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=-7.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, USER_AGENT_SANE_1 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 0CC48C433E0 for ; Fri, 22 Jan 2021 18:25:45 +0000 (UTC) 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 mail.kernel.org (Postfix) with ESMTPS id 9B64623A6A for ; Fri, 22 Jan 2021 18:25:44 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9B64623A6A Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Received: from localhost ([::1]:54414 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l317v-0000LP-Mn for qemu-devel@archiver.kernel.org; Fri, 22 Jan 2021 13:25:43 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:57924) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l313D-00011d-Dh for qemu-devel@nongnu.org; Fri, 22 Jan 2021 13:20:51 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:43428) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l313A-0007XY-W6 for qemu-devel@nongnu.org; Fri, 22 Jan 2021 13:20:50 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611339648; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=cccbrQUYKsX4M33r570q31j287A4JahzmcAjvmDhSaw=; b=EhIMW0dqfvNENLLL1rks4KzHljIaQqrkUj3xtLvP6e2j5Mzggdqv3MLww9am3QBWbFR04Q 1gcT8/Rp/qaUhJG/HCf0abmDOMyO+ihtx5GuvaWo3QPVYWfQ6GdBwqd8Hv4ASsxcIak30G VHSKXhNxYlsZWQi+1sBvTYingA2nwmI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-210-gsEK7lpiNWStq7pootChBg-1; Fri, 22 Jan 2021 13:20:42 -0500 X-MC-Unique: gsEK7lpiNWStq7pootChBg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B5644DB8F0 for ; Fri, 22 Jan 2021 18:19:49 +0000 (UTC) Received: from [10.3.113.116] (ovpn-113-116.phx2.redhat.com [10.3.113.116]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0C89A2BFE9; Fri, 22 Jan 2021 18:19:45 +0000 (UTC) To: Max Reitz , Laszlo Ersek , qemu-devel@nongnu.org References: <20210122102041.27031-1-mreitz@redhat.com> <10be5fcc-5e7a-3e44-3229-8526ad3b4547@redhat.com> <3e6b417c-eebb-dc6a-da7d-af2295118c6a@redhat.com> From: Eric Blake Organization: Red Hat, Inc. Subject: Re: [PATCH] coroutine-sigaltstack: Keep SIGUSR2 handler up Message-ID: <30d27bfa-770f-4edb-6f4b-9a1cd2eb2833@redhat.com> Date: Fri, 22 Jan 2021 12:19:45 -0600 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <3e6b417c-eebb-dc6a-da7d-af2295118c6a@redhat.com> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eblake@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=eblake@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -31 X-Spam_score: -3.2 X-Spam_bar: --- X-Spam_report: (-3.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.182, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.221, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Paolo Bonzini , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 1/22/21 11:58 AM, Max Reitz wrote: >>> +    if (!self) { >>> +        /* >>> +         * This SIGUSR2 came from an external source, not from >>> +         * qemu_coroutine_new(), so perform the default action. >>> +         */ >>> +        exit(0); >>> +    } >> >> (2) exit() is generally unsafe to call in signal handlers. >> >> We could reason whether or not it is safe in this particular case (POSIX >> describes the exact conditions -- >> ), >> >> but it's much simpler to just call _exit(). >> >> >> (3) "Performing the default action" would be slightly different from >> calling _exit(). When a process is terminated with a signal, the parent >> can distinguish that, when reaping the child. See waitpid() / >> WIFSIGNALED() / WTERMSIG(), versus WIFEXITED() / WEXITSTATUS(). >> >> So for the "default action", we'd have to: >> - restore the SIGUSR2 handler to SIG_DFL, and >> - re-raise the signal for the thread, and >> - because the signal being handled is automatically blocked unless >>    SA_NODEFER was set: unblock the signal for the thread. >> >> The pthread_sigmask() call, made for the last step, would be the one >> that would not return. >> >> *However*, all of this complexity is not really useful here. We don't >> really want to simulate being "forcefully terminated" by the unexpected >> (asynchronous) SIGUSR2. We just want to exit. >> >> Therefore, _exit() is fine. But, we should use an exit code different >> from 0, as this is definitely not a pristine exit from QEMU. I'm not >> sure if a convention exists for nonzero exit codes in QEMU; if not, then >> just pass EXIT_FAILURE to _exit(). > > I’m fine with calling _exit().  I hope, Eric is, too (as long as the > comment no longer claims this were the default behavior). Using _exit(nonzero) is fine by me as long as the comment is accurate. There are signals like SIGINT where you really DO want to terminate by signal rather than using _exit(SIGINT+128), because shells can tell the difference [1]; but SIGUSR2 is not one of the signals where shells special-case their behavior. [1] https://www.cons.org/cracauer/sigint.html > > Given that SIGUSR2 is not something that qemu is prepared to receive > from the outside, EXIT_FAILURE seems right to me.  (Even abort() could > be justified, but I don’t think generating a core dump does any good here.) > > (As for qemu-specific exit code conventions, the only ones I know of are > for certain qemu-img subcommands.  I’m sure you grepped, too, but I > can’t find anything for the system emulator.) > >> (4) Furthermore, please update the comment, because "perform the default >> action" is not precise. > > Sure, that’s definitely easier than to re-raise SIGUSR2. Works for me as well. -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org