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 A0A99C53200 for ; Wed, 29 Jul 2026 14:39:49 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists1p.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1wp5RI-0001PX-BI; Wed, 29 Jul 2026 10:39:20 -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 1wp5RD-0001PA-FD for qemu-devel@nongnu.org; Wed, 29 Jul 2026 10:39:16 -0400 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 1wp5RA-0005um-TF for qemu-devel@nongnu.org; Wed, 29 Jul 2026 10:39:15 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1785335951; 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=qOnTk6/yHiTcYITCKdeVC5C3GiW3dEC6Vexx6iygnPs=; b=RhikhoDbeQWhcg6TVjiNxIn8XOMEeG2v3oVOD8mRx5ynT+i6CTGLn6Vq8vB/yHXT9wY4kA mWD4D9difIwjG+98d2c5lugZk2TIuocXJbsY29igllo90N1n0U7zXaeaevgOrsTKenldgy qn4l/Nusu8bl0lO6D1Iws3d7EOnDJfA= Received: from mail-qv1-f69.google.com (mail-qv1-f69.google.com [209.85.219.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-75-eXCI95daOiuqwqyq3XX6aQ-1; Wed, 29 Jul 2026 10:39:10 -0400 X-MC-Unique: eXCI95daOiuqwqyq3XX6aQ-1 X-Mimecast-MFC-AGG-ID: eXCI95daOiuqwqyq3XX6aQ_1785335949 Received: by mail-qv1-f69.google.com with SMTP id 6a1803df08f44-8eeba1d9e47so12069326d6.2 for ; Wed, 29 Jul 2026 07:39:10 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=google; t=1785335949; x=1785940749; darn=nongnu.org; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:from:to:cc:subject:date:message-id:reply-to:content-type; bh=qOnTk6/yHiTcYITCKdeVC5C3GiW3dEC6Vexx6iygnPs=; b=Aslgrghs2HhhtRpZh0c9WrvvdDAGNe6AiEW4ARa7OGPM0VLqCGryOzFp56no5QrdsV clWKO3jHF4Z73fdVJsmYdefnYUW3YwubX6mwY5JEx/MMjakDN81OPHB0xB+c1MgFoD9h kZXKwBH0yOGUtDC4sJonxJLLkdb235OIweoehnqZHRJemk4B+UzlwwYvHamL/TTYECYF yA+E1CMZrtI26JS+JoAhbDDm5VFe83Y1GfE9WeQzvXbLuk4EylcQTQof3usKUnIxkvcf /qIRVCxLSuyHi5o1gzwVTvCxfXGa5gkvsCk98RHsAAinbv14Co5jnVW88CtxvNXdTxZJ xTTQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1785335949; x=1785940749; h=in-reply-to:content-transfer-encoding:content-disposition :content-type:mime-version:references:message-id:subject:cc:to:from :date:x-gm-gg:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to:content-type; bh=qOnTk6/yHiTcYITCKdeVC5C3GiW3dEC6Vexx6iygnPs=; b=AwajKyL0isbvkU/8KS9zgaIuhLD1S4MsNm0iBuj4wv1xF1gg0UGFjY3wHk/QMIfrva gBClQFgzxqe8niksdz1QUlHSc8/A74DzwuhU/0+NJL9SQn8FhEU+Z2CcUxDYKK81zGnI fLF2tR2vhM2I7SUNNIIBxkXl0r3QwgxEoxL280U2zuMB+IXSPJFZ5ORr3tWyJO+0ehyZ 69YdfaRRty7yOZUn+icdOhmdTtBlDkMcegCnAmuwBzC1xaAO+4zdZf1WZ1kBkOn20JrF +eXkSDxInKNLDMZGBwe+rb26uOcssG9uDQ12eZs2kgYVKsYhxf11Q8gKeEmZ6hbLYn0P Q/Sg== X-Gm-Message-State: AOJu0Yxl+eYXLmPx2wr8+elFeJ7OycX/GcvNdFRN+NqH+G8AuDhhAAR9 PwS/cg+Gt6NVHvPX0iv0M6EJb6r2TsC+OhyH8Szb9WSDkzIfS4KXkfmenfUbLF+5ndpu5sd0oBy GsrzHohzkYm5x49uKzVXdh0UHMrxyrnmWm0e04vtZlrGmxEy8pu4TACx+AZ3NQfdnn99H7HQqQr WNJj8RTPAXIJ95elKCWeio/VcGjZUMFIPZiGWkFA== X-Gm-Gg: AR+sD13MZ3688KHfv+VRy7xGmwGG9LRJuoPnwn5Lx72/3sFbwU8vguVvMwfVTk4+sw6 fEVFpjmq4e/Ego3muR3GlghXcBRIu7rNxRDwkncPZrFLNOCfHaBm//g1bkpvo0hrdUaTnIJ1qMd ZC/ZeyY/21O3WGcN1nJAXm5//yzjQ6JgCx8C3oyKpDpcJVYDVJOHr81rdfcnL7Mca5CiH3tbwJi xRjrixzZKFfkpP9+zbPN0UmDjnR9kE/rxv99y+xyEtMNd7OZ6GzB1QN5bV/RXlmpVSUOOMfH4mi BB5LeSG0Ds0YxZS6menuR6bUuJs78CGzemf9gsTmgbBXLze7wbwlA1vuHyqXCy+Nt/f/DcNVR9U