linux-kernel - [PATCH v2] locking/rwsem: Add __always_inline annotation to __down_write

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Date: Thu, 20 Jun 2024 10:41:53 -0700
From: John Stultz <jstultz@...gle.com>
To: LKML <linux-kernel@...r.kernel.org>
Cc: John Stultz <jstultz@...gle.com>, Tim Murray <timmurray@...gle.com>, 
	Nick Desaulniers <ndesaulniers@...gle.com>, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, 
	Boqun Feng <boqun.feng@...il.com>, kernel-team@...roid.com
Subject: [PATCH v2] locking/rwsem: Add __always_inline annotation to
 __down_write_common() and inlined callers

Apparently despite it being marked inline, the compiler
may not inline __down_write_common() which makes it difficult
to identify the cause of lock contention, as the wchan of the
blocked function will always be listed as __down_write_common().

So add __always_inline annotation to the common function (as
well as the inlined helper callers) to force it to be inlined
so a more useful blocking function will be listed (via wchan).

This mirrors commit 92cc5d00a431 ("locking/rwsem: Add
__always_inline annotation to __down_read_common() and inlined
callers") which did the same for __down_read_common.

I sort of worry that I'm playing wack-a-mole here, and talking
with compiler people, they tell me inline means nothing, which
makes me want to cry a little. So I'm wondering if we need to
replace all the inlines with __always_inline, or remove them
because either we mean something by it, or not.

Cc: Tim Murray <timmurray@...gle.com>
Cc: Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Will Deacon <will@...nel.org>
Cc: Waiman Long <longman@...hat.com>
Cc: Boqun Feng <boqun.feng@...il.com>
Cc: kernel-team@...roid.com
Fixes: c995e638ccbb ("locking/rwsem: Fold __down_{read,write}*()")
Reported-by: Tim Murray <timmurray@...gle.com>
Acked-by: Waiman Long <longman@...hat.com>
Signed-off-by: John Stultz <jstultz@...gle.com>
---
v2:
* Add ack tags & minor tweaks to commit message
---
 kernel/locking/rwsem.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index c6d17aee4209..33cac79e3994 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -1297,7 +1297,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 /*
  * lock for writing
  */
-static inline int __down_write_common(struct rw_semaphore *sem, int state)
+static __always_inline int __down_write_common(struct rw_semaphore *sem, int state)
 {
 	int ret = 0;
 
@@ -1310,12 +1310,12 @@ static inline int __down_write_common(struct rw_semaphore *sem, int state)
 	return ret;
 }
 
-static inline void __down_write(struct rw_semaphore *sem)
+static __always_inline void __down_write(struct rw_semaphore *sem)
 {
 	__down_write_common(sem, TASK_UNINTERRUPTIBLE);
 }
 
-static inline int __down_write_killable(struct rw_semaphore *sem)
+static __always_inline int __down_write_killable(struct rw_semaphore *sem)
 {
 	return __down_write_common(sem, TASK_KILLABLE);
 }
-- 
2.45.2.741.gdbec12cfda-goog