100644
--- a/include/linux/fs.h
+++ b/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
 }
 
-- 
2.31.1