From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp-out2.suse.de (smtp-out2.suse.de [IPv6:2001:67c:2178:6::1d]) by sourceware.org (Postfix) with ESMTPS id 6B3C33858424 for ; Mon, 18 Jul 2022 10:31:34 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.1 sourceware.org 6B3C33858424 Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by smtp-out2.suse.de (Postfix) with ESMTPS id B496D20DFB for ; Mon, 18 Jul 2022 10:31:32 +0000 (UTC) Received: from imap2.suse-dmz.suse.de (imap2.suse-dmz.suse.de [192.168.254.74]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (P-521) server-digest SHA512) (No client certificate requested) by imap2.suse-dmz.suse.de (Postfix) with ESMTPS id A374E13A37 for ; Mon, 18 Jul 2022 10:31:32 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap2.suse-dmz.suse.de with ESMTPSA id ZxyYJoQ21WJXTgAAMHmgww (envelope-from ) for ; Mon, 18 Jul 2022 10:31:32 +0000 Message-ID: <07df96c3-74ed-50ca-add8-cfb29b9eb63b@suse.de> Date: Mon, 18 Jul 2022 12:31:32 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.9.0 Subject: Re: [PATCH][gdbsupport] Avoid main thread in parallel for with thread sanitizer Content-Language: en-US From: Tom de Vries To: gdb-patches@sourceware.org References: <20220718100520.GA26230@delia.home> In-Reply-To: <20220718100520.GA26230@delia.home> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-12.4 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, GIT_PATCH_0, NICE_REPLY_A, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham 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: Mon, 18 Jul 2022 10:31:35 -0000 On 7/18/22 12:05, Tom de Vries wrote: > Hi, > > When running tasks in a parallel for, some elements may be handled by a worker > thread and others by the main thread. > > When running with -fsanitize=thread, avoid the main thread in the parallel > for, to prevent multi-threading problems being hidden. > FWIW, in terms of parallel_for_each_debug and the example I've been using, with -fsanitize=thread, we have: ... Parallel for: n_elements: 7271 Parallel for: minimum elements per thread: 10 Parallel for: elts_per_thread: 1817 Parallel for: elements on worker thread 0 : 1818 Parallel for: elements on worker thread 1 : 1818 Parallel for: elements on worker thread 2 : 1818 Parallel for: elements on worker thread 3 : 1817 Parallel for: elements on main thread : 0 ... and without: ... Parallel for: elements on worker thread 0 : 1818 Parallel for: elements on worker thread 1 : 1818 Parallel for: elements on worker thread 2 : 1818 Parallel for: elements on worker thread 3 : 0 Parallel for: elements on main thread : 1817 ... Thanks, - Tom > Tested on x86_64-linux, with -fsanitize=thread. > > Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29359 > > Any comments? > > Thanks, > - Tom > > [gdbsupport] Avoid main thread in parallel for with thread sanitizer > > --- > gdbsupport/parallel-for.h | 26 +++++++++++++++++++++++++- > 1 file changed, 25 insertions(+), 1 deletion(-) > > diff --git a/gdbsupport/parallel-for.h b/gdbsupport/parallel-for.h > index bf40f125f0f..81a2cfa215b 100644 > --- a/gdbsupport/parallel-for.h > +++ b/gdbsupport/parallel-for.h > @@ -24,6 +24,15 @@ > #include > #include "gdbsupport/thread-pool.h" > > +/* GCC does not understand __has_feature. */ > +#if !defined(__has_feature) > +# define __has_feature(x) 0 > +#endif > + > +#if __has_feature(thread_sanitizer) > +#define __SANITIZE_ADDRESS__ 1 > +#endif > + > namespace gdb > { > > @@ -161,7 +170,22 @@ parallel_for_each (unsigned n, RandomIt first, RandomIt last, > /* n_elements == n_threads * elts_per_thread + elts_left_over. */ > } > > - size_t count = n_threads == 0 ? 0 : n_threads - 1; > + /* Number of worker threads we're going to use. */ > + size_t count; > +#ifdef __SANITIZE_THREAD__ > + /* Avoid the main thread. */ > + count = (n_threads == 0 > + ? (/* Only use the first worker thread. */ > + 1) > + : (/* Only use worker threads. */ > + n_threads)); > +#else > + count = (n_threads == 0 > + ? (/* Only use the main thread. */ > + 0) > + : (/* Leave at least one worker thread unused. */ > + n_threads - 1)); > +#endif > gdb::detail::par_for_accumulator results (count); > > if (parallel_for_each_debug)