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 2D77FC07CB1 for ; Mon, 27 Nov 2023 11:56:36 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1r7aD7-0003N8-2L; Mon, 27 Nov 2023 06:55:33 -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 1r7aD2-0003Mj-EA for qemu-devel@nongnu.org; Mon, 27 Nov 2023 06:55:29 -0500 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 1r7aD0-0008Qj-Jx for qemu-devel@nongnu.org; Mon, 27 Nov 2023 06:55:28 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1701086124; 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: in-reply-to:in-reply-to:references:references; bh=Rfr1XOVjbK35EGD183JXF0Be1H8fgmAjVSqHKfP0x5k=; b=LbvyfcnccozxsOfW0aOHKZrOkJcBK7G3QsrXcYmdk6BJHj0Kj/BtaJ611jpFjsHpDljyEN 6rPzYggKQZq8Jh7AckmxRx77OZj2JwXRAsaDXvFnaBniv/eWfdUO1jOaRmfQPhUKqfw0cl 7CJBCERWqPnNt8jzSGCjD5i8p+/o81U= 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-58-EA8z-aXTM3GvGDh-tsOOkw-1; Mon, 27 Nov 2023 06:55:22 -0500 X-MC-Unique: EA8z-aXTM3GvGDh-tsOOkw-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (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 72DA1101A54C; Mon, 27 Nov 2023 11:55:22 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.177]) by smtp.corp.redhat.com (Postfix) with ESMTPS id CE6B05028; Mon, 27 Nov 2023 11:55:21 +0000 (UTC) Date: Mon, 27 Nov 2023 12:55:20 +0100 From: Kevin Wolf To: qemu-block@nongnu.org Cc: stefanha@redhat.com, qemu-devel@nongnu.org Subject: Re: [PATCH for-8.2] export/vhost-user-blk: Fix consecutive drains Message-ID: References: <20231124174436.46536-1-kwolf@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20231124174436.46536-1-kwolf@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, 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_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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 24.11.2023 um 18:44 hat Kevin Wolf geschrieben: > The vhost-user-blk export implement AioContext switches in its drain > implementation. This means that on drain_begin, it detaches the server > from its AioContext and on drain_end, attaches it again and schedules > the server->co_trip coroutine in the updated AioContext. > > However, nothing guarantees that server->co_trip is even safe to be > scheduled. Not only is it unclear that the coroutine is actually in a > state where it can be reentered externally without causing problems, but > with two consecutive drains, it is possible that the scheduled coroutine > didn't have a chance yet to run and trying to schedule an already > scheduled coroutine a second time crashes with an assertion failure. > > Following the model of NBD, this commit makes the vhost-user-blk export > shut down server->co_trip during drain so that resuming the export means > creating and scheduling a new coroutine, which is always safe. > > There is one exception: If the drain call didn't poll (for example, this > happens in the context of bdrv_graph_wrlock()), then the coroutine > didn't have a chance to shut down. However, in this case the AioContext > can't have changed; changing the AioContext always involves a polling > drain. So in this case we can simply assert that the AioContext is > unchanged and just leave the coroutine running or wake it up if it has > yielded to wait for the AioContext to be attached again. > > Fixes: e1054cd4aad03a493a5d1cded7508f7c348205bf > Fixes: https://issues.redhat.com/browse/RHEL-1708 > Signed-off-by: Kevin Wolf > --- > include/qemu/vhost-user-server.h | 2 ++ > block/export/vhost-user-blk-server.c | 9 +++++-- > util/vhost-user-server.c | 36 +++++++++++++++++++++++----- > 3 files changed, 39 insertions(+), 8 deletions(-) > > diff --git a/include/qemu/vhost-user-server.h b/include/qemu/vhost-user-server.h > index 64ad701015..ca1713b53e 100644 > --- a/include/qemu/vhost-user-server.h > +++ b/include/qemu/vhost-user-server.h > @@ -45,6 +45,8 @@ typedef struct { > /* Protected by ctx lock */ > bool in_qio_channel_yield; > bool wait_idle; > + bool quiescing; > + bool wake_on_ctx_attach; > VuDev vu_dev; > QIOChannel *ioc; /* The I/O channel with the client */ > QIOChannelSocket *sioc; /* The underlying data channel with the client */ > diff --git a/block/export/vhost-user-blk-server.c b/block/export/vhost-user-blk-server.c > index fe2cee3a78..16f48388d3 100644 > --- a/block/export/vhost-user-blk-server.c > +++ b/block/export/vhost-user-blk-server.c > @@ -283,6 +283,7 @@ static void vu_blk_drained_begin(void *opaque) > { > VuBlkExport *vexp = opaque; > > + vexp->vu_server.quiescing = true; > vhost_user_server_detach_aio_context(&vexp->vu_server); > } > > @@ -291,19 +292,23 @@ static void vu_blk_drained_end(void *opaque) > { > VuBlkExport *vexp = opaque; > > + vexp->vu_server.quiescing = false; > vhost_user_server_attach_aio_context(&vexp->vu_server, vexp->export.ctx); > } > > /* > - * Ensures that bdrv_drained_begin() waits until in-flight requests complete. > + * Ensures that bdrv_drained_begin() waits until in-flight requests complete > + * and the server->co_trip coroutine has terminated. It will be restarted in > + * vhost_user_server_attach_aio_context(). > * > * Called with vexp->export.ctx acquired. > */ > static bool vu_blk_drained_poll(void *opaque) > { > VuBlkExport *vexp = opaque; > + VuServer *server = &vexp->vu_server; > > - return vhost_user_server_has_in_flight(&vexp->vu_server); > + return server->co_trip || vhost_user_server_has_in_flight(server); > } > > static const BlockDevOps vu_blk_dev_ops = { > diff --git a/util/vhost-user-server.c b/util/vhost-user-server.c > index 5ccc6d24a0..23004d0c62 100644 > --- a/util/vhost-user-server.c > +++ b/util/vhost-user-server.c > @@ -133,7 +133,9 @@ vu_message_read(VuDev *vu_dev, int conn_fd, VhostUserMsg *vmsg) > server->in_qio_channel_yield = false; > } else { > /* Wait until attached to an AioContext again */ > + server->wake_on_ctx_attach = true; > qemu_coroutine_yield(); > + assert(!server->wake_on_ctx_attach); > } Yielding here isn't good enough as drained_poll waits for the coroutine to terminate, and if the coroutine is here, it will hang. v2 will return instead. Kevin