From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752735AbcELMMU (ORCPT <rfc822;w@1wt.eu>);
	Thu, 12 May 2016 08:12:20 -0400
Received: from merlin.infradead.org ([205.233.59.134]:60791 "EHLO
	merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1751781AbcELMMT (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 12 May 2016 08:12:19 -0400
Date: Thu, 12 May 2016 14:12:04 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Michal Hocko <mhocko@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        LKML <linux-kernel@vger.kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        "David S. Miller" <davem@davemloft.net>,
        Tony Luck <tony.luck@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Chris Zankel <chris@zankel.net>, Max Filippov <jcmvbkbc@gmail.com>,
        Davidlohr Bueso <dave@stgolabs.net>, Waiman Long <Waiman.Long@hpe.com>
Subject: Re: [PATCH 03/11] locking, rwsem: introduce basis for
 down_write_killable
Message-ID: <20160512121204.GQ3192@twins.programming.kicks-ass.net>
References: <20160510123806.GB3193@twins.programming.kicks-ass.net>
 <20160511072357.GC16677@dhcp22.suse.cz>
 <20160511082853.GF16677@dhcp22.suse.cz>
 <20160511084401.GH3193@twins.programming.kicks-ass.net>
 <20160511090442.GH16677@dhcp22.suse.cz>
 <20160511091733.GC3192@twins.programming.kicks-ass.net>
 <20160511093127.GI16677@dhcp22.suse.cz>
 <20160511094128.GB3190@twins.programming.kicks-ass.net>
 <20160511135938.GA19577@dhcp22.suse.cz>
 <20160511180345.GA27728@dhcp22.suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20160511180345.GA27728@dhcp22.suse.cz>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, May 11, 2016 at 08:03:46PM +0200, Michal Hocko wrote:
> I still cannot say I would understand why the pending
> RWSEM_WAITING_BIAS matters but I would probably need to look at the code
> again with a clean head, __rwsem_wake is quite tricky...

Ah, you're asking why an unconditional __rwsem_wake(ANY) isn't enough?

Because; if at that point there's nobody waiting, we're left with an
empty list and WAITER_BIAS set. This in turn will make all fast paths
fail.

Look at rwsem_down_read_failed() for instance; if we enter that we'll
unconditionally queue ourself, with nobody left to come wake us.