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=-13.6 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 062EEC433DB for ; Mon, 25 Jan 2021 21:56:31 +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 69D2621D94 for ; Mon, 25 Jan 2021 21:56:30 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 69D2621D94 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]:37636 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1l49qX-0002tr-40 for qemu-devel@archiver.kernel.org; Mon, 25 Jan 2021 16:56:29 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]:51804) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1l49pJ-0002L3-ES for qemu-devel@nongnu.org; Mon, 25 Jan 2021 16:55:13 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:50272) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_CBC_SHA1:256) (Exim 4.90_1) (envelope-from ) id 1l49pG-0005y9-F9 for qemu-devel@nongnu.org; Mon, 25 Jan 2021 16:55:12 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1611611709; 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=9nK72k/Folhbwd9IQb+ormU80OfBBQ2SsSN3XmF0vHM=; b=VTj7JhvS9kiK2unQqrqBQpcWtDfFMIQJbLn0/LDYGiNGYcqtcGAcJ1nbpqFzkNAUL6GjIE FoESzhnKxAhdEWLUtcB4ZaCqBIRJvaEvugNniyxuWyqDl6avhZRfln1wdVCsDDYyipXyFP DkQFgKjLu/VmEKEz/EeZ9RKCu/mqQhI= 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-88-0PUD1W_BPp6HG72su7ZEHw-1; Mon, 25 Jan 2021 16:55:06 -0500 X-MC-Unique: 0PUD1W_BPp6HG72su7ZEHw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 0307B107ACE3 for ; Mon, 25 Jan 2021 21:55:06 +0000 (UTC) Received: from lacos-laptop-7.usersys.redhat.com (ovpn-112-67.ams2.redhat.com [10.36.112.67]) by smtp.corp.redhat.com (Postfix) with ESMTP id ED34E19C78; Mon, 25 Jan 2021 21:55:01 +0000 (UTC) Subject: Re: [PATCH] coroutine-sigaltstack: Add SIGUSR2 mutex To: Max Reitz , qemu-devel@nongnu.org References: <20210125120305.19520-1-mreitz@redhat.com> From: Laszlo Ersek Message-ID: <4f36c7fc-9ff5-ddd7-de6b-ee6f8d896e23@redhat.com> Date: Mon, 25 Jan 2021 22:55:00 +0100 MIME-Version: 1.0 In-Reply-To: <20210125120305.19520-1-mreitz@redhat.com> X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=lersek@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: 7bit Received-SPF: pass client-ip=63.128.21.124; envelope-from=lersek@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -30 X-Spam_score: -3.1 X-Spam_bar: --- X-Spam_report: (-3.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.255, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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 , Stefan Hajnoczi Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 01/25/21 13:03, Max Reitz wrote: > Disposition (action) for any given signal is global for the process. > When two threads run coroutine-sigaltstack's qemu_coroutine_new() > concurrently, they may interfere with each other: One of them may revert > the SIGUSR2 handler to SIG_DFL, between the other thread (a) setting up > coroutine_trampoline() as the handler and (b) raising SIGUSR2. That > SIGUSR2 will then terminate the QEMU process abnormally. > > We have to ensure that only one thread at a time can modify the > process-global SIGUSR2 handler. To do so, wrap the whole section where > that is done in a mutex. > > Alternatively, we could for example have the SIGUSR2 handler always be > coroutine_trampoline(), so there would be no need to invoke sigaction() > in qemu_coroutine_new(). Laszlo has posted a patch to do so here: > > https://lists.nongnu.org/archive/html/qemu-devel/2021-01/msg05962.html > > However, given that coroutine-sigaltstack is more of a fallback > implementation for platforms that do not support ucontext, that change > may be a bit too invasive to be comfortable with it. The mutex proposed > here may negatively impact performance, but the change is much simpler. > > Signed-off-by: Max Reitz > --- > util/coroutine-sigaltstack.c | 9 +++++++++ > 1 file changed, 9 insertions(+) > > diff --git a/util/coroutine-sigaltstack.c b/util/coroutine-sigaltstack.c > index aade82afb8..e99b8a4f9c 100644 > --- a/util/coroutine-sigaltstack.c > +++ b/util/coroutine-sigaltstack.c > @@ -157,6 +157,7 @@ Coroutine *qemu_coroutine_new(void) > sigset_t sigs; > sigset_t osigs; > sigjmp_buf old_env; > + static pthread_mutex_t sigusr2_mutex = PTHREAD_MUTEX_INITIALIZER; > > /* The way to manipulate stack is with the sigaltstack function. We > * prepare a stack, with it delivering a signal to ourselves and then > @@ -186,6 +187,12 @@ Coroutine *qemu_coroutine_new(void) > sa.sa_handler = coroutine_trampoline; > sigfillset(&sa.sa_mask); > sa.sa_flags = SA_ONSTACK; > + > + /* > + * sigaction() is a process-global operation. We must not run > + * this code in multiple threads at once. > + */ > + pthread_mutex_lock(&sigusr2_mutex); > if (sigaction(SIGUSR2, &sa, &osa) != 0) { > abort(); > } > @@ -234,6 +241,8 @@ Coroutine *qemu_coroutine_new(void) > * Restore the old SIGUSR2 signal handler and mask > */ > sigaction(SIGUSR2, &osa, NULL); > + pthread_mutex_unlock(&sigusr2_mutex); > + > pthread_sigmask(SIG_SETMASK, &osigs, NULL); > > /* > Reviewed-by: Laszlo Ersek Thanks! Laszlo