linux-kernel - [PATCH REPOST] locking/local_lock: Make the empty local_lock

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Message-ID: <YgKjciR60fZft2l4@linutronix.de>
Date:   Tue, 8 Feb 2022 18:08:02 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH REPOST] locking/local_lock: Make the empty local_lock_*()
 function a macro.

It has been said that local_lock() does not add any overhead compared to
preempt_disable() in a !LOCKDEP configuration. A micro benchmark showed
an unexpected result which can be reduced to the fact that local_lock()
was not entirely optimized away.
In the !LOCKDEP configuration local_lock_acquire() is an empty static
inline function. On x86 the this_cpu_ptr() argument of that function is
fully evaluated leading to an additional mov+add instructions which are
not needed and not used.

Replace the static inline function with a macro. The typecheck() macro
ensures that the argument is of proper type while the resulting
disassembly shows no traces of this_cpu_ptr().

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Reviewed-by: Waiman Long <longman@...hat.com>
---
Repost of
   https://lkml.kernel.org/r/20220105202623.1118172-1-bigeasy@linutronix.de

 include/linux/local_lock_internal.h |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/include/linux/local_lock_internal.h
+++ b/include/linux/local_lock_internal.h
@@ -44,9 +44,9 @@ static inline void local_lock_debug_init
 }
 #else /* CONFIG_DEBUG_LOCK_ALLOC */
 # define LOCAL_LOCK_DEBUG_INIT(lockname)
-static inline void local_lock_acquire(local_lock_t *l) { }
-static inline void local_lock_release(local_lock_t *l) { }
-static inline void local_lock_debug_init(local_lock_t *l) { }
+# define local_lock_acquire(__ll)  do { typecheck(local_lock_t *, __ll); } while (0)
+# define local_lock_release(__ll)  do { typecheck(local_lock_t *, __ll); } while (0)
+# define local_lock_debug_init(__ll)  do { typecheck(local_lock_t *, __ll); } while (0)
 #endif /* !CONFIG_DEBUG_LOCK_ALLOC */
 
 #define INIT_LOCAL_LOCK(lockname)	{ LOCAL_LOCK_DEBUG_INIT(lockname) }