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=-16.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=unavailable 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 4EFB0C07E95 for ; Sat, 10 Jul 2021 12:35:54 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 0E1366113C for ; Sat, 10 Jul 2021 12:35:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 0E1366113C Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=intel.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date: Message-ID:From:References:To:Subject:Reply-To:Cc:Content-ID: Content-Description:Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc :Resent-Message-ID:List-Owner; bh=rEY316q3nx6Etv/kVD7ObQfaIsNftwhW0DpoHqOw75Q=; b=yQIFBbvseKkaQ0AxCelfW99e1k Gt3mLoFiBfU7uArUtPU9G3DTMExkcPs+tecDHjkos8Hs/5v5efm4Ge+5GBzo6dJI6HXax3gifZmHx FO9NlMi+PszpCVq46dIgDrkyD6FvRw5QDDhXJXikCP7qnv9RcSJnYNs7mXlumUSX319DbzY7djtYS S096PpEJyX5E5qAC2HmvcVldE3eogpncPm9G3TLepbaYTmU3qhnrb0Iqk+ez41Gw97OoVcUX5LBWt GSX4q4d1H++fXK2gllbV7ooyFw5yAAMgLVfOVZ5sC0OXbHwHKlqG2Acwwy5r/rhL9d6helZqMqFRX fQxBo8QA==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m2CBe-003dQH-Bb; Sat, 10 Jul 2021 12:34:26 +0000 Received: from mga12.intel.com ([192.55.52.136]) by bombadil.infradead.org with esmtps (Exim 4.94.2 #2 (Red Hat Linux)) id 1m2CBZ-003dPO-Eh for linux-arm-kernel@lists.infradead.org; Sat, 10 Jul 2021 12:34:24 +0000 X-IronPort-AV: E=McAfee;i="6200,9189,10040"; a="189506611" X-IronPort-AV: E=Sophos;i="5.84,229,1620716400"; d="scan'208";a="189506611" Received: from orsmga005.jf.intel.com ([10.7.209.41]) by fmsmga106.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Jul 2021 05:34:17 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.84,229,1620716400"; d="scan'208";a="629174636" Received: from ahunter-desktop.fi.intel.com (HELO [10.237.72.79]) ([10.237.72.79]) by orsmga005.jf.intel.com with ESMTP; 10 Jul 2021 05:34:12 -0700 Subject: Re: [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions To: Leo Yan , Arnaldo Carvalho de Melo , Peter Zijlstra , Ingo Molnar , Mark Rutland , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Thomas Gleixner , Borislav Petkov , x86@kernel.org, "H. Peter Anvin" , Mathieu Poirier , Suzuki K Poulose , Mike Leach , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, coresight@lists.linaro.org, linux-arm-kernel@lists.infradead.org References: <20210704071644.107397-1-leo.yan@linaro.org> <20210704071644.107397-6-leo.yan@linaro.org> From: Adrian Hunter Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki, Business Identity Code: 0357606 - 4, Domiciled in Helsinki Message-ID: <26efa59c-4bdd-b93a-7ab8-4bb270aaee93@intel.com> Date: Sat, 10 Jul 2021 15:34:24 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210704071644.107397-6-leo.yan@linaro.org> Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210710_053421_567923_D743C717 X-CRM114-Status: GOOD ( 29.16 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 4/07/21 10:16 am, Leo Yan wrote: > The main purpose for using __sync built-in functions is to support > compat mode for 32-bit perf with 64-bit kernel. But using these > built-in functions might cause couple potential issues. > > Firstly, __sync functions originally support Intel Itanium processoer [1] > but it cannot promise to support all 32-bit archs. Now these > functions have become the legacy functions. > > As Peter also pointed out the logic issue in the function > auxtrace_mmap__write_tail(), it does a cmpxchg with 0 values to load > old_tail, and then executes a further cmpxchg with old_tail to write > the new tail. If consider the aux_tail might be assigned to '0' in the > middle of loops, this can introduce mess for AUX buffer if the kernel > fetches the temporary value '0'. That is not exactly true. The definition of __sync_*_compare_and_swap is "if the current value of *ptr is oldval, then write newval into *pt" so replacing zero with zero won't make any difference, but it will return the old value in any case. Probably better to leave out that paragraph. > > Considering __sync functions cannot really fix the 64-bit value > atomicity on 32-bit archs, thus this patch drops __sync functions. > > Credits to Peter for detailed analysis. > > [1] https://gcc.gnu.org/onlinedocs/gcc/_005f_005fsync-Builtins.html#g_t_005f_005fsync-Builtins > > Suggested-by: Peter Zijlstra > Signed-off-by: Leo Yan > --- > tools/perf/util/auxtrace.h | 19 ------------------- > 1 file changed, 19 deletions(-) > > diff --git a/tools/perf/util/auxtrace.h b/tools/perf/util/auxtrace.h > index cc1c1b9cec9c..f489ca159997 100644 > --- a/tools/perf/util/auxtrace.h > +++ b/tools/perf/util/auxtrace.h > @@ -440,12 +440,6 @@ struct auxtrace_cache; > > #ifdef HAVE_AUXTRACE_SUPPORT > > -/* > - * In snapshot mode the mmapped page is read-only which makes using > - * __sync_val_compare_and_swap() problematic. However, snapshot mode expects > - * the buffer is not updated while the snapshot is made (e.g. Intel PT disables > - * the event) so there is not a race anyway. > - */ > static inline u64 auxtrace_mmap__read_snapshot_head(struct auxtrace_mmap *mm) > { > struct perf_event_mmap_page *pc = mm->userpg; > @@ -459,11 +453,7 @@ static inline u64 auxtrace_mmap__read_snapshot_head(struct auxtrace_mmap *mm) > static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm) > { > struct perf_event_mmap_page *pc = mm->userpg; > -#if BITS_PER_LONG == 64 || !defined(HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT) > u64 head = READ_ONCE(pc->aux_head); > -#else > - u64 head = __sync_val_compare_and_swap(&pc->aux_head, 0, 0); > -#endif > > /* Ensure all reads are done after we read the head */ > smp_rmb(); > @@ -473,19 +463,10 @@ static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm) > static inline void auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail) > { > struct perf_event_mmap_page *pc = mm->userpg; > -#if BITS_PER_LONG != 64 && defined(HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT) > - u64 old_tail; > -#endif > > /* Ensure all reads are done before we write the tail out */ > smp_mb(); > -#if BITS_PER_LONG == 64 || !defined(HAVE_SYNC_COMPARE_AND_SWAP_SUPPORT) > pc->aux_tail = tail; > -#else > - do { > - old_tail = __sync_val_compare_and_swap(&pc->aux_tail, 0, 0); > - } while (!__sync_bool_compare_and_swap(&pc->aux_tail, old_tail, tail)); > -#endif > } > > int auxtrace_mmap__mmap(struct auxtrace_mmap *mm, > _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel