Re: [PATCH v6 04/46] percpu_rwlock: Implement the core design of Per-CPU Reader-Writer Locks

[Lai Jiangshan <eag0628@gmail.com>]

Hi, Srivatsa,

The target of the whole patchset is nice for me.
A question: How did you find out the such usages of
"preempt_disable()" and convert them? did all are converted?

And I think the lock is too complex and reinvent the wheel, why don't
you reuse the lglock?
I wrote an untested draft here.

Thanks,
Lai

PS: Some HA tools(I'm writing one) which takes checkpoints of
virtual-machines frequently, I guess this patchset can speedup the
tools.

>From 01db542693a1b7fc6f9ece45d57cb529d9be5b66 Mon Sep 17 00:00:00 2001
From: Lai Jiangshan <laijs@cn.fujitsu.com>
Date: Mon, 25 Feb 2013 23:14:27 +0800
Subject: [PATCH] lglock: add read-preference local-global rwlock

locality via lglock(trylock)
read-preference read-write-lock via fallback rwlock_t

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
 include/linux/lglock.h |   31 +++++++++++++++++++++++++++++++
 kernel/lglock.c        |   45 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 76 insertions(+), 0 deletions(-)

diff --git a/include/linux/lglock.h b/include/linux/lglock.h
index 0d24e93..30fe887 100644
--- a/include/linux/lglock.h
+++ b/include/linux/lglock.h
@@ -67,4 +67,35 @@ void lg_local_unlock_cpu(struct lglock *lg, int cpu);
 void lg_global_lock(struct lglock *lg);
 void lg_global_unlock(struct lglock *lg);

+struct lgrwlock {
+	unsigned long __percpu *fallback_reader_refcnt;
+	struct lglock lglock;
+	rwlock_t fallback_rwlock;
+};
+
+#define DEFINE_LGRWLOCK(name)						\
+	static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock)		\
+	= __ARCH_SPIN_LOCK_UNLOCKED;					\
+	static DEFINE_PER_CPU(unsigned long, name ## _refcnt);		\
+	struct lgrwlock name = {					\
+		.fallback_reader_refcnt = &name ## _refcnt,		\
+		.lglock = { .lock = &name ## _lock } }
+
+#define DEFINE_STATIC_LGRWLOCK(name)					\
+	static DEFINE_PER_CPU(arch_spinlock_t, name ## _lock)		\
+	= __ARCH_SPIN_LOCK_UNLOCKED;					\
+	static DEFINE_PER_CPU(unsigned long, name ## _refcnt);		\
+	static struct lgrwlock name = {					\
+		.fallback_reader_refcnt = &name ## _refcnt,		\
+		.lglock = { .lock = &name ## _lock } }
+
+static inline void lg_rwlock_init(struct lgrwlock *lgrw, char *name)
+{
+	lg_lock_init(&lgrw->lglock, name);
+}
+
+void lg_rwlock_local_read_lock(struct lgrwlock *lgrw);
+void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw);
+void lg_rwlock_global_write_lock(struct lgrwlock *lgrw);
+void lg_rwlock_global_write_unlock(struct lgrwlock *lgrw);
 #endif
diff --git a/kernel/lglock.c b/kernel/lglock.c
index 6535a66..463543a 100644
--- a/kernel/lglock.c
+++ b/kernel/lglock.c
@@ -87,3 +87,48 @@ void lg_global_unlock(struct lglock *lg)
 	preempt_enable();
 }
 EXPORT_SYMBOL(lg_global_unlock);
+
+void lg_rwlock_local_read_lock(struct lgrwlock *lgrw)
+{
+	struct lglock *lg = &lgrw->lglock;
+
+	preempt_disable();
+	if (likely(!__this_cpu_read(*lgrw->fallback_reader_refcnt))) {
+		if (likely(arch_spin_trylock(this_cpu_ptr(lg->lock)))) {
+			rwlock_acquire_read(&lg->lock_dep_map, 0, 0, _RET_IP_);
+			return;
+		}
+		read_lock(&lgrw->fallback_rwlock);
+	}
+
+	__this_cpu_inc(*lgrw->fallback_reader_refcnt);
+}
+EXPORT_SYMBOL(lg_rwlock_local_read_lock);
+
+void lg_rwlock_local_read_unlock(struct lgrwlock *lgrw)
+{
+	if (likely(!__this_cpu_read(*lgrw->fallback_reader_refcnt))) {
+		lg_local_unlock(&lgrw->lglock);
+		return;
+	}
+
+	if (!__this_cpu_dec_return(*lgrw->fallback_reader_refcnt))
+		read_unlock(&lgrw->fallback_rwlock);
+
+	preempt_enable();
+}
+EXPORT_SYMBOL(lg_rwlock_local_read_unlock);
+
+void lg_rwlock_global_write_lock(struct lgrwlock *lgrw)
+{
+	lg_global_lock(&lgrw->lglock);
+	write_lock(&lgrw->fallback_rwlock);
+}
+EXPORT_SYMBOL(lg_rwlock_global_write_lock);
+
+void lg_rwlock_global_write_unlock(struct lgrwlock *lgrw)
+{
+	write_unlock(&lgrw->fallback_rwlock);
+	lg_global_unlock(&lgrw->lglock);
+}
+EXPORT_SYMBOL(lg_rwlock_global_write_unlock);
-- 
1.7.7.6