From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, will@kernel.org, oleg@redhat.com,
tglx@linutronix.de, linux-kernel@vger.kernel.org,
bigeasy@linutronix.de, juri.lelli@redhat.com,
williams@redhat.com, bristot@redhat.com, longman@redhat.com,
jack@suse.com
Subject: Re: [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem
Date: Mon, 18 Nov 2019 11:53:04 -0800 [thread overview]
Message-ID: <20191118195304.b3d6fg4jmmj7kmfh@linux-p48b> (raw)
In-Reply-To: <20191113102855.925208237@infradead.org>
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
next prev parent reply other threads:[~2019-11-18 19:57 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-13 10:21 [PATCH 0/5] locking: Percpu-rwsem rewrite Peter Zijlstra
2019-11-13 10:21 ` [PATCH 1/5] locking/percpu-rwsem, lockdep: Make percpu-rwsem use its own lockdep_map Peter Zijlstra
2019-11-15 20:39 ` Davidlohr Bueso
2020-01-08 1:33 ` [PATCH] locking/percpu-rwsem: Add might_sleep() for writer locking Davidlohr Bueso
2020-01-08 1:33 ` Davidlohr Bueso
2020-02-11 12:48 ` [tip: locking/core] " tip-bot2 for Davidlohr Bueso
2019-11-13 10:21 ` [PATCH 2/5] locking/percpu-rwsem: Convert to bool Peter Zijlstra
2019-11-13 10:21 ` [PATCH 3/5] locking/percpu-rwsem: Move __this_cpu_inc() into the slowpath Peter Zijlstra
2019-11-13 10:21 ` [PATCH 4/5] locking/percpu-rwsem: Extract __percpu_down_read_trylock() Peter Zijlstra
2019-11-18 16:28 ` Oleg Nesterov
2019-11-13 10:21 ` [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem Peter Zijlstra
2019-11-18 19:53 ` Davidlohr Bueso [this message]
2019-11-18 23:19 ` Davidlohr Bueso
2019-12-17 10:45 ` Peter Zijlstra
2019-12-17 10:35 ` Peter Zijlstra
2019-11-18 21:52 ` Waiman Long
2019-12-17 10:28 ` Peter Zijlstra
2019-11-19 13:50 ` Waiman Long
2019-11-19 15:58 ` Oleg Nesterov
2019-11-19 16:28 ` Waiman Long
2019-12-17 10:26 ` Peter Zijlstra
2019-12-17 10:28 ` Peter Zijlstra
2019-11-15 17:14 ` [PATCH 0/5] locking: Percpu-rwsem rewrite Juri Lelli
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