[PATCH 1/2] locking/arch: Avoid variable shadowing in local_try_cmpxchg()

[PATCH 1/2] locking/arch: Avoid variable shadowing in local_try_cmpxchg()

[Uros Bizjak]

Several architectures define arch_try_local_cmpxchg macro using
internal temporary variables named ___old, __old or _old. Remove
temporary varible in local_try_cmpxchg to avoid variable shadowing.

No functional change intended.

Fixes: d994f2c8e241 ("locking/arch: Wire up local_try_cmpxchg()")
Reported-by: Charlemagne Lasse <charlemagnelasse@gmail.com>
Closes: https://lore.kernel.org/lkml/CAFGhKbyxtuk=LoW-E3yLXgcmR93m+Dfo5-u9oQA_YC5Fcy_t9g@mail.gmail.com/
Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
Cc: Jun Yi <yijun@loongson.cn>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 arch/loongarch/include/asm/local.h | 4 ++--
 arch/mips/include/asm/local.h      | 4 ++--
 arch/x86/include/asm/local.h       | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/loongarch/include/asm/local.h b/arch/loongarch/include/asm/local.h
index 83e995b30e47..c49675852bdc 100644
--- a/arch/loongarch/include/asm/local.h
+++ b/arch/loongarch/include/asm/local.h
@@ -63,8 +63,8 @@ static inline long local_cmpxchg(local_t *l, long old, long new)
 
 static inline bool local_try_cmpxchg(local_t *l, long *old, long new)
 {
-	typeof(l->a.counter) *__old = (typeof(l->a.counter) *) old;
-	return try_cmpxchg_local(&l->a.counter, __old, new);
+	return try_cmpxchg_local(&l->a.counter,
+				 (typeof(l->a.counter) *) old, new);
 }
 
 #define local_xchg(l, n) (atomic_long_xchg((&(l)->a), (n)))
diff --git a/arch/mips/include/asm/local.h b/arch/mips/include/asm/local.h
index 5daf6fe8e3e9..e6ae3df0349d 100644
--- a/arch/mips/include/asm/local.h
+++ b/arch/mips/include/asm/local.h
@@ -101,8 +101,8 @@ static __inline__ long local_cmpxchg(local_t *l, long old, long new)
 
 static __inline__ bool local_try_cmpxchg(local_t *l, long *old, long new)
 {
-	typeof(l->a.counter) *__old = (typeof(l->a.counter) *) old;
-	return try_cmpxchg_local(&l->a.counter, __old, new);
+	return try_cmpxchg_local(&l->a.counter,
+				 (typeof(l->a.counter) *) old, new);
 }
 
 #define local_xchg(l, n) (atomic_long_xchg((&(l)->a), (n)))
diff --git a/arch/x86/include/asm/local.h b/arch/x86/include/asm/local.h
index 56d4ef604b91..635132a12778 100644
--- a/arch/x86/include/asm/local.h
+++ b/arch/x86/include/asm/local.h
@@ -127,8 +127,8 @@ static inline long local_cmpxchg(local_t *l, long old, long new)
 
 static inline bool local_try_cmpxchg(local_t *l, long *old, long new)
 {
-	typeof(l->a.counter) *__old = (typeof(l->a.counter) *) old;
-	return try_cmpxchg_local(&l->a.counter, __old, new);
+	return try_cmpxchg_local(&l->a.counter,
+				 (typeof(l->a.counter) *) old, new);
 }
 
 /* Always has a lock prefix */
-- 
2.41.0

[PATCH 2/2] perf/ring_buffer: Use local_try_cmpxchg in __perf_output_begin

[Uros Bizjak]

Use local_try_cmpxchg instead of local_cmpxchg (*ptr, old, new) == old
in __perf_output_begin.  x86 CMPXCHG instruction returns success in ZF
flag, so this change saves a compare after cmpxchg (and related move
instruction in front of cmpxchg).

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails. There is no need to re-read the value in the loop.

No functional change intended.

Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
 kernel/events/ring_buffer.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/events/ring_buffer.c b/kernel/events/ring_buffer.c
index a0433f37b024..fb1e180b5f0a 100644
--- a/kernel/events/ring_buffer.c
+++ b/kernel/events/ring_buffer.c
@@ -191,9 +191,10 @@ __perf_output_begin(struct perf_output_handle *handle,
 
 	perf_output_get_handle(handle);
 
+	offset = local_read(&rb->head);
 	do {
+		head = offset;
 		tail = READ_ONCE(rb->user_page->data_tail);
-		offset = head = local_read(&rb->head);
 		if (!rb->overwrite) {
 			if (unlikely(!ring_buffer_has_space(head, tail,
 							    perf_data_size(rb),
@@ -217,7 +218,7 @@ __perf_output_begin(struct perf_output_handle *handle,
 			head += size;
 		else
 			head -= size;
-	} while (local_cmpxchg(&rb->head, offset, head) != offset);
+	} while (!local_try_cmpxchg(&rb->head, &offset, head));
 
 	if (backward) {
 		offset = head;
-- 
2.41.0