From: Sebastian Andrzej Siewior Date: Fri, 8 Sep 2023 18:22:49 +0200 Subject: [PATCH 2/7] locking/rtmutex: Avoid unconditional slowpath for DEBUG_RT_MUTEXES Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.6/older/patches-6.6.7-rt18.tar.xz 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 Signed-off-by: Thomas Gleixner Signed-off-by: Sebastian Andrzej Siewior Signed-off-by: Peter Zijlstra (Intel) 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(-) --- 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 @@ static int __sched } 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;