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

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Date: Fri,  5 Apr 2024 13:05:26 -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: [RFC][PATCH] 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 blocked
function in traceevents 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 the blocking function will be listed (via Wchan) in
traceevents.

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>
Signed-off-by: John Stultz <jstultz@...gle.com>
---
 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);
 }
-- 
2.44.0.478.gd926399ef9-goog