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 lists1p.gnu.org (lists1p.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 11D6DC531F9 for ; Tue, 28 Jul 2026 16:25:04 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wokbt-0005CD-3b; Tue, 28 Jul 2026 12:24:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists1p.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wokbr-0005Bc-9l for qemu-devel@nongnu.org; Tue, 28 Jul 2026 12:24:51 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1wokbp-0002s8-I8 for qemu-devel@nongnu.org; Tue, 28 Jul 2026 12:24:51 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1785255888; 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=HS8il8U5k/gDj35IYlMWAB/ZvnxXF5xTgJfwDonB5tk=; b=ion5Qf5665/mnp3WsYovhHIyBfRfIz3Ssc/fw7S/0krxhyo54TaZSDHzWs+qkw9ahF8ixh eiYHExJmN1Jf8+VHfOOmYik07QfBvIGshftRJXHAYaLMBXw1WA55fgATYQ2WXCFoVaLcrD tFDv47Sx0g8OAxdtFeM5D5Qn2OELLfc= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-358-0HLiithJNaGJ9Ur_3P5aVw-1; Tue, 28 Jul 2026 12:24:43 -0400 X-MC-Unique: 0HLiithJNaGJ9Ur_3P5aVw-1 X-Mimecast-MFC-AGG-ID: 0HLiithJNaGJ9Ur_3P5aVw_1785255882 Received: from mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.95]) (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 mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id DB04F18002E2; Tue, 28 Jul 2026 16:24:41 +0000 (UTC) Received: from redhat.com (unknown [10.44.50.113]) by mx-prod-int-10.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id D7FB3404; Tue, 28 Jul 2026 16:24:39 +0000 (UTC) Date: Tue, 28 Jul 2026 17:24:36 +0100 From: Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= To: Peter Xu Cc: qemu-devel@nongnu.org, Fabiano Rosas , Juraj Marcin , Feifan Qian Subject: Re: [PATCH 4/5] migration: Fix rare hang of migration_channel_read_peek() Message-ID: References: <20260728155247.1894355-1-peterx@redhat.com> <20260728155247.1894355-5-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260728155247.1894355-5-peterx@redhat.com> User-Agent: Mutt/2.4.0 (2026-06-19) X-Scanned-By: MIMEDefang 3.6 on 10.30.177.95 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: -36 X-Spam_score: -3.7 X-Spam_bar: --- X-Spam_report: (-3.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.58, 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_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-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: qemu development 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 Tue, Jul 28, 2026 at 11:52:46AM -0400, Peter Xu wrote: > In an unlikely case, when a migration stream is attached to the destination > QEMU and only send <4 bytes to the channel as magic, it's possible that > migration_channel_read_peek() may spin forever without yielding in the main > thread causing two unwanted consequences: > > - CPU will spin 100% waiting for the rest bytes until it reaches 4 > - (more importantly..) Main thread is stuck during this process as the qio > operation won't really yield the coroutine > > Fix it by consuming the bytes that arrived. > > Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3889 > Cc: Daniel P. Berrangé > Reported-by: Feifan Qian > Signed-off-by: Peter Xu > --- > migration/channel.c | 26 +++++++++++++++++++++++--- > 1 file changed, 23 insertions(+), 3 deletions(-) > > diff --git a/migration/channel.c b/migration/channel.c > index 1e2935f926..28fe1d2906 100644 > --- a/migration/channel.c > +++ b/migration/channel.c > @@ -294,11 +294,31 @@ int migration_channel_read_peek(QIOChannel *ioc, > return -1; > } > > - if (len == buflen) { > + if (len == iov.iov_len) { > break; > - } > + } else if (len == 0) { > + qio_channel_wait_cond(ioc, G_IO_IN); > + } else { > + ssize_t received = len; > > - qio_channel_wait_cond(ioc, G_IO_IN); > + /* > + * Partially arrived, read out to make qio_channel_wait_cond() > + * won't return immediately, causing an unwanted spin on this > + * CPU. > + */ > + iov.iov_len = len; > + len = qio_channel_readv_full(ioc, &iov, 1, NULL, NULL, 0, errp); Sure this breaks the API behaviour that the caller is expecting to see. migration_channel_identify will call migration_channel_read_peek to match the magic bytes. But something later in the flow will actually try to read the magic bytes. By consuming them in this migration_channel_read_peek method, surely we're breaking the code that wants to read the bytes later. > + /* > + * QIO_CHANNEL_ERR_BLOCK also shouldn't happen, due to the > + * prior peek just happened. We should be pretty sure we will > + * read what we peeked, or the channel was broken. > + */ > + if (len != received) { > + return -1; > + } > + iov.iov_base += received; > + iov.iov_len = buflen - received; > + } > } > > return 0; > -- > 2.54.0 > With regards, Daniel -- |: https://berrange.com ~~ https://hachyderm.io/@berrange :| |: https://libvirt.org ~~ https://entangle-photo.org :| |: https://pixelfed.art/berrange ~~ https://fstop138.berrange.com :|