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SubjectRe: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
On Wed, 13 Nov 2019, Peter Zijlstra wrote:
>@@ -54,23 +52,23 @@ static bool __percpu_down_read_trylock(s
> * the same CPU as the increment, avoiding the
> * increment-on-one-CPU-and-decrement-on-another problem.

Nit: Now that you've made read_count more symmetric, maybe this first
paragraph can be moved down to __percpu_rwsem_trylock() reader side,
as such:

/*
* Due to having preemption disabled the decrement happens on
* the same CPU as the increment, avoiding the
* increment-on-one-CPU-and-decrement-on-another problem.
*/
preempt_disable();
ret = __percpu_down_read_trylock(sem);
preempt_enable();

> *
>- * If the reader misses the writer's assignment of readers_block, then
>- * the writer is guaranteed to see the reader's increment.
>+ * If the reader misses the writer's assignment of sem->block, then the
>+ * writer is guaranteed to see the reader's increment.

...

> 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.

>@@ -117,7 +174,7 @@ void __percpu_up_read(struct percpu_rw_s
> */
> __this_cpu_dec(*sem->read_count);
>
>- /* Prod writer to recheck readers_active */
>+ /* Prod writer to re-evaluate readers_active_check() */
> rcuwait_wake_up(&sem->writer);
> }
> EXPORT_SYMBOL_GPL(__percpu_up_read);
>@@ -137,6 +194,8 @@ EXPORT_SYMBOL_GPL(__percpu_up_read);
> * zero. If this sum is zero, then it is stable due to the fact that if any
> * newly arriving readers increment a given counter, they will immediately
> * decrement that same counter.
>+ *
>+ * Assumes sem->block is set.
> */
> static bool readers_active_check(struct percpu_rw_semaphore *sem)
> {
>@@ -160,23 +219,22 @@ void percpu_down_write(struct percpu_rw_
> /* Notify readers to take the slow path. */
> rcu_sync_enter(&sem->rss);
>
>- __down_write(&sem->rw_sem);
>-
> /*
>- * Notify new readers to block; up until now, and thus throughout the
>- * longish rcu_sync_enter() above, new readers could still come in.
>+ * Try set sem->block; this provides writer-writer exclusion.
>+ * Having sem->block set makes new readers block.
> */
>- WRITE_ONCE(sem->readers_block, 1);
>+ if (!__percpu_down_write_trylock(sem))
>+ percpu_rwsem_wait(sem, /* .reader = */ false);
>
>- smp_mb(); /* D matches A */
>+ /* smp_mb() implied by __percpu_down_writer_trylock() on success -- D matches A */
^^^
write
...

>--- a/kernel/locking/rwsem.h
>+++ b/kernel/locking/rwsem.h
>@@ -1,12 +0,0 @@
>-/* SPDX-License-Identifier: GPL-2.0 */
>-
>-#ifndef __INTERNAL_RWSEM_H
>-#define __INTERNAL_RWSEM_H
>-#include <linux/rwsem.h>
>-
>-extern void __down_read(struct rw_semaphore *sem);
>-extern void __up_read(struct rw_semaphore *sem);
>-extern void __down_write(struct rw_semaphore *sem);
>-extern void __up_write(struct rw_semaphore *sem);

This is a nice side effect.

Thanks,
Davidlohr

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