Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754518AbcKUMzm (ORCPT ); Mon, 21 Nov 2016 07:55:42 -0500 Received: from mx1.redhat.com ([209.132.183.28]:47986 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754446AbcKUMzk (ORCPT ); Mon, 21 Nov 2016 07:55:40 -0500 Date: Mon, 21 Nov 2016 13:55:35 +0100 From: Oleg Nesterov To: Davidlohr Bueso Cc: mingo@kernel.org, peterz@infradead.org, john.stultz@linaro.org, dimitrysh@google.com, linux-kernel@vger.kernel.org, Davidlohr Bueso Subject: Re: [PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues with swait Message-ID: <20161121125535.GB1459@redhat.com> References: <1479495277-9075-1-git-send-email-dave@stgolabs.net> <1479495277-9075-3-git-send-email-dave@stgolabs.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1479495277-9075-3-git-send-email-dave@stgolabs.net> User-Agent: Mutt/1.5.18 (2008-05-17) X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.29]); Mon, 21 Nov 2016 12:55:40 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 734 Lines: 28 On 11/18, Davidlohr Bueso wrote: > > @@ -12,7 +12,7 @@ struct percpu_rw_semaphore { > struct rcu_sync rss; > unsigned int __percpu *read_count; > struct rw_semaphore rw_sem; > - wait_queue_head_t writer; > + struct swait_queue_head writer; I won't argue, but even swait_queue_head is overkill in this case. We can just add "struct task_struct *writer" into percpu_rw_semaphore, __percpu_up_read: rcu_read_lock(); writer = task_rcu_dereference(&sem->writer); if (writer) wake_up_process(writer); rcu_read_unlock(); percpu_down_write() can set sem->writer == current and do the simple while-not-condition-schedule() loop. But this probably needs a couple of new helpers, and probably they can have more users. Oleg.