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=-8.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY, SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 31E7BC35247 for ; Tue, 4 Feb 2020 15:40:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ECE4C2087E for ; Tue, 4 Feb 2020 15:40:17 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580830818; bh=XjO/rypf4gOojGf0oXOQkll6+73OmV+xBEcS+uJX0Ac=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=u9aJwnpWMG8CpWCpEnOMaQPT17FlKHF2WMIj4fcVEIyxlkzs+npnJvshSKyaXIAVE 3NuhvRBP+P7GHidM7OkiiDkRXTMRPIo40EqQJKqI7EhbOjM1R4eLb/RwHCIiqpr8s8 2MJQuDUWanbwpc96pBkq93CRmBaUR0DZKdK3Pk2M= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727385AbgBDPkR (ORCPT ); Tue, 4 Feb 2020 10:40:17 -0500 Received: from mail.kernel.org ([198.145.29.99]:56538 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727290AbgBDPkQ (ORCPT ); Tue, 4 Feb 2020 10:40:16 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id CAB1720674; Tue, 4 Feb 2020 15:40:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1580830815; bh=XjO/rypf4gOojGf0oXOQkll6+73OmV+xBEcS+uJX0Ac=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=vodTEsLi+gyzU1kEcPqnqOzgfXe7tDAP6mg488qR+wwKflCraZFaf1nRX5gBLiPlM k0WOtxiHHQaY/qHSKZUlBbYzEXNWfj4Sf+cN8xFNPXXu87KVUUJ/nMXXpK5Qc8k7nw RO7JNt9amNpSfJw3V4ttf203mW86CZPqL5u0Bx9Q= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 5D8DD352270F; Tue, 4 Feb 2020 07:40:15 -0800 (PST) Date: Tue, 4 Feb 2020 07:40:15 -0800 From: "Paul E. McKenney" To: Marco Elver Cc: Andrey Konovalov , Alexander Potapenko , Dmitry Vyukov , kasan-dev , LKML Subject: Re: [PATCH 1/3] kcsan: Add option to assume plain writes up to word size are atomic Message-ID: <20200204154015.GQ2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200204140353.177797-1-elver@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 04, 2020 at 04:28:47PM +0100, Marco Elver wrote: > On Tue, 4 Feb 2020 at 15:04, Marco Elver wrote: > > > > This adds option KCSAN_ASSUME_PLAIN_WRITES_ATOMIC. If enabled, plain > > writes up to word size are also assumed to be atomic, and also not > > subject to other unsafe compiler optimizations resulting in data races. > > I just realized we should probably also check for alignedness. Would > this be fair to add as an additional constraint? It would be my > preference. Checking for alignment makes a lot of sense to me! Otherwise, write tearing is expected behavior on some systems. Thanx, Paul > Thanks, > -- Marco > > > This option has been enabled by default to reflect current kernel-wide > > preferences. > > > > Signed-off-by: Marco Elver > > --- > > kernel/kcsan/core.c | 20 +++++++++++++++----- > > lib/Kconfig.kcsan | 26 +++++++++++++++++++------- > > 2 files changed, 34 insertions(+), 12 deletions(-) > > > > diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c > > index 64b30f7716a12..3bd1bf8d6bfeb 100644 > > --- a/kernel/kcsan/core.c > > +++ b/kernel/kcsan/core.c > > @@ -169,10 +169,19 @@ static __always_inline struct kcsan_ctx *get_ctx(void) > > return in_task() ? ¤t->kcsan_ctx : raw_cpu_ptr(&kcsan_cpu_ctx); > > } > > > > -static __always_inline bool is_atomic(const volatile void *ptr) > > +static __always_inline bool > > +is_atomic(const volatile void *ptr, size_t size, int type) > > { > > - struct kcsan_ctx *ctx = get_ctx(); > > + struct kcsan_ctx *ctx; > > + > > + if ((type & KCSAN_ACCESS_ATOMIC) != 0) > > + return true; > > > > + if (IS_ENABLED(CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC) && > > + (type & KCSAN_ACCESS_WRITE) != 0 && size <= sizeof(long)) > > + return true; /* Assume all writes up to word size are atomic. */ > > + > > + ctx = get_ctx(); > > if (unlikely(ctx->atomic_next > 0)) { > > /* > > * Because we do not have separate contexts for nested > > @@ -193,7 +202,8 @@ static __always_inline bool is_atomic(const volatile void *ptr) > > return kcsan_is_atomic(ptr); > > } > > > > -static __always_inline bool should_watch(const volatile void *ptr, int type) > > +static __always_inline bool > > +should_watch(const volatile void *ptr, size_t size, int type) > > { > > /* > > * Never set up watchpoints when memory operations are atomic. > > @@ -202,7 +212,7 @@ static __always_inline bool should_watch(const volatile void *ptr, int type) > > * should not count towards skipped instructions, and (2) to actually > > * decrement kcsan_atomic_next for consecutive instruction stream. > > */ > > - if ((type & KCSAN_ACCESS_ATOMIC) != 0 || is_atomic(ptr)) > > + if (is_atomic(ptr, size, type)) > > return false; > > > > if (this_cpu_dec_return(kcsan_skip) >= 0) > > @@ -460,7 +470,7 @@ static __always_inline void check_access(const volatile void *ptr, size_t size, > > if (unlikely(watchpoint != NULL)) > > kcsan_found_watchpoint(ptr, size, type, watchpoint, > > encoded_watchpoint); > > - else if (unlikely(should_watch(ptr, type))) > > + else if (unlikely(should_watch(ptr, size, type))) > > kcsan_setup_watchpoint(ptr, size, type); > > } > > > > diff --git a/lib/Kconfig.kcsan b/lib/Kconfig.kcsan > > index 3552990abcfe5..08972376f0454 100644 > > --- a/lib/Kconfig.kcsan > > +++ b/lib/Kconfig.kcsan > > @@ -91,13 +91,13 @@ config KCSAN_REPORT_ONCE_IN_MS > > limiting reporting to avoid flooding the console with reports. > > Setting this to 0 disables rate limiting. > > > > -# Note that, while some of the below options could be turned into boot > > -# parameters, to optimize for the common use-case, we avoid this because: (a) > > -# it would impact performance (and we want to avoid static branch for all > > -# {READ,WRITE}_ONCE, atomic_*, bitops, etc.), and (b) complicate the design > > -# without real benefit. The main purpose of the below options is for use in > > -# fuzzer configs to control reported data races, and they are not expected > > -# to be switched frequently by a user. > > +# The main purpose of the below options is to control reported data races (e.g. > > +# in fuzzer configs), and are not expected to be switched frequently by other > > +# users. We could turn some of them into boot parameters, but given they should > > +# not be switched normally, let's keep them here to simplify configuration. > > +# > > +# The defaults below are chosen to be very conservative, and may miss certain > > +# bugs. > > > > config KCSAN_REPORT_RACE_UNKNOWN_ORIGIN > > bool "Report races of unknown origin" > > @@ -116,6 +116,18 @@ config KCSAN_REPORT_VALUE_CHANGE_ONLY > > the data value of the memory location was observed to remain > > unchanged, do not report the data race. > > > > +config KCSAN_ASSUME_PLAIN_WRITES_ATOMIC > > + bool "Assume that plain writes up to word size are atomic" > > + default y > > + help > > + Assume that plain writes up to word size are atomic by default, and > > + also not subject to other unsafe compiler optimizations resulting in > > + data races. This will cause KCSAN to not report data races due to > > + conflicts where the only plain accesses are writes up to word size: > > + conflicts between marked reads and plain writes up to word size will > > + not be reported as data races; notice that data races between two > > + conflicting plain writes will also not be reported. > > + > > config KCSAN_IGNORE_ATOMICS > > bool "Do not instrument marked atomic accesses" > > help > > -- > > 2.25.0.341.g760bfbb309-goog > >