From nobody Tue May 14 15:20:22 2024 Delivered-To: importer@patchew.org Authentication-Results: mx.zohomail.com; dkim=fail; spf=pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) smtp.mailfrom=qemu-devel-bounces+importer=patchew.org@nongnu.org; dmarc=fail(p=none dis=none) header.from=redhat.com Return-Path: Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) by mx.zohomail.com with SMTPS id 1645631956304200.58997278934703; Wed, 23 Feb 2022 07:59:16 -0800 (PST) Received: from localhost ([::1]:56916 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1nMu2s-0000IZ-Qz for importer@patchew.org; Wed, 23 Feb 2022 10:59:14 -0500 Received: from eggs.gnu.org ([209.51.188.92]:44662) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nMu1O-00071a-W4 for qemu-devel@nongnu.org; Wed, 23 Feb 2022 10:57:43 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]:34704) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1nMu1F-00035w-EM for qemu-devel@nongnu.org; Wed, 23 Feb 2022 10:57:42 -0500 Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-439-sLtOB-OlNO60NvVdvF_TUQ-1; Wed, 23 Feb 2022 10:57:30 -0500 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id B32C21091DA2; Wed, 23 Feb 2022 15:57:29 +0000 (UTC) Received: from localhost (unknown [10.39.195.159]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4B5AC83790; Wed, 23 Feb 2022 15:57:04 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1645631852; 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; bh=bFX6I6Zco2OnlKD5mCsWjbTnHqtoaa0seYWQbKqIhRY=; b=K6kEddnM9HZU+4NufS7VKaC4uPVow/DtstgQoplyUlYk9HFGS3rms3Wsa4B1kHz6G/75pv R0DKZaKSpRITgaEK1GMyUs5IucP7PQUDlUH8M3GIpcveXzAMJFkv8HUDV9MQE00/5s2c3u KG+M2CnYfPyf8ig5/nw3ggHnI2IGCqs= X-MC-Unique: sLtOB-OlNO60NvVdvF_TUQ-1 From: Stefan Hajnoczi To: qemu-devel@nongnu.org Subject: [PATCH] aio-posix: fix spurious ->poll_ready() callbacks in main loop Date: Wed, 23 Feb 2022 15:57:03 +0000 Message-Id: <20220223155703.136833-1-stefanha@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=stefanha@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=stefanha@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H5=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=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: , Cc: Fam Zheng , Paolo Bonzini , Jason Wang , qemu-block@nongnu.org, Stefan Hajnoczi Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: fail (Header signature does not verify) X-ZM-MESSAGEID: 1645631958563100001 Content-Type: text/plain; charset="utf-8" When ->poll() succeeds the AioHandler is placed on the ready list with revents set to the magic value 0. This magic value causes aio_dispatch_handler() to invoke ->poll_ready() instead of ->io_read() for G_IO_IN or ->io_write() for G_IO_OUT. This magic value 0 hack works for the IOThread where AioHandlers are placed on ->ready_list and processed by aio_dispatch_ready_handlers(). It does not work for the main loop where all AioHandlers are processed by aio_dispatch_handlers(), even those that are not ready and have a revents value of 0. As a result the main loop invokes ->poll_ready() on AioHandlers that are not ready. These spurious ->poll_ready() calls waste CPU cycles and could lead to crashes if the code assumes ->poll() must have succeeded before ->poll_ready() is called (a reasonable asumption but I haven't seen it in practice). Stop using revents to track whether ->poll_ready() will be called on an AioHandler. Introduce a separate AioHandler->poll_ready field instead. This eliminates spurious ->poll_ready() calls in the main loop. Fixes: 826cc32423db2a99d184dbf4f507c737d7e7a4ae ("aio-posix: split poll che= ck from ready handler") Signed-off-by: Stefan Hajnoczi Reported-by: Jason Wang Tested-by: Jason Wang --- util/aio-posix.h | 1 + util/aio-posix.c | 32 ++++++++++++++++++-------------- 2 files changed, 19 insertions(+), 14 deletions(-) diff --git a/util/aio-posix.h b/util/aio-posix.h index 7f2c37a684..80b927c7f4 100644 --- a/util/aio-posix.h +++ b/util/aio-posix.h @@ -37,6 +37,7 @@ struct AioHandler { unsigned flags; /* see fdmon-io_uring.c */ #endif int64_t poll_idle_timeout; /* when to stop userspace polling */ + bool poll_ready; /* has polling detected an event? */ bool is_external; }; =20 diff --git a/util/aio-posix.c b/util/aio-posix.c index 7b9f629218..be0182a3c6 100644 --- a/util/aio-posix.c +++ b/util/aio-posix.c @@ -23,15 +23,6 @@ #include "trace.h" #include "aio-posix.h" =20 -/* - * G_IO_IN and G_IO_OUT are not appropriate revents values for polling, si= nce - * the handler may not need to access the file descriptor. For example, the - * handler doesn't need to read from an EventNotifier if it polled a memory - * location and a read syscall would be slow. Define our own unique revents - * value to indicate that polling determined this AioHandler is ready. - */ -#define REVENTS_POLL_READY 0 - /* Stop userspace polling on a handler if it isn't active for some time */ #define POLL_IDLE_INTERVAL_NS (7 * NANOSECONDS_PER_SECOND) =20 @@ -49,6 +40,14 @@ void aio_add_ready_handler(AioHandlerList *ready_list, QLIST_INSERT_HEAD(ready_list, node, node_ready); } =20 +static void aio_add_poll_ready_handler(AioHandlerList *ready_list, + AioHandler *node) +{ + QLIST_SAFE_REMOVE(node, node_ready); /* remove from nested parent's li= st */ + node->poll_ready =3D true; + QLIST_INSERT_HEAD(ready_list, node, node_ready); +} + static AioHandler *find_aio_handler(AioContext *ctx, int fd) { AioHandler *node; @@ -76,6 +75,7 @@ static bool aio_remove_fd_handler(AioContext *ctx, AioHan= dler *node) } =20 node->pfd.revents =3D 0; + node->poll_ready =3D false; =20 /* If the fd monitor has already marked it deleted, leave it alone */ if (QLIST_IS_INSERTED(node, node_deleted)) { @@ -247,7 +247,7 @@ static bool poll_set_started(AioContext *ctx, AioHandle= rList *ready_list, =20 /* Poll one last time in case ->io_poll_end() raced with the event= */ if (!started && node->io_poll(node->opaque)) { - aio_add_ready_handler(ready_list, node, REVENTS_POLL_READY); + aio_add_poll_ready_handler(ready_list, node); progress =3D true; } } @@ -282,6 +282,7 @@ bool aio_pending(AioContext *ctx) QLIST_FOREACH_RCU(node, &ctx->aio_handlers, node) { int revents; =20 + /* TODO should this check poll ready? */ revents =3D node->pfd.revents & node->pfd.events; if (revents & (G_IO_IN | G_IO_HUP | G_IO_ERR) && node->io_read && aio_node_check(ctx, node->is_external)) { @@ -323,11 +324,15 @@ static void aio_free_deleted_handlers(AioContext *ctx) static bool aio_dispatch_handler(AioContext *ctx, AioHandler *node) { bool progress =3D false; + bool poll_ready; int revents; =20 revents =3D node->pfd.revents & node->pfd.events; node->pfd.revents =3D 0; =20 + poll_ready =3D node->poll_ready; + node->poll_ready =3D false; + /* * Start polling AioHandlers when they become ready because activity is * likely to continue. Note that starvation is theoretically possible= when @@ -344,7 +349,7 @@ static bool aio_dispatch_handler(AioContext *ctx, AioHa= ndler *node) QLIST_INSERT_HEAD(&ctx->poll_aio_handlers, node, node_poll); } if (!QLIST_IS_INSERTED(node, node_deleted) && - revents =3D=3D 0 && + poll_ready && revents =3D=3D 0 && aio_node_check(ctx, node->is_external) && node->io_poll_ready) { node->io_poll_ready(node->opaque); @@ -432,7 +437,7 @@ static bool run_poll_handlers_once(AioContext *ctx, QLIST_FOREACH_SAFE(node, &ctx->poll_aio_handlers, node_poll, tmp) { if (aio_node_check(ctx, node->is_external) && node->io_poll(node->opaque)) { - aio_add_ready_handler(ready_list, node, REVENTS_POLL_READY); + aio_add_poll_ready_handler(ready_list, node); =20 node->poll_idle_timeout =3D now + POLL_IDLE_INTERVAL_NS; =20 @@ -491,8 +496,7 @@ static bool remove_idle_poll_handlers(AioContext *ctx, * this causes progress. */ if (node->io_poll(node->opaque)) { - aio_add_ready_handler(ready_list, node, - REVENTS_POLL_READY); + aio_add_poll_ready_handler(ready_list, node); progress =3D true; } } --=20 2.34.1