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 0524BC83F04 for ; Wed, 2 Jul 2025 12:11:55 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1uWwIv-0001bm-6I; Wed, 02 Jul 2025 08:11:09 -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 1uWwIu-0001bX-4y for qemu-devel@nongnu.org; Wed, 02 Jul 2025 08:11:08 -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 1uWwIn-0004gl-ST for qemu-devel@nongnu.org; Wed, 02 Jul 2025 08:11:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1751458258; 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=YnhrzkU5vZ7tGM0aDBqmL79oz9iz1TPWWmJimRwsUPo=; b=OvPsxtIMy4AndUysIX+LNlSLWuY3MHbq1yfDw8qYVB6TmJelJOybfW3zG+y3KzMP52BWoC aL1M9Fsb2amp6bF58uJZQ9CKRaVZ1bQ+SRq15cC+6mRB3fr8vu/HVCvKPervG7LeejfDYA CkYSmTPem99KCQoMCPBulThOX8ggjMo= Received: from mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-593-7h0qnEr-NuKtn2-lC0EiFw-1; Wed, 02 Jul 2025 08:10:57 -0400 X-MC-Unique: 7h0qnEr-NuKtn2-lC0EiFw-1 X-Mimecast-MFC-AGG-ID: 7h0qnEr-NuKtn2-lC0EiFw_1751458252 Received: from mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.93]) (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 mx-prod-mc-01.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id E39A7195F175; Wed, 2 Jul 2025 12:10:51 +0000 (UTC) Received: from redhat.com (unknown [10.44.32.171]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 256E318003FC; Wed, 2 Jul 2025 12:10:47 +0000 (UTC) Date: Wed, 2 Jul 2025 14:10:45 +0200 From: Kevin Wolf To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Hanna Reitz , Stefan Weil , Paolo Bonzini , Fam Zheng , eblake@redhat.com, Stefano Garzarella , qemu-block@nongnu.org, Aarushi Mehta , hibriansong@gmail.com Subject: Re: [PATCH v2 01/12] aio-posix: fix race between io_uring CQE and AioHandler deletion Message-ID: References: <20250620000829.1426291-1-stefanha@redhat.com> <20250620000829.1426291-2-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250620000829.1426291-2-stefanha@redhat.com> X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 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: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 5.0 requ) BAYES_00=-1.9, 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_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_CERTIFIED_BLOCKED=0.237, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, SPF_HELO_NONE=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: 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 20.06.2025 um 02:08 hat Stefan Hajnoczi geschrieben: > When an AioHandler is enqueued on ctx->submit_list for removal, the > fill_sq_ring() function will submit an io_uring POLL_REMOVE operation to > cancel the in-flight POLL_ADD operation. > > There is a race when another thread enqueues an AioHandler for deletion > on ctx->submit_list when the POLL_ADD CQE has already appeared. In that > case POLL_REMOVE is unnecessary. The code already handled this, but > forgot that the AioHandler itself is still on ctx->submit_list when the > POLL_ADD CQE is being processed. It's unsafe to delete the AioHandler at > that point in time (use-after-free). > > Solve this problem by keeping the AioHandler alive but setting a flag so > that it will be deleted by fill_sq_ring() when it runs. > > Signed-off-by: Stefan Hajnoczi > --- > util/fdmon-io_uring.c | 26 +++++++++++++++++++------- > 1 file changed, 19 insertions(+), 7 deletions(-) > > diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c > index b0d68bdc44..2e40fff09a 100644 > --- a/util/fdmon-io_uring.c > +++ b/util/fdmon-io_uring.c > @@ -52,9 +52,10 @@ enum { > FDMON_IO_URING_ENTRIES = 128, /* sq/cq ring size */ > > /* AioHandler::flags */ > - FDMON_IO_URING_PENDING = (1 << 0), > - FDMON_IO_URING_ADD = (1 << 1), > - FDMON_IO_URING_REMOVE = (1 << 2), > + FDMON_IO_URING_PENDING = (1 << 0), > + FDMON_IO_URING_ADD = (1 << 1), > + FDMON_IO_URING_REMOVE = (1 << 2), > + FDMON_IO_URING_DELETE_AIO_HANDLER = (1 << 3), > }; > > static inline int poll_events_from_pfd(int pfd_events) > @@ -218,6 +219,9 @@ static void fill_sq_ring(AioContext *ctx) > if (flags & FDMON_IO_URING_REMOVE) { > add_poll_remove_sqe(ctx, node); > } > + if (flags & FDMON_IO_URING_DELETE_AIO_HANDLER) { > + QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted); > + } > } > } Why is it safe to add new SQEs for the node and then add it to ctx->deleted_aio_handlers without waiting for the CQEs first? I expected this to be the first check in the loop iteration and to contain a 'continue;' statement. The POLL_REMOVE case is clear when looking at more context, it doesn't pass the node. As for POLL_ADD, I suppose both flags are actually never set together in practice because FDMON_IO_URING_DELETE_AIO_HANDLER is only set when processing the CQE of POLL_ADD, so no new POLL_ADD for the same node will be pending yet. And checking the callers, I see that adding is only ever done with newly allocated nodes, so something like removing and re-adding the same node doesn't happen either. Could we then assert that FDMON_IO_URING_DELETE_AIO_HANDLER is never combined with FDMON_IO_URING_ADD, but always with FDMON_IO_URING_REMOVE, to make the assumptions more explicit? > @@ -347,10 +356,13 @@ void fdmon_io_uring_destroy(AioContext *ctx) > unsigned flags = qatomic_fetch_and(&node->flags, > ~(FDMON_IO_URING_PENDING | > FDMON_IO_URING_ADD | > - FDMON_IO_URING_REMOVE)); > + FDMON_IO_URING_REMOVE | > + FDMON_IO_URING_DELETE_AIO_HANDLER)); > > - if (flags & FDMON_IO_URING_REMOVE) { > - QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, node, node_deleted); > + if ((flags & FDMON_IO_URING_REMOVE) || > + (flags & FDMON_IO_URING_DELETE_AIO_HANDLER)) { If my conclusion above is right, FDMON_IO_URING_REMOVE will be set in both cases, so checking FDMON_IO_URING_DELETE_AIO_HANDLER is redundant. Maybe assert this, too, when setting FDMON_IO_URING_DELETE_AIO_HANDLER. > + QLIST_INSERT_HEAD_RCU(&ctx->deleted_aio_handlers, > + node, node_deleted); > } > > QSLIST_REMOVE_HEAD_RCU(&ctx->submit_list, node_submitted); Kevin