From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752894AbbFNTZh (ORCPT ); Sun, 14 Jun 2015 15:25:37 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36918 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752109AbbFNTZa (ORCPT ); Sun, 14 Jun 2015 15:25:30 -0400 Date: Sun, 14 Jun 2015 21:24:22 +0200 From: Oleg Nesterov To: Ingo Molnar Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andy Lutomirski , Andrew Morton , Denys Vlasenko , Brian Gerst , Peter Zijlstra , Borislav Petkov , "H. Peter Anvin" , Linus Torvalds , Thomas Gleixner , Waiman Long , "Paul E. McKenney" Subject: Re: [PATCH 02/12] x86/mm/hotplug: Remove pgd_list use from the memory hotplug code Message-ID: <20150614192422.GA18477@redhat.com> References: <1434188955-31397-1-git-send-email-mingo@kernel.org> <1434188955-31397-3-git-send-email-mingo@kernel.org> <20150613192454.GA1735@redhat.com> <20150614073652.GA5923@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150614073652.GA5923@gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 06/14, Ingo Molnar wrote: > > * Oleg Nesterov wrote: > > > > + spin_lock(&pgd_lock); /* Implies rcu_read_lock() for the task list iteration: */ > > ^^^^^^^^^^^^^^^^^^^^^^^ > > > > Hmm, but it doesn't if PREEMPT_RCU? No, no, I do not pretend I understand how it > > actually works ;) But, say, rcu_check_callbacks() can be called from irq and > > since spin_lock() doesn't increment current->rcu_read_lock_nesting this can lead > > to rcu_preempt_qs()? > > No, RCU grace periods are still defined by 'heavy' context boundaries such as > context switches, entering idle or user-space mode. > > PREEMPT_RCU is like traditional RCU, except that blocking is allowed within the > RCU read critical section - that is why it uses a separate nesting counter > (current->rcu_read_lock_nesting), not the preempt count. Yes. > But if a piece of kernel code is non-preemptible, such as a spinlocked region or > an irqs-off region, then those are still natural RCU read lock regions, regardless > of the RCU model, and need no additional RCU locking. I do not think so. Yes I understand that rcu_preempt_qs() itself doesn't finish the gp, but if there are no other rcu-read-lock holders then it seems synchronize_rcu() on another CPU can return _before_ spin_unlock(), this CPU no longer needs rcu_preempt_note_context_switch(). OK, I can be easily wrong, I do not really understand the implementation of PREEMPT_RCU. Perhaps preempt_disable() can actually act as rcu_read_lock() with the _current_ implementation. Still this doesn't look right even if happens to work, and Documentation/RCU/checklist.txt says: 11. Note that synchronize_rcu() -only- guarantees to wait until all currently executing rcu_read_lock()-protected RCU read-side critical sections complete. It does -not- necessarily guarantee that all currently running interrupts, NMIs, preempt_disable() code, or idle loops will complete. Therefore, if your read-side critical sections are protected by something other than rcu_read_lock(), do -not- use synchronize_rcu(). Oleg.