Re: [RFC PATCH] locking/percpu-rwsem: use this_cpu_{inc|dec}() for read_count

peterz
2020-09-18T10:01:33Z

On Fri, Sep 18, 2020 at 11:07:02AM +0200, Jan Kara wrote:
> If people really wanted to avoid irq-safe inc/dec for archs where it is
> more expensive, one idea I had was that we could add 'read_count_in_irq' to
> percpu_rw_semaphore. So callers in normal context would use read_count and
> callers in irq context would use read_count_in_irq. And the writer side
> would sum over both but we don't care about performance of that one much.

That's not a bad idea... something like so I suppose.

(completely untested)

---
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index 5fda40f97fe9..9c847490a86a 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -11,7 +11,7 @@

struct percpu_rw_semaphore {
struct rcu_sync rss;
- unsigned int __percpu *read_count;
+ u32 __percpu *read_count;
struct rcuwait writer;
wait_queue_head_t waiters;
atomic_t block;
@@ -60,7 +60,7 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
* anything we did within this RCU-sched read-size critical section.
*/
if (likely(rcu_sync_is_idle(&sem->rss)))
- this_cpu_inc(*sem->read_count);
+ __this_cpu_inc(sem->read_count[0]);
else
__percpu_down_read(sem, false); /* Unconditional memory barrier */
/*
@@ -74,12 +74,16 @@ static inline bool percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
bool ret = true;

+#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
+ WARN_ON_ONCE(!in_task());
+#endif
+
preempt_disable();
/*
* Same as in percpu_down_read().
*/
if (likely(rcu_sync_is_idle(&sem->rss)))
- this_cpu_inc(*sem->read_count);
+ __this_cpu_inc(sem->read_count[0]);
else
ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
preempt_enable();
@@ -98,12 +102,16 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
{
rwsem_release(&sem->dep_map, _RET_IP_);

+#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
+ WARN_ON_ONCE(!in_task());
+#endif
+
preempt_disable();
/*
* Same as in percpu_down_read().
*/
if (likely(rcu_sync_is_idle(&sem->rss))) {
- this_cpu_dec(*sem->read_count);
+ __this_cpu_dec(sem->read_count[0]);
} else {
/*
* slowpath; reader will only ever wake a single blocked
@@ -115,12 +123,39 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
* aggregate zero, as that is the only time it matters) they
* will also see our critical section.
*/
- this_cpu_dec(*sem->read_count);
+ __this_cpu_dec(sem->read_count[0]);
rcuwait_wake_up(&sem->writer);
}
preempt_enable();
}

+static inline void __percpu_up_read_irqsafe(struct percpu_rw_semaphore *sem)
+{
+ unsigned long flags;
+
+ local_irq_save(flags);
+ /*
+ * Same as in percpu_down_read().
+ */
+ if (likely(rcu_sync_is_idle(&sem->rss))) {
+ __this_cpu_dec(sem->read_count[1]);
+ } else {
+ /*
+ * slowpath; reader will only ever wake a single blocked
+ * writer.
+ */
+ smp_mb(); /* B matches C */
+ /*
+ * In other words, if they see our decrement (presumably to
+ * aggregate zero, as that is the only time it matters) they
+ * will also see our critical section.
+ */
+ __this_cpu_dec(sem->read_count[1]);
+ rcuwait_wake_up(&sem->writer);
+ }
+ local_irq_restore(flags);
+}
+
extern void percpu_down_write(struct percpu_rw_semaphore *);
extern void percpu_up_write(struct percpu_rw_semaphore *);

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 70a32a576f3f..00741216a7f6 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -12,7 +12,7 @@
int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
const char *name, struct lock_class_key *key)
{
- sem->read_count = alloc_percpu(int);
+ sem->read_count = (u32 *)alloc_percpu(u64);
if (unlikely(!sem->read_count))
return -ENOMEM;

@@ -45,7 +45,7 @@ EXPORT_SYMBOL_GPL(percpu_free_rwsem);

static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
{
- this_cpu_inc(*sem->read_count);
+ __this_cpu_inc(sem->read_count[0]);

/*
* Due to having preemption disabled the decrement happens on
@@ -71,7 +71,7 @@ static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
if (likely(!atomic_read_acquire(&sem->block)))
return true;

- this_cpu_dec(*sem->read_count);
+ __this_cpu_dec(sem->read_count[0]);

/* Prod writer to re-evaluate readers_active_check() */
rcuwait_wake_up(&sem->writer);
@@ -198,7 +198,9 @@ EXPORT_SYMBOL_GPL(__percpu_down_read);
*/
static bool readers_active_check(struct percpu_rw_semaphore *sem)
{
- if (per_cpu_sum(*sem->read_count) != 0)
+ u64 sum = per_cpu_sum(*(u64 *)sem->read_count);
+
+ if (sum + (sum >> 32))
return false;

/*