From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753747AbcEIHjh (ORCPT ); Mon, 9 May 2016 03:39:37 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:48621 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753728AbcEIHjg (ORCPT ); Mon, 9 May 2016 03:39:36 -0400 Date: Mon, 9 May 2016 09:39:33 +0200 From: Peter Zijlstra To: Davidlohr Bueso Cc: mingo@kernel.org, tglx@linutronix.de, Waiman.Long@hpe.com, jason.low2@hp.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/4] locking/rwsem: Drop superfluous waiter refcount Message-ID: <20160509073933.GC3430@twins.programming.kicks-ass.net> References: <1462769770-29363-1-git-send-email-dave@stgolabs.net> <1462769770-29363-3-git-send-email-dave@stgolabs.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1462769770-29363-3-git-send-email-dave@stgolabs.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, May 08, 2016 at 09:56:08PM -0700, Davidlohr Bueso wrote: > Read waiters are currently reference counted from the time it enters > the slowpath until the lock is released and the waiter is awoken. This > is fragile and superfluous considering everything occurs within down_read() > without returning to the caller, and the very nature of the primitive does > not suggest that the task can disappear from underneath us. In addition, > spurious wakeups can make the whole refcount useless as get_task_struct() > is only called when setting up the waiter. So I think you're wrong here; imagine this: rwsem_down_read_failed() rwsem_wake() get_task_struct(); raw_spin_lock_irq(&wait_lock); list_add_tail(&waiter.list, &wait_list); raw_spin_unlock_irq(&wait_lock); raw_spin_lock_irqsave(&wait_lock) __rwsem_do_wake() while (true) { set_task_state(tsk, TASK_UNINTERRUPTIBLE); waiter->task = NULL if (!waiter.task) // true break; __set_task_state(tsk, TASK_RUNNING); do_exit(); wake_up_process(tsk); /* BOOM */