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SubjectRe: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
On Mon, 18 Nov 2019, Davidlohr Bueso wrote:

>On Wed, 13 Nov 2019, Peter Zijlstra wrote:
3>>bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
>>{
>> if (__percpu_down_read_trylock(sem))
>>@@ -89,20 +156,10 @@ bool __percpu_down_read(struct percpu_rw
>> if (try)
>> return false;
>>
>>- /*
>>- * We either call schedule() in the wait, or we'll fall through
>>- * and reschedule on the preempt_enable() in percpu_down_read().
>>- */
>>- preempt_enable_no_resched();
>>-
>>- /*
>>- * Avoid lockdep for the down/up_read() we already have them.
>>- */
>>- __down_read(&sem->rw_sem);
>>- this_cpu_inc(*sem->read_count);
>>- __up_read(&sem->rw_sem);
>>-
>>+ preempt_enable();
>>+ percpu_rwsem_wait(sem, /* .reader = */ true );
>> preempt_disable();
>>+
>> return true;
>>}
>>EXPORT_SYMBOL_GPL(__percpu_down_read);
>
>Do we really need to export symbol here? This function is only called
>from percpu-rwsem.h.

Similarly, afaict we can get rid of __percpu_up_read() and put the
slowpath all into percpu_up_read(). Also explicitly mention the
single task nature of the writer (which is a better comment for
the rcuwait_wake_up()).

diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h
index f5ecf6a8a1dd..eda545f42fb8 100644
--- a/include/linux/percpu-rwsem.h
+++ b/include/linux/percpu-rwsem.h
@@ -43,7 +43,6 @@ is_static struct percpu_rw_semaphore name = { \
__DEFINE_PERCPU_RWSEM(name, static)

extern bool __percpu_down_read(struct percpu_rw_semaphore *, bool);
-extern void __percpu_up_read(struct percpu_rw_semaphore *);

static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
{
@@ -103,10 +102,23 @@ static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
/*
* Same as in percpu_down_read().
*/
- if (likely(rcu_sync_is_idle(&sem->rss)))
+ if (likely(rcu_sync_is_idle(&sem->rss))) {
__this_cpu_dec(*sem->read_count);
- else
- __percpu_up_read(sem); /* Unconditional memory barrier */
+ goto done;
+ }
+
+ /*
+ * 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);
+ rcuwait_wake_up(&sem->writer);
+done:
preempt_enable();
}

diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
index 851038468efb..a5150a876626 100644
--- a/kernel/locking/percpu-rwsem.c
+++ b/kernel/locking/percpu-rwsem.c
@@ -164,21 +164,6 @@ bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
}
EXPORT_SYMBOL_GPL(__percpu_down_read);

-void __percpu_up_read(struct percpu_rw_semaphore *sem)
-{
- 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);
-
- /* Prod writer to re-evaluate readers_active_check() */
- rcuwait_wake_up(&sem->writer);
-}
-EXPORT_SYMBOL_GPL(__percpu_up_read);
-
#define per_cpu_sum(var) \
({ \
typeof(var) __sum = 0; \
\
 
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