sgjnL9NafP78rRCVfdkyFVc2lzvo= X-Received: by 2002:a05:6214:29c1:b0:8ef:ec2c:6b70 with SMTP id 6a1803df08f44-90817379643mr79305296d6.53.1785335949303; Wed, 29 Jul 2026 07:39:09 -0700 (PDT) X-Received: by 2002:a05:6214:29c1:b0:8ef:ec2c:6b70 with SMTP id 6a1803df08f44-90817379643mr79304276d6.53.1785335947843; Wed, 29 Jul 2026 07:39:07 -0700 (PDT) Received: from x1.local (67-208-31-164.ip.tor.radiant.net. [67.208.31.164]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-9081dd6b55asm25599566d6.36.2026.07.29.07.39.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 29 Jul 2026 07:39:07 -0700 (PDT) Date: Wed, 29 Jul 2026 10:38:59 -0400 From: Peter Xu To: qemu-devel@nongnu.org Cc: Fabiano Rosas , Juraj Marcin , Feifan Qian , Daniel =?utf-8?B?UC4gQmVycmFuZ8Op?= Subject: Re: [PATCH v2 4/5] migration: Fix rare hang of migration_channel_read_peek() Message-ID: References: <20260728210417.1925078-1-peterx@redhat.com> <20260728210417.1925078-5-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@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_H2=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: , 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 05:29:35PM -0400, Peter Xu wrote: > On Tue, Jul 28, 2026 at 05:04:16PM -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. > > > > Fix it by adding a manual sleep for partial read. > > > > Since the path isn't attached to a coroutine, it means when partial read > > happens, there's yet not much we can do but hang the main thread, it will > > happen even for len==0 case. It means monitors can hang due to this, > > either partial read or no data arrived (but connection established). > > > > Leave this for later, the hope is this is extremely rare in production. > > > > Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3889 > > Reported-by: Feifan Qian > > Cc: Daniel P. Berrangé > > Signed-off-by: Peter Xu > > --- > > migration/channel.c | 11 +++++++++-- > > 1 file changed, 9 insertions(+), 2 deletions(-) > > > > diff --git a/migration/channel.c b/migration/channel.c > > index 1e2935f926..f446561b59 100644 > > --- a/migration/channel.c > > +++ b/migration/channel.c > > @@ -296,9 +296,16 @@ int migration_channel_read_peek(QIOChannel *ioc, > > > > if (len == buflen) { > > break; > > + } else if (len == 0) { > > I think this should be QIO_CHANNEL_ERR_BLOCK, not 0.. I'll fix it when I > post v3, and I'll do some more tests. I just found that I cannot really hit this path.. because essentially migration_channel_read_peek() is too early, and we haven't set iochannel to be in non-blocking (QEMU only does it for the main channel; for the rest channels they're always in blocking mode). But still, instead of asserting it, I plan to keep this line so this code is generic to blocking mode too in the future. > > > + qio_channel_wait_cond(ioc, G_IO_IN); > > + } else { > > + /* > > + * When partially ready, we can't use qio_channel_wait_cond() > > + * because it will return immediately. Apply a manual wait. > > + */ > > + assert(!qemu_in_coroutine()); > > + g_usleep(1000); > > } > > - > > - qio_channel_wait_cond(ioc, G_IO_IN); > > } > > > > return 0; > > -- > > 2.54.0 > > > > -- > Peter Xu -- Peter Xu