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 12F56C4332F for ; Mon, 6 Nov 2023 10:11:15 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qzwZ6-0006v7-83; Mon, 06 Nov 2023 05:10:40 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qzwZ4-0006uc-C9 for qemu-devel@nongnu.org; Mon, 06 Nov 2023 05:10:38 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qzwZ2-0002L1-8X for qemu-devel@nongnu.org; Mon, 06 Nov 2023 05:10:38 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1699265434; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=MokpWFlmV1Tn5A62l2zEJkfJh1k9MgusHtDfFxa+wXw=; b=Uofsutyb+fIsO/8dNNHOgZCItvUQ+oe6PGd2hemwx+p86Igb2RgrdunskwNMKGEFRBvCqA +wpUp8lSxyzQ/F9RwSprl3ZOHE8zIix8OXVhr2NpEuihwOeaqh4nF+TTt0huTGlrGqs8w8 7PCzBS9f2O76DMIPfEuN1oXx1ij12YU= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-459-OwXqxHH5OJaMk2ZGw7V_kg-1; Mon, 06 Nov 2023 05:10:29 -0500 X-MC-Unique: OwXqxHH5OJaMk2ZGw7V_kg-1 Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.rdu2.redhat.com [10.11.54.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id EFC25185A783; Mon, 6 Nov 2023 10:10:28 +0000 (UTC) Received: from redhat.com (unknown [10.42.28.95]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 0F260492A; Mon, 6 Nov 2023 10:10:26 +0000 (UTC) Date: Mon, 6 Nov 2023 10:10:19 +0000 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Steven Sistare Cc: qemu-devel@nongnu.org, Markus Armbruster , Michael Roth , "Dr. David Alan Gilbert" Subject: Re: [PATCH] monitor: flush messages on abort Message-ID: References: <1699027289-213995-1-git-send-email-steven.sistare@oracle.com> <3d45ebc0-de9f-4051-9c08-47e40fea65da@oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <3d45ebc0-de9f-4051-9c08-47e40fea65da@oracle.com> User-Agent: Mutt/2.2.9 (2022-11-12) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.1 Received-SPF: pass client-ip=170.10.129.124; envelope-from=berrange@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -26 X-Spam_score: -2.7 X-Spam_bar: -- X-Spam_report: (-2.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.581, 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_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 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-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Fri, Nov 03, 2023 at 03:51:00PM -0400, Steven Sistare wrote: > On 11/3/2023 1:33 PM, Daniel P. Berrangé wrote: > > On Fri, Nov 03, 2023 at 09:01:29AM -0700, Steve Sistare wrote: > >> Buffered monitor output is lost when abort() is called. The pattern > >> error_report() followed by abort() occurs about 60 times, so valuable > >> information is being lost when the abort is called in the context of a > >> monitor command. > > > > I'm curious, was there a particular abort() scenario that you hit ? > > Yes, while tweaking the suspended state, and forgetting to add transitions: > > error_report("invalid runstate transition: '%s' -> '%s'", > abort(); > > But I have previously hit this for other errors. > > > For some crude statistics: > > > > $ for i in abort return exit goto ; do echo -n "$i: " ; git grep --after 1 error_report | grep $i | wc -l ; done > > abort: 47 > > return: 512 > > exit: 458 > > goto: 177 > > > > to me those numbers say that calling "abort()" after error_report > > should be considered a bug, and we can blanket replace all the > > abort() calls with exit(EXIT_FAILURE), and thus avoid the need to > > special case flushing the monitor. > > And presumably add an atexit handler to flush the monitor ala monitor_abort. > AFAICT currently no destructor is called for the monitor at exit time. The HMP monitor flushes at each newline, and exit() will take care of flushing stdout, so I don't think there's anything else needed. > > Also I think there's a decent case to be made for error_report() > > to call monitor_flush(). > > A good start, but that would not help for monitors with skip_flush=true, which > need to format the buffered string in a json response, which is the case I > tripped over. 'skip_flush' is only set to 'true' when using a QMP monitor and invoking "hmp-monitor-command". In such a case, the error message needs to be built into a JSON error reply and sent over the socket. Your patch doesn't help this case since you've just printed to stderr. I don't think it is reasonable to expect QMP monitors to send replies on SIG_ABRT anyway. So I don't think the skip_flush=true scenario is a problem to be concerned with. > >> To fix, install a SIGABRT handler to flush the monitor buffer to stderr. > >> > >> Signed-off-by: Steve Sistare > >> --- > >> monitor/monitor.c | 38 ++++++++++++++++++++++++++++++++++++++ > >> 1 file changed, 38 insertions(+) > >> > >> diff --git a/monitor/monitor.c b/monitor/monitor.c > >> index dc352f9..65dace0 100644 > >> --- a/monitor/monitor.c > >> +++ b/monitor/monitor.c > >> @@ -701,6 +701,43 @@ void monitor_cleanup(void) > >> } > >> } > >> > >> +#ifdef CONFIG_LINUX > >> + > >> +static void monitor_abort(int signal, siginfo_t *info, void *c) > >> +{ > >> + Monitor *mon = monitor_cur(); > >> + > >> + if (!mon || qemu_mutex_trylock(&mon->mon_lock)) { > >> + return; > >> + } > >> + > >> + if (mon->outbuf && mon->outbuf->len) { > >> + fputs("SIGABRT received: ", stderr); > >> + fputs(mon->outbuf->str, stderr); > >> + if (mon->outbuf->str[mon->outbuf->len - 1] != '\n') { > >> + fputc('\n', stderr); > >> + } > >> + } > >> + > >> + qemu_mutex_unlock(&mon->mon_lock); > > > > The SIGABRT handling does not only fire in response to abort() > > calls, but also in response to bad memory scenarios, so we have > > to be careful what we do in signal handlers. > > > > In particular using mutexes in signal handlers is a big red > > flag generally. Mutex APIs are not declare async signal > > safe, so this code is technically a POSIX compliance > > violation. > > Righto. I would need to mask all signals in the sigaction to be on the safe(r) side. This is still doomed, because SIGABRT could fire while 'mon_lock' is already held, and so this code would deadlock trying to acquire the lock. > > So I think we'd be safer just eliminating the explicit abort() > > calls and adding monitor_flush call to error_report. > > I like adding a handler because it is future proof. No need to play whack-a-mole when > developers re-introduce abort() calls in the future. A minor benefit is I would not > need ack's from 50 maintainers to change 50 call sites from abort to exit. That's a bit of a crazy exaggeration. THe aborts() don't cover 50 different subsystems, and we don't require explicit acks from every subsystem maintainer for trivial cleanups like this. > A slight risk of the exit solution is that something bad happened at the call site, so > qemu state can no longer be trusted. Calling abort immediately may be safer than calling > exit which will call the existing atexit handlers and could have side effects. If that was a real problem, then we already face it because we have ~500 places already calling exit() and only 50 calling abort(). > A third option is to define qemu_abort() which flushes the monitor, and replaces all abort > calls. That avoids async-signal-mutex hand wringing, but is still subject to whack-a-mole. > > So: atexit, signal handler, or qemu_abort? I will go with your preference. Just replace abort -> exit. I'm not seeing a need for an atexit handler on top. 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 :|