From owner-freebsd-hackers@FreeBSD.ORG Tue Jun 9 10:55:37 2015 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [8.8.178.115]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by hub.freebsd.org (Postfix) with ESMTPS id C3CA2D42 for ; Tue, 9 Jun 2015 10:55:37 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from kib.kiev.ua (kib.kiev.ua [IPv6:2001:470:d5e7:1::1]) (using TLSv1 with cipher DHE-RSA-CAMELLIA256-SHA (256/256 bits)) (Client did not present a certificate) by mx1.freebsd.org (Postfix) with ESMTPS id 37EB41867 for ; Tue, 9 Jun 2015 10:55:37 +0000 (UTC) (envelope-from kostikbel@gmail.com) Received: from tom.home (kostik@localhost [127.0.0.1]) by kib.kiev.ua (8.14.9/8.14.9) with ESMTP id t59AtRvj012214 (version=TLSv1/SSLv3 cipher=DHE-RSA-CAMELLIA256-SHA bits=256 verify=NO); Tue, 9 Jun 2015 13:55:27 +0300 (EEST) (envelope-from kostikbel@gmail.com) DKIM-Filter: OpenDKIM Filter v2.9.2 kib.kiev.ua t59AtRvj012214 Received: (from kostik@localhost) by tom.home (8.14.9/8.14.9/Submit) id t59AtQXB012205; Tue, 9 Jun 2015 13:55:26 +0300 (EEST) (envelope-from kostikbel@gmail.com) X-Authentication-Warning: tom.home: kostik set sender to kostikbel@gmail.com using -f Date: Tue, 9 Jun 2015 13:55:26 +0300 From: Konstantin Belousov To: Sebastian Huber Cc: freebsd-hackers@freebsd.org, phk@phk.freebsd.dk Subject: Re: [PATCH v3] timecounters: Fix timehand generation read/write Message-ID: <20150609105526.GK2499@kib.kiev.ua> References: <1433331966-27548-1-git-send-email-sebastian.huber@embedded-brains.de> <1433838715-22850-1-git-send-email-sebastian.huber@embedded-brains.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1433838715-22850-1-git-send-email-sebastian.huber@embedded-brains.de> User-Agent: Mutt/1.5.23 (2014-03-12) X-Spam-Status: No, score=-2.0 required=5.0 tests=ALL_TRUSTED,BAYES_00, DKIM_ADSP_CUSTOM_MED,FREEMAIL_FROM,NML_ADSP_CUSTOM_MED autolearn=no autolearn_force=no version=3.4.1 X-Spam-Checker-Version: SpamAssassin 3.4.1 (2015-04-28) on tom.home X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.20 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 09 Jun 2015 10:55:37 -0000 On Tue, Jun 09, 2015 at 10:31:55AM +0200, Sebastian Huber wrote: > The compiler is free to re-order load/store instructions to non-volatile > variables around a load/store of a volatile variable. So the volatile > generation counter is insufficent. In addition tests on a Freescale > T4240 platform with 24 PowerPC processors showed that real memory > barriers are required. Compiler memory barriers are not enough. This looks fine. The only detail I changed is that I do not see a reason to use atomics or barriers on UP machines. See the fragment at the end of the message for the exact diff. > > For the test the timehand count was reduced to one with 10000 > tc_windup() calls per second. The timehand memory location was adjusted > so that the th_generation field was on its own cache line. You mean, that the 'th_generation for its own cache line' was done for testing only ? > --- > > v2: Don't use tc_getgen() in tc_windup() since in the only writer there is no > need for a read memory barrier. > > v3: Use atomic load/store instead of explicit memory barriers. > If you do not have any comments, I will commit this version. diff --git a/sys/kern/kern_tc.c b/sys/kern/kern_tc.c index 9dca0e8..fabfe03 100644 --- a/sys/kern/kern_tc.c +++ b/sys/kern/kern_tc.c @@ -189,6 +190,28 @@ tc_delta(struct timehands *th) tc->tc_counter_mask); } +static u_int +tc_getgen(struct timehands *th) +{ + +#ifdef SMP + return (atomic_load_acq_int(&th->th_generation)); +#else + return (th->th_generation); +#endif +} + +static void +tc_setgen(struct timehands *th, u_int newgen) +{ + +#ifdef SMP + atomic_store_rel_int(&th->th_generation, newgen); +#else + th->th_generation = newgen; +#endif +} + /* * Functions for reading the time. We have to loop until we are sure that * the timehands that we operated on was not updated under our feet. See