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=-0.6 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 1375AC10F25 for ; Fri, 6 Mar 2020 16:53:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id DE830206E2 for ; Fri, 6 Mar 2020 16:53:04 +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="TK+sI/Ld" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726676AbgCFQxE (ORCPT ); Fri, 6 Mar 2020 11:53:04 -0500 Received: from bombadil.infradead.org ([198.137.202.133]:54230 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726083AbgCFQxD (ORCPT ); Fri, 6 Mar 2020 11:53:03 -0500 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; bh=Hc8EBMbziFCPflui+tXZT2dPAWJHoF3kP2tPXgp7AkU=; b=TK+sI/Ld4B82lkfwn/6fJoY4P+ gkj8zAwtDF7zS03QcZEyuZaFrQkkqJ8TMExYdioC8sYW7FBqt2Go+0VwlvQagXXs0SPCYIVdBcH1n Yy0Mf4ADrOFSbQPFxYmAtS8oULu3+gOG1IqcrLpRwzhePOC2CWGVGkCHJcXNoyWlun5UpH2NOaXW/ YmYx3SsXWWdkoX4jLsxEYIV5twmHnvFYxG7n8ZX/j6UbX6SwRI74i0m58oiezWmfBUTu2xiXmP4gX AHVkS4s8Gq28FyFNtQ9lwpD2pWdu0CdFm1j02F65pvDkjsGCHbG/UiT9Ktekzl1MZAvzqC/QJwLgW tW1dsiSw==; Received: from willy by bombadil.infradead.org with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1jAGDd-0005Ys-2b; Fri, 06 Mar 2020 16:53:01 +0000 Date: Fri, 6 Mar 2020 08:53:00 -0800 From: Matthew Wilcox To: Marco Elver Cc: "Paul E. McKenney" , Qian Cai , LKML Subject: Re: [PATCH -next] lib: disable KCSAN for XArray Message-ID: <20200306165300.GC25710@bombadil.infradead.org> References: <20200304031551.1326-1-cai@lca.pw> <20200304033329.GZ29971@bombadil.infradead.org> <20200304040515.GX2935@paulmck-ThinkPad-P72> <20200304043356.GC29971@bombadil.infradead.org> <20200304141021.GY2935@paulmck-ThinkPad-P72> <20200305151831.GM29971@bombadil.infradead.org> <20200305213946.GL2935@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 06, 2020 at 02:38:39PM +0100, Marco Elver wrote: > On Thu, 5 Mar 2020 at 22:39, Paul E. McKenney wrote: > > On Thu, Mar 05, 2020 at 07:18:31AM -0800, Matthew Wilcox wrote: > > > I have found three locations where we use the ->marks array: > > > > > > 1. > > > unsigned long data = *addr & (~0UL << offset); > > > if (data) > > > return __ffs(data); > > > > > > 2. > > > return find_next_bit(addr, XA_CHUNK_SIZE, offset); > > > 3. > > > return test_bit(offset, node_marks(node, mark)); > > > > > > The modifications -- all done with the spinlock held -- use the non-atomic > > > bitops: > > > return __test_and_set_bit(offset, node_marks(node, mark)); > > > return __test_and_clear_bit(offset, node_marks(node, mark)); > > > bitmap_fill(node_marks(node, mark), XA_CHUNK_SIZE); > > > (that last one doesn't really count -- it's done prior to placing the node > > > in the tree) > > > > > > The first read seems straightforward; I can place a READ_ONCE around > > > *addr. The second & third reads are rather less straightforward. > > > find_next_bit() and test_bit() are common code and use plain loads today. > > > > Yes, those last two are a bit annoying, aren't they? I guess the first > > thing would be placing READ_ONCE() inside them, and if that results in > > regressions, have an alternative API for concurrent access? > > FWIW test_bit() is an "atomic" bitop (per atomic_bitops.txt), and > KCSAN treats it as such. On x86 arch_test_bit() is not instrumented, > and then in asm-generic/bitops/instrumented-non-atomic.h test_bit() is > instrumented with instrument_atomic_read(). So on x86, things should > already be fine for test_bit(). Not sure about other architectures. Hum. It may well be documented as atomic, but is it? Here's the generic implementation: static inline int test_bit(int nr, const volatile unsigned long *addr) { return 1UL & (addr[BIT_WORD(nr)] >> (nr & (BITS_PER_LONG-1))); } arch_test_bit is only used by the instrumented variants: $ git grep arch_test_bit include include/asm-generic/bitops/instrumented-non-atomic.h: return arch_test_bit(nr, addr); As far as I can tell, the generic version is what's used on x86. Does the 'volatile' qualifier save us here? find_next_bit() doesn't have the 'volatile' qualifier, so may still be a problem?