From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wg0-f47.google.com (mail-wg0-f47.google.com [74.125.82.47]) by kanga.kvack.org (Postfix) with ESMTP id 7A0A06B0093 for ; Fri, 19 Jun 2015 13:11:41 -0400 (EDT) Received: by wguu7 with SMTP id u7so22550764wgu.3 for ; Fri, 19 Jun 2015 10:11:40 -0700 (PDT) Received: from gum.cmpxchg.org (gum.cmpxchg.org. [85.214.110.215]) by mx.google.com with ESMTPS id d4si5711885wiy.1.2015.06.19.10.11.39 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 19 Jun 2015 10:11:40 -0700 (PDT) Date: Fri, 19 Jun 2015 13:11:18 -0400 From: Johannes Weiner Subject: Re: [PATCH] mm: memcontrol: correct the comment in mem_cgroup_swapout() Message-ID: <20150619171118.GA11423@cmpxchg.org> References: <20150619163418.GA21040@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150619163418.GA21040@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 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. > 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. -- 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