linux-kernel - [PATCH] fs: load the ->i_sb pointer once in inode_sb_list

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Message-ID: <20250319004635.1820589-1-mjguzik@gmail.com>
Date: Wed, 19 Mar 2025 01:46:35 +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] fs: load the ->i_sb pointer once in inode_sb_list_{add,del}

While this may sound like a pedantic clean up, it does in fact impact
code generation -- the patched add routine is slightly smaller.

Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
---

Below is disasm before/after. I did not want to pull this into the
commit message because of the total length vs long term usefulness ratio.

can be moved up into the commit message no problem if someone insists on
it:

(gdb) disassemble inode_sb_list_add
before:
 <+0>:     endbr64
 <+4>:     call   0xffffffff8130e9b0 <__fentry__>
 <+9>:     push   %rbx
 <+10>:    mov    0x28(%rdi),%rax
 <+14>:    mov    %rdi,%rbx
 <+17>:    lea    0x540(%rax),%rdi
 <+24>:    call   0xffffffff8225cf20 <_raw_spin_lock>
 <+29>:    mov    0x28(%rbx),%rax
 <+33>:    lea    0x110(%rbx),%rdx
 <+40>:    mov    0x548(%rax),%rcx
 <+47>:    mov    %rdx,0x8(%rcx)
 <+51>:    mov    %rcx,0x110(%rbx)
 <+58>:    lea    0x548(%rax),%rcx
 <+65>:    mov    %rcx,0x118(%rbx)
 <+72>:    mov    %rdx,0x548(%rax)
 <+79>:    mov    0x28(%rbx),%rdi
 <+83>:    pop    %rbx
 <+84>:    add    $0x540,%rdi
 <+91>:    jmp    0xffffffff8225d020 <_raw_spin_unlock>

after:
 <+0>:     endbr64
 <+4>:     call   0xffffffff8130e9b0 <__fentry__>
 <+9>:     push   %r12
 <+11>:    push   %rbp
 <+12>:    push   %rbx
 <+13>:    mov    0x28(%rdi),%rbp
 <+17>:    mov    %rdi,%rbx
 <+20>:    lea    0x540(%rbp),%r12
 <+27>:    mov    %r12,%rdi
 <+30>:    call   0xffffffff8225cf20 <_raw_spin_lock>
 <+35>:    mov    0x548(%rbp),%rdx
 <+42>:    lea    0x110(%rbx),%rax
 <+49>:    mov    %r12,%rdi
 <+52>:    mov    %rax,0x8(%rdx)
 <+56>:    mov    %rdx,0x110(%rbx)
 <+63>:    lea    0x548(%rbp),%rdx
 <+70>:    mov    %rdx,0x118(%rbx)
 <+77>:    mov    %rax,0x548(%rbp)
 <+84>:    pop    %rbx
 <+85>:    pop    %rbp
 <+86>:    pop    %r12
 <+88>:    jmp    0xffffffff8225d020 <_raw_spin_unlock>

 fs/inode.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/fs/inode.c b/fs/inode.c
index 10121fc7b87e..e188bb1eb07a 100644
--- a/fs/inode.c
+++ b/fs/inode.c
@@ -623,18 +623,22 @@ static void inode_wait_for_lru_isolating(struct inode *inode)
  */
 void inode_sb_list_add(struct inode *inode)
 {
-	spin_lock(&inode->i_sb->s_inode_list_lock);
-	list_add(&inode->i_sb_list, &inode->i_sb->s_inodes);
-	spin_unlock(&inode->i_sb->s_inode_list_lock);
+	struct super_block *sb = inode->i_sb;
+
+	spin_lock(&sb->s_inode_list_lock);
+	list_add(&inode->i_sb_list, &sb->s_inodes);
+	spin_unlock(&sb->s_inode_list_lock);
 }
 EXPORT_SYMBOL_GPL(inode_sb_list_add);
 
 static inline void inode_sb_list_del(struct inode *inode)
 {
+	struct super_block *sb = inode->i_sb;
+
 	if (!list_empty(&inode->i_sb_list)) {
-		spin_lock(&inode->i_sb->s_inode_list_lock);
+		spin_lock(&sb->s_inode_list_lock);
 		list_del_init(&inode->i_sb_list);
-		spin_unlock(&inode->i_sb->s_inode_list_lock);
+		spin_unlock(&sb->s_inode_list_lock);
 	}
 }
 
-- 
2.43.0