linux-kernel - [PATCH 0/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG

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Message-Id: <20220525144013.6481-1-ubizjak@gmail.com>
Date:   Wed, 25 May 2022 16:40:11 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>, Waiman.Long@...com,
        paulmck@...ux.vnet.ibm.com
Subject: [PATCH 0/2] locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro

This patch series introduces try_cmpxchg64 into CMPXCHG_LOOP macro
to improve generated code a bit. x86 CMPXCHG instruction returns success
in ZF flag, so a compare after CMPXCHG and a related move instruction
can be eliminated.

The second patch (optionally) enables lockless reference count updates
for X86_32 target with X86_CMPXCHG64 config flag set. When try_cmpxchg64
is used in CMPXCHG_LOOP macro in lib/lockref.c, the compiler avoids
double-word compare and related move and produces quite optimal code
around CMPXCHG8B for a register starved X86_32 target.

Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Waiman.Long@...com
Cc: paulmck@...ux.vnet.ibm.com
---
Uros Bizjak (2):
  locking/lockref: Use try_cmpxchg64 in CMPXCHG_LOOP macro
  locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86_32 &&
    X86_CMPXCHG64

 arch/x86/Kconfig | 2 +-
 lib/lockref.c    | 9 ++++-----
 2 files changed, 5 insertions(+), 6 deletions(-)

-- 
2.35.3