From patchwork Wed May 10 20:36:01 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Kevin Wolf X-Patchwork-Id: 13237237 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 1D492C77B7C for ; Wed, 10 May 2023 20:38:03 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1pwqYK-0005KC-1g; Wed, 10 May 2023 16:36:48 -0400 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 1pwqYB-0005FJ-AE for qemu-devel@nongnu.org; Wed, 10 May 2023 16:36:40 -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 1pwqY9-0003NY-Qh for qemu-devel@nongnu.org; Wed, 10 May 2023 16:36:39 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1683750997; 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=N1c/27ddh+1DM1DKbHPLEkGEAW689Mmjcp7QLU92/U4=; b=M76IaXT4NC5TxHDKZ7/WTi7sxHt4pocKHWIwW5nGQ/H8a8FyUjRTSJTFmMbruq957FzGQq oACraiv2S5lSiRyORMUZp1dcZNX2iRAFiKU0R58ISy0PfPJj0Czqt6GFfflmXQJcjD8MP7 fQurpHMzNp1OJlFiGcZMLVJmylHTVKY= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-447-dkwqwCdwN52ga9lji9WkYQ-1; Wed, 10 May 2023 16:36:33 -0400 X-MC-Unique: dkwqwCdwN52ga9lji9WkYQ-1 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 829C380269A; Wed, 10 May 2023 20:36:33 +0000 (UTC) Received: from merkur.fritz.box (unknown [10.39.194.148]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4E9931121314; Wed, 10 May 2023 20:36:32 +0000 (UTC) From: Kevin Wolf To: qemu-block@nongnu.org Cc: kwolf@redhat.com, hreitz@redhat.com, stefanha@redhat.com, f.ebner@proxmox.com, qemu-devel@nongnu.org Subject: [PATCH 8/8] graph-lock: Honour read locks even in the main thread Date: Wed, 10 May 2023 22:36:01 +0200 Message-Id: <20230510203601.418015-9-kwolf@redhat.com> In-Reply-To: <20230510203601.418015-1-kwolf@redhat.com> References: <20230510203601.418015-1-kwolf@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.1 on 10.11.54.3 Received-SPF: pass client-ip=170.10.129.124; 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, 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 There are some conditions under which we don't actually need to do anything for taking a reader lock: Writing the graph is only possible from the main context while holding the BQL. So if a reader is running in the main context under the BQL and knows that it won't be interrupted until the next writer runs, we don't actually need to do anything. This is the case if the reader code neither has a nested event loop (this is forbidden anyway while you hold the lock) nor is a coroutine (because a writer could run when the coroutine has yielded). These conditions are exactly what bdrv_graph_rdlock_main_loop() asserts. They are not fulfilled in bdrv_graph_co_rdlock(), which always runs in a coroutine. This deletes the shortcuts in bdrv_graph_co_rdlock() that skip taking the reader lock in the main thread. Reported-by: Fiona Ebner Signed-off-by: Kevin Wolf Reviewed-by: Eric Blake --- block/graph-lock.c | 10 ---------- 1 file changed, 10 deletions(-) diff --git a/block/graph-lock.c b/block/graph-lock.c index 377884c3a9..9c42bd9799 100644 --- a/block/graph-lock.c +++ b/block/graph-lock.c @@ -162,11 +162,6 @@ void coroutine_fn bdrv_graph_co_rdlock(void) BdrvGraphRWlock *bdrv_graph; bdrv_graph = qemu_get_current_aio_context()->bdrv_graph; - /* Do not lock if in main thread */ - if (qemu_in_main_thread()) { - return; - } - for (;;) { qatomic_set(&bdrv_graph->reader_count, bdrv_graph->reader_count + 1); @@ -230,11 +225,6 @@ void coroutine_fn bdrv_graph_co_rdunlock(void) BdrvGraphRWlock *bdrv_graph; bdrv_graph = qemu_get_current_aio_context()->bdrv_graph; - /* Do not lock if in main thread */ - if (qemu_in_main_thread()) { - return; - } - qatomic_store_release(&bdrv_graph->reader_count, bdrv_graph->reader_count - 1); /* make sure writer sees reader_count before we check has_writer */