From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm1-f41.google.com (mail-wm1-f41.google.com [209.85.128.41]) by sourceware.org (Postfix) with ESMTPS id 3C76438460B3 for ; Wed, 29 Jun 2022 18:28:31 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 3C76438460B3 Authentication-Results: sourceware.org; dmarc=none (p=none dis=none) header.from=palves.net Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=gmail.com Received: by mail-wm1-f41.google.com with SMTP id t17-20020a1c7711000000b003a0434b0af7so173148wmi.0 for ; Wed, 29 Jun 2022 11:28:31 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:message-id:date:mime-version:user-agent:subject :content-language:from:to:cc:references:in-reply-to :content-transfer-encoding; bh=dobWq6TYY8c4ArtV0NkIH7El8Mvl6450GGvE7j7YJ1s=; b=XEtIvD50h0okWO1W+eNbXk1FDrFc1hBc6sRcj2wwBMe2YZVTVJZR383l0ci/t2Eou8 cTPURrEyyR4WgBWPEyRGf5xlsbb8Xb5sypFztIOmaPOpFh4qXYEEZZ9VpAiG98l9NiKt dh/+/w2Wikmqp3VQt7hoLmMmkWGX7LRqEWTE50k5fJ717z8BXxLeV6id7IWWzzvEJ6ER PNqVC89+3y8jKoc1CGEDSZPjamSmTefjG2gqK2G8N6CUTaFV8PyG6B9yx7enStBZXWUr m7Mejs2w/UATynnKsIgJWN1IeuuvO0XvJXi1Lk8DX89/lUzTudX1+gc0Xxb1EDFDsew/ CEZg== X-Gm-Message-State: AJIora8NSkIJkyY2qIcfKgAogIl+1sMcM4+UQlEznbtdKfsRQbEjU24m qt3nV7Xs8LJ13898yAkU/AS3e0EKC74= X-Google-Smtp-Source: AGRyM1sFSX0QqINgjmBOg1nhdarTSKRg7Er8A0xYFqujyyqrgrzxdm44Fb4YXOcvKeN6Zeq8YmXDWQ== X-Received: by 2002:a05:600c:c5:b0:3a0:3ef3:838a with SMTP id u5-20020a05600c00c500b003a03ef3838amr7291777wmm.50.1656527310112; Wed, 29 Jun 2022 11:28:30 -0700 (PDT) Received: from ?IPV6:2001:8a0:f924:2600:209d:85e2:409e:8726? ([2001:8a0:f924:2600:209d:85e2:409e:8726]) by smtp.gmail.com with ESMTPSA id e3-20020adfef03000000b0021bbd525b8esm16763803wro.45.2022.06.29.11.28.28 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 29 Jun 2022 11:28:29 -0700 (PDT) Message-ID: <62bc9947-ac55-a976-d225-5c828207f558@palves.net> Date: Wed, 29 Jun 2022 19:28:28 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.10.0 Subject: Re: [PATCH 3/5] [gdb/symtab] Work around fsanitize=address false positive for per_cu->lang Content-Language: en-US From: Pedro Alves To: Tom de Vries , gdb-patches@sourceware.org Cc: Tom Tromey References: <20220629152914.13149-1-tdevries@suse.de> <20220629152914.13149-3-tdevries@suse.de> <1cdbd4f9-4da7-0dd3-ea91-496797f2ad72@palves.net> In-Reply-To: <1cdbd4f9-4da7-0dd3-ea91-496797f2ad72@palves.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Spam-Status: No, score=-4.1 required=5.0 tests=BAYES_00, FREEMAIL_FORGED_FROMDOMAIN, FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS, KAM_DMARC_STATUS, NICE_REPLY_A, RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H2, SPF_HELO_NONE, SPF_PASS, TXREP, T_SCC_BODY_TEXT_LINE autolearn=no autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org X-BeenThere: gdb-patches@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gdb-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 29 Jun 2022 18:28:34 -0000 On 2022-06-29 19:25, Pedro Alves wrote: > On 2022-06-29 18:38, Pedro Alves wrote: >> On 2022-06-29 16:29, Tom de Vries via Gdb-patches wrote: >>> When building gdb with -fsanitize=thread and gcc 12, and running test-case >>> gdb.dwarf2/dwz.exp, we run into a data race between: >>> ... >>> Read of size 1 at 0x7b200000300d by thread T2:^M >>> #0 cutu_reader::cutu_reader(dwarf2_per_cu_data*, dwarf2_per_objfile*, \ >>> abbrev_table*, dwarf2_cu*, bool, abbrev_cache*) gdb/dwarf2/read.c:6164 \ >>> (gdb+0x82ec95)^M >>> ... >>> and: >>> ... >>> Previous write of size 1 at 0x7b200000300d by main thread:^M >>> #0 prepare_one_comp_unit gdb/dwarf2/read.c:23588 (gdb+0x86f973)^M >>> ... >>> >>> In other words, between: >>> ... >>> if (this_cu->reading_dwo_directly) >>> ... >>> and: >>> ... >>> cu->per_cu->lang = pretend_language; >>> ... >>> >>> Both fields are part of the same bitfield, and writing to one field while >>> reading from another is not a problem, so this is a false positive. >> >> I don't understand this sentence. Particularly "same bitfield", or >> really "Both fields are part of the same bitfield,". How can two bitfields >> be part of the same bitfield? >> >> Anyhow, both bitfields are part of a sequence of contiguous bitfields, here >> stripped of comments: >> >> unsigned int queued : 1; >> unsigned int is_debug_types : 1; >> unsigned int is_dwz : 1; >> unsigned int reading_dwo_directly : 1; >> unsigned int tu_read : 1; >> mutable bool m_header_read_in : 1; >> bool addresses_seen : 1; >> unsigned int mark : 1; >> bool files_read : 1; >> ENUM_BITFIELD (dwarf_unit_type) unit_type : 8; >> ENUM_BITFIELD (language) lang : LANGUAGE_BITS; >> >> Per C++11, they're all part of the same _memory location_. From N3253 (C++11), intro.memory: >> >> "A memory location is either an object of scalar type or a maximal sequence of adjacent bit-fields all having >> non-zero width. (...) Two threads of execution (1.10) can update and access separate memory locations >> without interfering with each other. >> (...) >> [ Note: Thus a bit-field and an adjacent non-bit-field are in separate memory locations, and therefore can be >> concurrently updated by two threads of execution without interference. The same applies to two bit-fields, >> if one is declared inside a nested struct declaration and the other is not, or if the two are separated by >> a zero-length bit-field declaration, or if they are separated by a non-bit-field declaration. It is not safe to >> concurrently update two bit-fields in the same struct if all fields between them are also bit-fields of non-zero >> width. — end note ]" >> >> And while it is true that in practice writing to one bit-field from one thread and reading from another, >> if they reside on the same location, is OK in practice, it is still undefined behavior. >> >> Note the escape hatch mentioned above: >> >> "if the two are separated by a zero-length bit-field declaration" >> >> Thus, a change like this: >> >> unsigned int queued : 1; >> unsigned int is_debug_types : 1; >> unsigned int is_dwz : 1; >> unsigned int reading_dwo_directly : 1; >> unsigned int tu_read : 1; >> mutable bool m_header_read_in : 1; >> bool addresses_seen : 1; >> unsigned int mark : 1; >> bool files_read : 1; >> ENUM_BITFIELD (dwarf_unit_type) unit_type : 8; >> + >> + /* Ensure lang is a separate memory location, so we can update >> + it concurrently with other bitfields. */ >> + char :0; >> + >> ENUM_BITFIELD (language) lang : LANGUAGE_BITS; >> >> >> ... should work. > > The "if one is declared inside a nested struct declaration and the other > is not" escape hatch may be interesting too, as in, we'd write: > > struct { > ENUM_BITFIELD (language) lang : LANGUAGE_BITS; > }; > > ... and since the struct is anonymous, nothing else needs to change. > > In patch #4, you'd just do this too: > > struct { > ENUM_BITFIELD (dwarf_unit_type) unit_type : 8; > }; > > The "wrapping" syntax seems to read a bit better, particularly since this > way you don't have to worry about putting a :0 bitfield before and > another after. I keep coming back, sorry... :-P Another thought is that in both patches #3 and #4, it's reading_dwo_directly that is racing with two other bitfields. So I wonder whether it's _that_ one that should be moved to a separate memory location.