Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752854AbbFNTjd (ORCPT ); Sun, 14 Jun 2015 15:39:33 -0400 Received: from mx1.redhat.com ([209.132.183.28]:58115 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752355AbbFNTjc (ORCPT ); Sun, 14 Jun 2015 15:39:32 -0400 Date: Sun, 14 Jun 2015 21:38:25 +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: <20150614193825.GA19582@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> <20150614192422.GA18477@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150614192422.GA18477@redhat.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 Content-Length: 3548 Lines: 102 On 06/14, Oleg Nesterov wrote: > > 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(). I've even checked this ;) I applied the stupid patch below and then $ taskset 2 perl -e 'syscall 157, 666, 5000' & [1] 565 $ taskset 1 perl -e 'syscall 157, 777' $ [1]+ Done taskset 2 perl -e 'syscall 157, 666, 5000' $ dmesg -c SPIN start SYNC start SYNC done! SPIN done! Oleg. --- a/kernel/sys.c +++ b/kernel/sys.c @@ -2049,6 +2049,9 @@ static int prctl_get_tid_address(struct task_struct *me, int __user **tid_addr) } #endif +#include + + SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, unsigned long, arg4, unsigned long, arg5) { @@ -2062,6 +2065,19 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3, error = 0; switch (option) { + case 666: + preempt_disable(); + pr_crit("SPIN start\n"); + while (arg2--) + mdelay(1); + pr_crit("SPIN done!\n"); + preempt_enable(); + break; + case 777: + pr_crit("SYNC start\n"); + synchronize_rcu(); + pr_crit("SYNC done!\n"); + break; case PR_SET_PDEATHSIG: if (!valid_signal(arg2)) { error = -EINVAL; -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/