From 3d628f90592cd2e55319ce38380617b726f1fee9 Mon Sep 17 00:00:00 2001
From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Fri, 8 Sep 2023 18:22:49 +0200
Subject: [PATCH 002/204] locking/rtmutex: Avoid unconditional slowpath for
 DEBUG_RT_MUTEXES

With DEBUG_RT_MUTEXES enabled the fast-path rt_mutex_cmpxchg_acquire()
always fails and all lock operations take the slow path.

Provide a new helper inline rt_mutex_try_acquire() which maps to
rt_mutex_cmpxchg_acquire() in the non-debug case. For the debug case
it invokes rt_mutex_slowtrylock() which can acquire a non-contended
rtmutex under full debug coverage.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230908162254.999499-3-bigeasy@linutronix.de
---
 kernel/locking/rtmutex.c     |   21 ++++++++++++++++++++-
 kernel/locking/ww_rt_mutex.c |    2 +-
 2 files changed, 21 insertions(+), 2 deletions(-)

Index: linux-6.6.58-rt45/kernel/locking/rtmutex.c
===================================================================
@ linux-6.6.58-rt45/kernel/locking/rtmutex.c:221 @ static __always_inline bool rt_mutex_cmp
 	return try_cmpxchg_acquire(&lock->owner, &old, new);
 }
 
+static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)
+{
+	return rt_mutex_cmpxchg_acquire(lock, NULL, current);
+}
+
 static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
 						     struct task_struct *old,
 						     struct task_struct *new)
@ linux-6.6.58-rt45/kernel/locking/rtmutex.c:305 @ static __always_inline bool rt_mutex_cmp
 
 }
 
+static int __sched rt_mutex_slowtrylock(struct rt_mutex_base *lock);
+
+static __always_inline bool rt_mutex_try_acquire(struct rt_mutex_base *lock)
+{
+	/*
+	 * With debug enabled rt_mutex_cmpxchg trylock() will always fail.
+	 *
+	 * Avoid unconditionally taking the slow path by using
+	 * rt_mutex_slow_trylock() which is covered by the debug code and can
+	 * acquire a non-contended rtmutex.
+	 */
+	return rt_mutex_slowtrylock(lock);
+}
+
 static __always_inline bool rt_mutex_cmpxchg_release(struct rt_mutex_base *lock,
 						     struct task_struct *old,
 						     struct task_struct *new)
@ linux-6.6.58-rt45/kernel/locking/rtmutex.c:1778 @ static int __sched rt_mutex_slowlock(str
 static __always_inline int __rt_mutex_lock(struct rt_mutex_base *lock,
 					   unsigned int state)
 {
-	if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current)))
+	if (likely(rt_mutex_try_acquire(lock)))
 		return 0;
 
 	return rt_mutex_slowlock(lock, NULL, state);
Index: linux-6.6.58-rt45/kernel/locking/ww_rt_mutex.c
===================================================================
--- linux-6.6.58-rt45.orig/kernel/locking/ww_rt_mutex.c
+++ linux-6.6.58-rt45/kernel/locking/ww_rt_mutex.c
@ linux-6.6.58-rt45/kernel/locking/rtmutex.c:65 @ __ww_rt_mutex_lock(struct ww_mutex *lock
 	}
 	mutex_acquire_nest(&rtm->dep_map, 0, 0, nest_lock, ip);
 
-	if (likely(rt_mutex_cmpxchg_acquire(&rtm->rtmutex, NULL, current))) {
+	if (likely(rt_mutex_try_acquire(&rtm->rtmutex))) {
 		if (ww_ctx)
 			ww_mutex_set_context_fastpath(lock, ww_ctx);
 		return 0;