From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f173.google.com (mail-wi0-f173.google.com [209.85.212.173]) by kanga.kvack.org (Postfix) with ESMTP id 64AF46B0099 for ; Fri, 19 Jun 2015 13:28:21 -0400 (EDT) Received: by wibee9 with SMTP id ee9so15856190wib.0 for ; Fri, 19 Jun 2015 10:28:21 -0700 (PDT) Received: from gum.cmpxchg.org (gum.cmpxchg.org. [85.214.110.215]) by mx.google.com with ESMTPS id s1si5810988wiy.4.2015.06.19.10.28.19 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 19 Jun 2015 10:28:20 -0700 (PDT) Date: Fri, 19 Jun 2015 13:28:02 -0400 From: Johannes Weiner Subject: Re: [PATCH] mm: memcontrol: correct the comment in mem_cgroup_swapout() Message-ID: <20150619172802.GA11492@cmpxchg.org> References: <20150619163418.GA21040@linutronix.de> <20150619171118.GA11423@cmpxchg.org> <55844EE7.7070508@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <55844EE7.7070508@linutronix.de> Sender: owner-linux-mm@kvack.org List-ID: To: Sebastian Andrzej Siewior Cc: akpm@linux-foundation.org, linux-mm@kvack.org, tglx@linutronix.de, rostedt@goodmis.org, williams@redhat.com On Fri, Jun 19, 2015 at 07:18:31PM +0200, Sebastian Andrzej Siewior wrote: > On 06/19/2015 07:11 PM, Johannes Weiner wrote: > > On Fri, Jun 19, 2015 at 06:34:18PM +0200, Sebastian Andrzej Siewior wrote: > >> Clark stumbled over a VM_BUG_ON() in -RT which was then was removed by > >> Johannes in commit f371763a79d ("mm: memcontrol: fix false-positive > >> VM_BUG_ON() on -rt"). The comment before that patch was a tiny bit > >> better than it is now. While the patch claimed to fix a false-postive on > >> -RT this was not the case. None of the -RT folks ACKed it and it was not a > >> false positive report. That was a *real* problem. > > > > The real problem is that irqs_disabled() on -rt is returning false > > negatives. Having it return false within a spin_lock_irq() section is > > broken. > > As I explained it in > http://www.spinics.net/lists/linux-rt-users/msg13499.html > it is not. I missed this email, sorry about that. > >> This patch updates the comment that is improper because it refers to > >> "disabled preemption" as a consequence of that lock being taken. A > >> spin_lock() disables preemption, true, but in this case the code relies on > >> the fact that the lock _also_ disables interrupts once it is acquired. And > >> this is the important detail (which was checked the VM_BUG_ON()) which needs > >> to be pointed out. This is the hint one needs while looking at the code. It > >> was explained by Johannes on the list that the per-CPU variables are protected > >> by local_irq_save(). The BUG_ON() was helpful. This code has been workarounded > >> in -RT in the meantime. I wouldn't mind running into more of those if the code > >> in question uses *special* kind of locking since now there is no no > >> verification (in terms of lockdep or BUG_ON()). > > > > I'd be happy to re-instate the VM_BUG_ON that checks for disabled > > interrupts as before, that was the most obvious documentation. > > sure thing, patch follows in a jiffy or two. Thanks. -- 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