From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f197.google.com (mail-io0-f197.google.com [209.85.223.197]) by kanga.kvack.org (Postfix) with ESMTP id E20756B0253 for ; Sat, 4 Nov 2017 04:45:10 -0400 (EDT) Received: by mail-io0-f197.google.com with SMTP id k9so15967846iok.4 for ; Sat, 04 Nov 2017 01:45:10 -0700 (PDT) Received: from www262.sakura.ne.jp (www262.sakura.ne.jp. [2001:e42:101:1:202:181:97:72]) by mx.google.com with ESMTPS id 78si7432137ioo.217.2017.11.04.01.45.09 for (version=TLS1 cipher=AES128-SHA bits=128/128); Sat, 04 Nov 2017 01:45:10 -0700 (PDT) Subject: Re: [PATCH v3] printk: Add console owner and waiter logic to loadbalance console writes From: Tetsuo Handa References: <20171103072121.3c2fd5ab@vmware.local.home> <20171103075404.14f9058a@vmware.local.home> <6b1cda44-126d-bf47-66cc-fc80bdb7eb7d@nvidia.com> <201711041732.BFE78178.OFFLOtVQMFHSJO@I-love.SAKURA.ne.jp> In-Reply-To: <201711041732.BFE78178.OFFLOtVQMFHSJO@I-love.SAKURA.ne.jp> Message-Id: <201711041743.GCG95335.OQFLJMFSHOVFtO@I-love.SAKURA.ne.jp> Date: Sat, 4 Nov 2017 17:43:53 +0900 Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Sender: owner-linux-mm@kvack.org List-ID: To: jhubbard@nvidia.com, rostedt@goodmis.org Cc: vbabka@suse.cz, linux-kernel@vger.kernel.org, peterz@infradead.org, akpm@linux-foundation.org, linux-mm@kvack.org, xiyou.wangcong@gmail.com, dave.hansen@intel.com, hannes@cmpxchg.org, mgorman@suse.de, mhocko@kernel.org, pmladek@suse.com, sergey.senozhatsky@gmail.com, yuwang.yuwang@alibaba-inc.com, torvalds@linux-foundation.org, jack@suse.cz, mathieu.desnoyers@efficios.com, penguin-kernel@I-love.SAKURA.ne.jp Tetsuo Handa wrote: > John Hubbard wrote: > > On 11/03/2017 02:46 PM, John Hubbard wrote: > > > On 11/03/2017 04:54 AM, Steven Rostedt wrote: > > >> On Fri, 3 Nov 2017 07:21:21 -0400 > > >> Steven Rostedt wrote: > > [...] > > >> > > >> I'll condense the patch to show what I mean: > > >> > > >> To become a waiter, a task must do the following: > > >> > > >> + printk_safe_enter_irqsave(flags); > > >> + > > >> + raw_spin_lock(&console_owner_lock); > > >> + owner = READ_ONCE(console_owner); > > >> + waiter = READ_ONCE(console_waiter); > > When CPU0 is writing to consoles after "console_owner = current;", > what prevents from CPU1 and CPU2 concurrently reached this line from > seeing waiter == false && owner != NULL && owner != current (which will > concurrently set console_waiter = true and spin = true) without > using atomic instructions? Oops. I overlooked that console_owner_lock is held. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org