[PATCH 2/6] locking/rtmutex: Avoid unconditional slowpath for DEBUG_RT_MUTEXES

Peter Zijlstra
2023-08-15T11:18:06Z

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

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/20230427111937.2745231-4-bigeasy@linutronix.de
---
kernel/locking/rtmutex.c | 21 ++++++++++++++++++++-
kernel/locking/ww_rt_mutex.c | 2 +-
2 files changed, 21 insertions(+), 2 deletions(-)

--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -218,6 +218,11 @@ 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)
@@ -297,6 +302,20 @@ 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)
@@ -1755,7 +1774,7 @@ 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);
--- a/kernel/locking/ww_rt_mutex.c
+++ b/kernel/locking/ww_rt_mutex.c
@@ -62,7 +62,7 @@ __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;