From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-ot1-f70.google.com (mail-ot1-f70.google.com [209.85.210.70]) by kanga.kvack.org (Postfix) with ESMTP id 394D16B310F for ; Fri, 23 Nov 2018 06:31:38 -0500 (EST) Received: by mail-ot1-f70.google.com with SMTP id q11so187265otl.23 for ; Fri, 23 Nov 2018 03:31:38 -0800 (PST) Received: from foss.arm.com (usa-sjc-mx-foss1.foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id 32si19978233otw.207.2018.11.23.03.31.36 for ; Fri, 23 Nov 2018 03:31:37 -0800 (PST) Date: Fri, 23 Nov 2018 11:31:31 +0000 From: Catalin Marinas Subject: Re: [PATCH v2] kmemleak: Turn kmemleak_lock to raw spinlock on RT Message-ID: <20181123113130.GA3360@arrakis.emea.arm.com> References: <1542877459-144382-1-git-send-email-zhe.he@windriver.com> <20181123095314.hervxkxtqoixovro@linutronix.de> <20181123110226.GA5125@andrea> <20181123110611.s2gmd237j7docrxt@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181123110611.s2gmd237j7docrxt@linutronix.de> Sender: owner-linux-mm@kvack.org List-ID: To: Sebastian Andrzej Siewior Cc: Andrea Parri , Peter Zijlstra , zhe.he@windriver.com, tglx@linutronix.de, rostedt@goodmis.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, boqun.feng@gmail.com On Fri, Nov 23, 2018 at 12:06:11PM +0100, Sebastian Andrzej Siewior wrote: > On 2018-11-23 12:02:55 [+0100], Andrea Parri wrote: > > > is this an RT-only problem? Because mainline should not allow read->read > > > locking or read->write locking for reader-writer locks. If this only > > > happens on v4.18 and not on v4.19 then something must have fixed it. > > > > Probably misunderstanding, but I'd say that read->read locking is "the > > norm"...? > > > > If you don't use qrwlock, readers are also "recursive", in part., > > > > P0 P1 > > read_lock(l) > > write_lock(l) > > read_lock(l) > > > > won't block P0 on the second read_lock(). (qrwlock somehow complicate > > the analysis; IIUC, they are recursive if and only if in_interrupt().). > > ehm, peterz, is that true? My memory on that is that all readers will > block if there is a writer pending. With qwrlocks, the readers will normally block if there is a pending writer (to avoid starving the writer), unless in_interrupt() when the readers are allowed to starve a pending writer. TLA+/PlusCal model here: ;) https://git.kernel.org/pub/scm/linux/kernel/git/cmarinas/kernel-tla.git/tree/qrwlock.tla -- Catalin