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Wed, 2 May 2018 14:27:36 -0400 Received: from mail-io0-f194.google.com ([209.85.223.194]:36736 "EHLO mail-io0-f194.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751991AbeEBS1e (ORCPT ); Wed, 2 May 2018 14:27:34 -0400 X-Google-Smtp-Source: AB8JxZokXsAdMWsCeDE4RpigQi+7taF8phGuH8cU9AuCJMnPs8f8irsFKtQ+sfw2kPvrApimGSbtLuItc56ZuYkIX+s= MIME-Version: 1.0 References: <20180430224433.17407-1-mathieu.desnoyers@efficios.com> <20180502172218.GL12180@hirez.programming.kicks-ass.net> In-Reply-To: <20180502172218.GL12180@hirez.programming.kicks-ass.net> From: Daniel Colascione Date: Wed, 02 May 2018 18:27:22 +0000 Message-ID: Subject: Re: [RFC PATCH for 4.18 00/14] Restartable Sequences To: Peter Zijlstra Cc: Mathieu Desnoyers , Paul McKenney , boqun.feng@gmail.com, luto@amacapital.net, davejwatson@fb.com, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org, Paul Turner , Andrew Morton , linux@arm.linux.org.uk, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, Andrew Hunter , andi@firstfloor.org, cl@linux.com, bmaurer@fb.com, rostedt@goodmis.org, josh@joshtriplett.org, torvalds@linux-foundation.org, catalin.marinas@arm.com, will.deacon@arm.com, Michael Kerrisk-manpages , Joel Fernandes Content-Type: text/plain; charset="UTF-8" Sender: linux-api-owner@vger.kernel.org X-Mailing-List: linux-api@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Wed, May 2, 2018 at 10:22 AM Peter Zijlstra wrote: >> On Wed, May 02, 2018 at 03:53:47AM +0000, Daniel Colascione wrote: > > Suppose we make a userspace mutex implemented with a lock word having three > > bits: acquired, sleep_mode, and wait_pending, with the rest of the word not > > being relevant at the moment. > So ideally we'd kill FUTEX_WAIT/FUTEX_WAKE for mutexes entirely, and go > with FUTEX_LOCK/FUTEX_UNLOCK that have the same semantics as the > existing FUTEX_LOCK_PI/FUTEX_UNLOCK_PI, namely, the word contains the > owner TID. That doesn't work if you want to use the rest of the word for something else, like a recursion count. With FUTEX_WAIT and FUTEX_WAKE, you can make a lock with two bits. > As brought up in the last time we talked about spin loops, why do we > care if the spin loop is in userspace or not? Aside from the whole PTI > thing, the syscall cost was around 150 cycle or so, while a LOCK CMPXCHG > is around 20 cycles. So ~7 spins gets you the cost of entry. That's pre-KPTI, isn't it?