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From: Oleg Nesterov <oleg@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: mingo@kernel.org, will@kernel.org, tglx@linutronix.de,
	linux-kernel@vger.kernel.org, bigeasy@linutronix.de,
	juri.lelli@redhat.com, williams@redhat.com, bristot@redhat.com,
	longman@redhat.com, dave@stgolabs.net, jack@suse.com
Subject: Re: [PATCH 4/5] locking/percpu-rwsem: Extract __percpu_down_read_trylock()
Date: Mon, 18 Nov 2019 17:28:38 +0100	[thread overview]
Message-ID: <20191118162837.GA3025@redhat.com> (raw)
In-Reply-To: <20191113102855.868390100@infradead.org>

Hi Peter, sorry for delay.

I'll re-read this series tomorrow, but everything looks correct at first
glance...

Except one very minor problem in this patch, see below.

On 11/13, Peter Zijlstra wrote:
>
> -bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
> +static bool __percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
>  {
>  	__this_cpu_inc(*sem->read_count);
>  
> @@ -70,14 +70,21 @@ bool __percpu_down_read(struct percpu_rw
>  	 * If !readers_block the critical section starts here, matched by the
>  	 * release in percpu_up_write().
>  	 */
> -	if (likely(!smp_load_acquire(&sem->readers_block)))
> +	if (likely(!atomic_read_acquire(&sem->readers_block)))

I don't think this can be compiled ;) ->readers_block is "int" until the next
patch makes it atomic_t and renames to ->block.

And. I think __percpu_down_read_trylock() should do

	if (atomic_read(&sem->block))
		return false;

at the start, before __this_cpu_inc(read_count).

Suppose that the pending writer sleeps in rcuwait_wait_event(readers_active_check).
If the new reader comes, it is better to not wake up that writer.

Oleg.


  reply	other threads:[~2019-11-18 16:28 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 [this message]
2019-11-13 10:21 ` [PATCH 5/5] locking/percpu-rwsem: Remove the embedded rwsem Peter Zijlstra
2019-11-18 19:53   ` Davidlohr Bueso
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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