From patchwork Thu Dec 21 15:35:47 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Stefan Hajnoczi X-Patchwork-Id: 1879260 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@legolas.ozlabs.org Authentication-Results: legolas.ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=Tbv+2zG8; dkim-atps=neutral Authentication-Results: legolas.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=patchwork.ozlabs.org) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by legolas.ozlabs.org (Postfix) with ESMTPS id 4Swvhg575mz1ydZ for ; Fri, 22 Dec 2023 02:38:11 +1100 (AEDT) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rGL5x-0005y4-Jw; Thu, 21 Dec 2023 10:36:21 -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 1rGL5w-0005xd-9U for qemu-devel@nongnu.org; Thu, 21 Dec 2023 10:36:20 -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 1rGL5i-0005az-7T for qemu-devel@nongnu.org; Thu, 21 Dec 2023 10:36:19 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1703172965; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=mK+NLIDFhmr0TUKTwrjSQoiP7pMtYM6wCDVaFG6Vy34=; b=Tbv+2zG8GyJU5gt/jRlF344So06LcFFFGqvvNRWcDrv3cUCOe6UNPXzeil1KIkGAYcqegx ts8YaJ95akOOhWeVYeHZkyE/Zle9zCre6piAfwLM6OaMBXpnji+5AyuS/aqfeQ7Otj83pq xZg1FpMWDTs43o4576Qos8JgYrRnQrk= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-494-l9vTLMsMN86Rkf14PpPTzg-1; Thu, 21 Dec 2023 10:36:02 -0500 X-MC-Unique: l9vTLMsMN86Rkf14PpPTzg-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 D5CEE3C29A6F; Thu, 21 Dec 2023 15:36:01 +0000 (UTC) Received: from localhost (unknown [10.39.194.145]) by smtp.corp.redhat.com (Postfix) with ESMTP id 573FA51D5; Thu, 21 Dec 2023 15:36:01 +0000 (UTC) From: Stefan Hajnoczi To: Kevin Wolf , qemu-devel@nongnu.org Cc: Juan Quintela , Fabiano Rosas , Vladimir Sementsov-Ogievskiy , Hanna Reitz , Paolo Bonzini , Leonardo Bras , Stefan Hajnoczi , Peter Xu , Fam Zheng , Eric Blake , qemu-block@nongnu.org Subject: [PATCH v2 5/6] nbd/server: only traverse NBDExport->clients from main loop thread Date: Thu, 21 Dec 2023 10:35:47 -0500 Message-ID: <20231221153548.1752005-6-stefanha@redhat.com> In-Reply-To: <20231221153548.1752005-1-stefanha@redhat.com> References: <20231221153548.1752005-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.133.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.061, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, SPF_HELO_NONE=0.001, T_SCC_BODY_TEXT_LINE=-0.01, T_SPF_TEMPERROR=0.01 autolearn=unavailable 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+incoming=patchwork.ozlabs.org@nongnu.org Sender: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org The NBD clients list is currently accessed from both the export AioContext and the main loop thread. When the AioContext lock is removed there will be nothing protecting the clients list. Adding a lock around the clients list is tricky because NBDClient structs are refcounted and may be freed from the export AioContext or the main loop thread. nbd_export_request_shutdown() -> client_close() -> nbd_client_put() is also tricky because the list lock would be held while indirectly dropping references to NDBClients. A simpler approach is to only allow nbd_client_put() and client_close() calls from the main loop thread. Then the NBD clients list is only accessed from the main loop thread and no fancy locking is needed. nbd_trip() just needs to reschedule itself in the main loop AioContext before calling nbd_client_put() and client_close(). This costs more CPU cycles per NBD request so add nbd_client_put_nonzero() to optimize the common case where more references to NBDClient remain. Note that nbd_client_get() can still be called from either thread, so make NBDClient->refcount atomic. Signed-off-by: Stefan Hajnoczi --- nbd/server.c | 61 +++++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 51 insertions(+), 10 deletions(-) diff --git a/nbd/server.c b/nbd/server.c index 0b09ccc8dc..e91e2e0903 100644 --- a/nbd/server.c +++ b/nbd/server.c @@ -122,7 +122,7 @@ struct NBDMetaContexts { }; struct NBDClient { - int refcount; + int refcount; /* atomic */ void (*close_fn)(NBDClient *client, bool negotiated); NBDExport *exp; @@ -1501,14 +1501,17 @@ static int coroutine_fn nbd_receive_request(NBDClient *client, NBDRequest *reque #define MAX_NBD_REQUESTS 16 +/* Runs in export AioContext and main loop thread */ void nbd_client_get(NBDClient *client) { - client->refcount++; + qatomic_inc(&client->refcount); } void nbd_client_put(NBDClient *client) { - if (--client->refcount == 0) { + assert(qemu_in_main_thread()); + + if (qatomic_fetch_dec(&client->refcount) == 1) { /* The last reference should be dropped by client->close, * which is called by client_close. */ @@ -1529,8 +1532,35 @@ void nbd_client_put(NBDClient *client) } } +/* + * Tries to release the reference to @client, but only if other references + * remain. This is an optimization for the common case where we want to avoid + * the expense of scheduling nbd_client_put() in the main loop thread. + * + * Returns true upon success or false if the reference was not released because + * it is the last reference. + */ +static bool nbd_client_put_nonzero(NBDClient *client) +{ + int old = qatomic_read(&client->refcount); + int expected; + + do { + if (old == 1) { + return false; + } + + expected = old; + old = qatomic_cmpxchg(&client->refcount, expected, expected - 1); + } while (old != expected); + + return true; +} + static void client_close(NBDClient *client, bool negotiated) { + assert(qemu_in_main_thread()); + if (client->closing) { return; } @@ -2933,15 +2963,20 @@ static coroutine_fn int nbd_handle_request(NBDClient *client, static coroutine_fn void nbd_trip(void *opaque) { NBDClient *client = opaque; - NBDRequestData *req; + NBDRequestData *req = NULL; NBDRequest request = { 0 }; /* GCC thinks it can be used uninitialized */ int ret; Error *local_err = NULL; + /* + * Note that nbd_client_put() and client_close() must be called from the + * main loop thread. Use aio_co_reschedule_self() to switch AioContext + * before calling these functions. + */ + trace_nbd_trip(); if (client->closing) { - nbd_client_put(client); - return; + goto done; } if (client->quiescing) { @@ -2949,10 +2984,9 @@ static coroutine_fn void nbd_trip(void *opaque) * We're switching between AIO contexts. Don't attempt to receive a new * request and kick the main context which may be waiting for us. */ - nbd_client_put(client); client->recv_coroutine = NULL; aio_wait_kick(); - return; + goto done; } req = nbd_request_get(client); @@ -3012,8 +3046,13 @@ static coroutine_fn void nbd_trip(void *opaque) qio_channel_set_cork(client->ioc, false); done: - nbd_request_put(req); - nbd_client_put(client); + if (req) { + nbd_request_put(req); + } + if (!nbd_client_put_nonzero(client)) { + aio_co_reschedule_self(qemu_get_aio_context()); + nbd_client_put(client); + } return; disconnect: @@ -3021,6 +3060,8 @@ disconnect: error_reportf_err(local_err, "Disconnect client, due to: "); } nbd_request_put(req); + + aio_co_reschedule_self(qemu_get_aio_context()); client_close(client, true); nbd_client_put(client); }