From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7B830C43218 for ; Tue, 11 Jun 2019 13:11:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 4DC0E20673 for ; Tue, 11 Jun 2019 13:11:48 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="aLUiKbmu" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390311AbfFKNLr (ORCPT ); Tue, 11 Jun 2019 09:11:47 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:52698 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2390242AbfFKNLr (ORCPT ); Tue, 11 Jun 2019 09:11:47 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=mXp7GfOu1REQRIFlq9C+d+O/GeRzxbASf2pcGXMeKhE=; b=aLUiKbmukQrL/U1LPPsjrJ+0O ChvbrLtjGXC9SZMNEwtoYmpcNyyDqle2WwDidzWiQWLzhoa9T32xbRnaVLVfxFK74e+Anw3A5VEjt jDaP5+mB0EJ06W8qaX2r7RPVnEuQQJsxzypyMOJBHFk7N4HTy70/y5+3yiPba78ttKqxOqh4dtjgH oUEjUgsgyIt1qrKrQejBbhQaIyihLg/BgSbQHlQcMo9ZjWw9bLo6xzQuNA0j35JKGZfVaA1anIUQl +1CucbkmJ7uiPq0KleCAQAlstwyoaO3/7siMLbq33B0/vqbIBEiKQKI71xc64Gqipdd5QTm2ulEn3 7/pygxh+w==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.92 #3 (Red Hat Linux)) id 1hagYn-0003q6-H1; Tue, 11 Jun 2019 13:11:34 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 661F920234FA3; Tue, 11 Jun 2019 15:11:31 +0200 (CEST) Date: Tue, 11 Jun 2019 15:11:31 +0200 From: Peter Zijlstra To: Waiman Long Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , Borislav Petkov , "H. Peter Anvin" , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying Subject: Re: [PATCH v8 16/19] locking/rwsem: Guard against making count negative Message-ID: <20190611131131.GG3402@hirez.programming.kicks-ass.net> References: <20190520205918.22251-1-longman@redhat.com> <20190520205918.22251-17-longman@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190520205918.22251-17-longman@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, May 20, 2019 at 04:59:15PM -0400, Waiman Long wrote: > +static inline long rwsem_read_trylock(struct rw_semaphore *sem, long *cnt) > +{ > + long adjustment = -RWSEM_READER_BIAS; > + > + *cnt = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count); I'm thinking we'd actually want add_return_acquire() here. > + if (unlikely(*cnt < 0)) { > + atomic_long_add(-RWSEM_READER_BIAS, &sem->count); > + adjustment = 0; > + } > + return adjustment; > +} > @@ -1271,9 +1332,10 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem) > */ > inline void __down_read(struct rw_semaphore *sem) > { > + long tmp, adjustment = rwsem_read_trylock(sem, &tmp); > + > + if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) { > + rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, adjustment); > DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); > } else { > rwsem_set_reader_owned(sem); > @@ -1282,9 +1344,11 @@ inline void __down_read(struct rw_semaphore *sem) > > static inline int __down_read_killable(struct rw_semaphore *sem) > { > + long tmp, adjustment = rwsem_read_trylock(sem, &tmp); > + > + if (unlikely(tmp & RWSEM_READ_FAILED_MASK)) { > + if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE, > + adjustment))) > return -EINTR; > DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem); > } else { I'm confused by the need for @tmp; isn't that returning the exact same state !adjustment is? Also; half the patch seems to do cnt<0, while the other half (above) does &READ_FAILED, what gives?