From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH v2] kmemleak: Turn kmemleak_lock to raw spinlock on RT Date: Mon, 26 Nov 2018 09:40:42 +0100 Message-ID: <20181126084042.GK2113@hirez.programming.kicks-ass.net> 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 Cc: Andrea Parri , zhe.he@windriver.com, catalin.marinas@arm.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 To: Sebastian Andrzej Siewior Return-path: Content-Disposition: inline In-Reply-To: <20181123110611.s2gmd237j7docrxt@linutronix.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org 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. Since qrwlock is the more strict, all users should use its semantics. Just like we cannot 'rely' on the unfairness of some lock implementations.