sched: Simplify mutex_spin_on_owner() (307bf980) · Commits · 戴 / test

It does not make sense to rcu_read_lock/unlock() in every loop iteration while spinning on the mutex. Move the rcu protection outside the loop. Also simplify the return path to always check for lock->owner == NULL which meets the requirements of both owner changed and need_resched() caused loop exits. Signed-off-by:

Thomas Gleixner <tglx@linutronix.de> Signed-off-by:

Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Andrew Morton <akpm@linux-foundation.org> Link: http://lkml.kernel.org/r/alpine.LFD.2.02.1106101458350.11814@ionos Signed-off-by:

Ingo Molnar <mingo@elte.hu>

kernel/sched.c

+9 −16

Original line number	Diff line number	Diff line
		@@ -4306,11 +4306,8 @@ EXPORT_SYMBOL(schedule);

		static inline bool owner_running(struct mutex lock, struct task_struct owner)
		{
		bool ret = false;

		rcu_read_lock();
		if (lock->owner != owner)
		goto fail;
		return false;

		/*
		* Ensure we emit the owner->on_cpu, dereference _after_ checking
		@@ -4320,11 +4317,7 @@ static inline bool owner_running(struct mutex lock, struct task_struct owner)
		*/
		barrier();

		ret = owner->on_cpu;
		fail:
		rcu_read_unlock();

		return ret;
		return owner->on_cpu;
		}

		/*
		@@ -4336,21 +4329,21 @@ int mutex_spin_on_owner(struct mutex lock, struct task_struct owner)
		if (!sched_feat(OWNER_SPIN))
		return 0;

		rcu_read_lock();
		while (owner_running(lock, owner)) {
		if (need_resched())
		return 0;
		break;

		arch_mutex_cpu_relax();
		}
		rcu_read_unlock();

		/*
		* If the owner changed to another task there is likely
		* heavy contention, stop spinning.
		* We break out the loop above on need_resched() and when the
		* owner changed, which is a sign for heavy contention. Return
		* success only when lock->owner is NULL.
		*/
		if (lock->owner)
		return 0;

		return 1;
		return lock->owner == NULL;
		}
		#endif

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