linux-kernel - [PATCH] locking/local_lock: Pretend to use the per-CPU variable if not needed.

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Date:   Wed, 23 Mar 2022 12:09:40 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH] locking/local_lock: Pretend to use the per-CPU variable if
 not needed.

In the !RT && !LOCKDEP case the per-CPU variables aren't used. The commit
mentioned below tried to avoid the usage of this_cpu_ptr() because it
generates code, clobbers registers which is not needed.
This change generated so little reference to the variable that llvm
assumed that it is not used and created a warning.

Revert local_lock_*() to its previous static inline implementation for
type checking.
Replace this_cpu_ptr() with __ll_cpu_ptr() which points to
this_cpu_ptr() when it is used.
In the !RT && !LOCKDEP case use per_cpu_ptr(, 0) which does not leave
any code behind and llvm does not complain either. It also ensures that
it is a per-CPU pointer. The assembly output in this case is unchanged.

Fixes: 9983a9d577db4 ("locking/local_lock: Make the empty local_lock_*() function a macro.")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/local_lock_internal.h | 21 ++++++++++++---------
 1 file changed, 12 insertions(+), 9 deletions(-)

diff --git a/include/linux/local_lock_internal.h b/include/linux/local_lock_internal.h
index 6d635e8306d64..e671ead5fbad5 100644
--- a/include/linux/local_lock_internal.h
+++ b/include/linux/local_lock_internal.h
@@ -24,6 +24,8 @@ typedef struct {
 	},						\
 	.owner = NULL,
 
+#define __ll_cpu_ptr(__ll_cpuptr) (this_cpu_ptr(__ll_cpuptr))
+
 static inline void local_lock_acquire(local_lock_t *l)
 {
 	lock_map_acquire(&l->dep_map);
@@ -44,9 +46,10 @@ static inline void local_lock_debug_init(local_lock_t *l)
 }
 #else /* CONFIG_DEBUG_LOCK_ALLOC */
 # define LOCAL_LOCK_DEBUG_INIT(lockname)
-# 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)
+# define __ll_cpu_ptr(__ll_cpuptr) per_cpu_ptr(__ll_cpuptr, 0)
+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) { }
 #endif /* !CONFIG_DEBUG_LOCK_ALLOC */
 
 #define INIT_LOCAL_LOCK(lockname)	{ LOCAL_LOCK_DEBUG_INIT(lockname) }
@@ -65,36 +68,36 @@ do {								\
 #define __local_lock(lock)					\
 	do {							\
 		preempt_disable();				\
-		local_lock_acquire(this_cpu_ptr(lock));		\
+		local_lock_acquire(__ll_cpu_ptr(lock));		\
 	} while (0)
 
 #define __local_lock_irq(lock)					\
 	do {							\
 		local_irq_disable();				\
-		local_lock_acquire(this_cpu_ptr(lock));		\
+		local_lock_acquire(__ll_cpu_ptr(lock));		\
 	} while (0)
 
 #define __local_lock_irqsave(lock, flags)			\
 	do {							\
 		local_irq_save(flags);				\
-		local_lock_acquire(this_cpu_ptr(lock));		\
+		local_lock_acquire(__ll_cpu_ptr(lock));		\
 	} while (0)
 
 #define __local_unlock(lock)					\
 	do {							\
-		local_lock_release(this_cpu_ptr(lock));		\
+		local_lock_release(__ll_cpu_ptr(lock));		\
 		preempt_enable();				\
 	} while (0)
 
 #define __local_unlock_irq(lock)				\
 	do {							\
-		local_lock_release(this_cpu_ptr(lock));		\
+		local_lock_release(__ll_cpu_ptr(lock));		\
 		local_irq_enable();				\
 	} while (0)
 
 #define __local_unlock_irqrestore(lock, flags)			\
 	do {							\
-		local_lock_release(this_cpu_ptr(lock));		\
+		local_lock_release(__ll_cpu_ptr(lock));		\
 		local_irq_restore(flags);			\
 	} while (0)
 
-- 
2.35.1