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 Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 550A1C4332F for ; Wed, 19 Oct 2022 16:05:32 +0000 (UTC) Received: from localhost ([::1]:50450 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1olBZG-0004uo-5V for qemu-devel@archiver.kernel.org; Wed, 19 Oct 2022 12:05:26 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:59182) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1olBST-0000S8-L0 for qemu-devel@nongnu.org; Wed, 19 Oct 2022 11:58:18 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:25026) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1olBSO-0000fw-0X for qemu-devel@nongnu.org; Wed, 19 Oct 2022 11:58:17 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1666195090; h=from:from:reply-to:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:in-reply-to:in-reply-to: references:references; bh=iRnlB8kXCZEFF8GkMhD0+xXcb49Eju+WDNBiM3rxI1U=; b=aGbrMY8eSoco4BV6Rf1cecQK5QoIGuBpB899hnLaZnoYcLd30UOkcw9NA7bzm+DyKJ7xj0 xh4+Lk4jjeDU153Fo1ndwaMHz6kuE0TGOVRFQz+VOeD7bV0JZsK+le1gVDNp96WRKG7jJc zFQ8bQQ33ef9QMeDDQ5ACqErm8w+BJo= Received: from mimecast-mx02.redhat.com (mx3-rdu2.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-662-Ix7MvLAuOpmnN0yYFDRIlw-1; Wed, 19 Oct 2022 11:58:09 -0400 X-MC-Unique: Ix7MvLAuOpmnN0yYFDRIlw-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.rdu2.redhat.com [10.11.54.2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id E11A91C0A585; Wed, 19 Oct 2022 15:58:02 +0000 (UTC) Received: from redhat.com (unknown [10.33.36.69]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 21ACF40C94EC; Wed, 19 Oct 2022 15:57:58 +0000 (UTC) Date: Wed, 19 Oct 2022 16:57:54 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Greg Kurz Cc: qemu-devel@nongnu.org, Alex =?utf-8?Q?Benn=C3=A9e?= , Richard Henderson , Paolo Bonzini Subject: Re: [PATCH v2 1/2] util/log: Derive thread id from getpid() on hosts w/o gettid() syscall Message-ID: References: <20221019151651.334334-1-groug@kaod.org> <20221019151651.334334-2-groug@kaod.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20221019151651.334334-2-groug@kaod.org> User-Agent: Mutt/2.2.7 (2022-08-07) X-Scanned-By: MIMEDefang 3.1 on 10.11.54.2 Received-SPF: pass client-ip=170.10.133.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -23 X-Spam_score: -2.4 X-Spam_bar: -- X-Spam_report: (-2.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.256, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Wed, Oct 19, 2022 at 05:16:50PM +0200, Greg Kurz wrote: > A subsequent patch needs to be able to differentiate the main QEMU > thread from other threads. An obvious way to do so is to compare > log_thread_id() and getpid(), based on the fact that they are equal > for the main thread on systems that have the gettid() syscall (e.g. > linux). > > Adapt the fallback code for systems without gettid() to provide the > same assumption. > > Suggested-by: Paolo Bonzini > Signed-off-by: Greg Kurz > --- > util/log.c | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/util/log.c b/util/log.c > index d6eb0378c3a3..e1c2535cfcd2 100644 > --- a/util/log.c > +++ b/util/log.c > @@ -72,8 +72,13 @@ static int log_thread_id(void) > #elif defined(SYS_gettid) > return syscall(SYS_gettid); > #else > + static __thread int my_id = -1; > static int counter; > - return qatomic_fetch_inc(&counter); > + > + if (my_id == -1) { > + my_id = getpid() + qatomic_fetch_inc(&counter); > + } > + return my_id; This doesn't look safe for linux-user when we fork, but don't exec. The getpid() will change after the fork, but counter won't be reset, so a thread in the parent could clash with a thread in the forked child. I feel like if we want to check for the main thread, we should be using pthread_self(), and compare result against the value cached from main. Or cache in a __constructor__ function in log.c to keep it isolated from main(). With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|