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.xenproject.org (lists.xenproject.org [192.237.175.120]) (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 695C7C41535 for ; Tue, 19 Dec 2023 18:20:35 +0000 (UTC) Received: from list by lists.xenproject.org with outflank-mailman.657071.1025731 (Exim 4.92) (envelope-from ) id 1rFehM-0006ln-U4; Tue, 19 Dec 2023 18:20:08 +0000 X-Outflank-Mailman: Message body and most headers restored to incoming version Received: by outflank-mailman (output) from mailman id 657071.1025731; Tue, 19 Dec 2023 18:20:08 +0000 Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1rFehM-0006lg-RI; Tue, 19 Dec 2023 18:20:08 +0000 Received: by outflank-mailman (input) for mailman id 657071; Tue, 19 Dec 2023 18:20:07 +0000 Received: from se1-gles-flk1-in.inumbo.com ([94.247.172.50] helo=se1-gles-flk1.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1rFehL-0006cR-CJ for xen-devel@lists.xenproject.org; Tue, 19 Dec 2023 18:20:07 +0000 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by se1-gles-flk1.inumbo.com (Halon) with ESMTPS id 39f8db44-9e9b-11ee-9b0f-b553b5be7939; Tue, 19 Dec 2023 19:20:03 +0100 (CET) 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-557-ng6tSX5TO9q6N0AkdP052Q-1; Tue, 19 Dec 2023 13:19:54 -0500 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.rdu2.redhat.com [10.11.54.3]) (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 9391184B04B; Tue, 19 Dec 2023 18:19:53 +0000 (UTC) Received: from redhat.com (unknown [10.39.194.175]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 81F601121306; Tue, 19 Dec 2023 18:19:48 +0000 (UTC) X-BeenThere: xen-devel@lists.xenproject.org List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Errors-To: xen-devel-bounces@lists.xenproject.org Precedence: list Sender: "Xen-devel" X-Inumbo-ID: 39f8db44-9e9b-11ee-9b0f-b553b5be7939 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1703010002; 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=kdHrCBh+aFwerdanskMZMdWR+mM1k+MIbCkQXlcqavA=; b=YUzEEySBxcELbNg2caIEBGbXo1YJnCd4+VMKq/hxIgtYu/AO5v1qTcQUzzyeo6WM89UuYQ 68rHFSIGHAtjX7SF3uC/yzLJArla9GFtWnpPYx2KMP6QWHadUmWQKN3qxWWLd6DY2fbtEb f3DNBL9RSzQtmLo9s8iuPGAWf7/deyw= X-MC-Unique: ng6tSX5TO9q6N0AkdP052Q-1 Date: Tue, 19 Dec 2023 19:19:47 +0100 From: Kevin Wolf To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= , Vladimir Sementsov-Ogievskiy , Cleber Rosa , Xie Changlong , Paul Durrant , Ari Sundholm , Jason Wang , Eric Blake , John Snow , Eduardo Habkost , Wen Congyang , Alberto Garcia , Anthony Perard , "Michael S. Tsirkin" , Stefano Stabellini , qemu-block@nongnu.org, Juan Quintela , Paolo Bonzini , Coiby Xu , Fabiano Rosas , Hanna Reitz , Zhang Chen , Daniel =?iso-8859-1?Q?P=2E_Berrang=E9?= , Pavel Dovgalyuk , Peter Xu , Emanuele Giuseppe Esposito , Fam Zheng , Leonardo Bras , David Hildenbrand , Li Zhijian , xen-devel@lists.xenproject.org Subject: Re: [PATCH v2 04/14] aio: make aio_context_acquire()/aio_context_release() a no-op Message-ID: References: <20231205182011.1976568-1-stefanha@redhat.com> <20231205182011.1976568-5-stefanha@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.3 Am 19.12.2023 um 16:28 hat Kevin Wolf geschrieben: > Am 05.12.2023 um 19:20 hat Stefan Hajnoczi geschrieben: > > aio_context_acquire()/aio_context_release() has been replaced by > > fine-grained locking to protect state shared by multiple threads. The > > AioContext lock still plays the role of balancing locking in > > AIO_WAIT_WHILE() and many functions in QEMU either require that the > > AioContext lock is held or not held for this reason. In other words, the > > AioContext lock is purely there for consistency with itself and serves > > no real purpose anymore. > > > > Stop actually acquiring/releasing the lock in > > aio_context_acquire()/aio_context_release() so that subsequent patches > > can remove callers across the codebase incrementally. > > > > I have performed "make check" and qemu-iotests stress tests across > > x86-64, ppc64le, and aarch64 to confirm that there are no failures as a > > result of eliminating the lock. > > > > Signed-off-by: Stefan Hajnoczi > > Reviewed-by: Eric Blake > > Acked-by: Kevin Wolf > > I knew why I wasn't confident enough to give a R-b... This crashes > qemu-storage-daemon in the qemu-iotests case graph-changes-while-io. > > qemu-storage-daemon: ../nbd/server.c:2542: nbd_co_receive_request: Assertion `client->recv_coroutine == qemu_coroutine_self()' failed. > > (gdb) bt > #0 0x00007fdb00529884 in __pthread_kill_implementation () from /lib64/libc.so.6 > #1 0x00007fdb004d8afe in raise () from /lib64/libc.so.6 > #2 0x00007fdb004c187f in abort () from /lib64/libc.so.6 > #3 0x00007fdb004c179b in __assert_fail_base.cold () from /lib64/libc.so.6 > #4 0x00007fdb004d1187 in __assert_fail () from /lib64/libc.so.6 > #5 0x0000557f9f9534eb in nbd_co_receive_request (errp=0x7fdafc25eec0, request=0x7fdafc25ef10, req=0x7fdaf00159c0) at ../nbd/server.c:2542 > #6 nbd_trip (opaque=0x557fa0b33fa0) at ../nbd/server.c:2962 > #7 0x0000557f9faa416b in coroutine_trampoline (i0=, i1=) at ../util/coroutine-ucontext.c:177 > #8 0x00007fdb004efe90 in ?? () from /lib64/libc.so.6 > #9 0x00007fdafc35f680 in ?? () > #10 0x0000000000000000 in ?? () > (gdb) p *client > $2 = {refcount = 4, close_fn = 0x557f9f95dc40 , exp = 0x557fa0b30590, tlscreds = 0x0, tlsauthz = 0x0, sioc = 0x557fa0b33d90, ioc = 0x557fa0b33d90, > recv_coroutine = 0x7fdaf0015eb0, send_lock = {locked = 0, ctx = 0x0, from_push = {slh_first = 0x0}, to_pop = {slh_first = 0x0}, handoff = 0, sequence = 0, holder = 0x0}, > send_coroutine = 0x0, read_yielding = false, quiescing = false, next = {tqe_next = 0x0, tqe_circ = {tql_next = 0x0, tql_prev = 0x557fa0b305e8}}, nb_requests = 1, closing = false, > check_align = 1, mode = NBD_MODE_EXTENDED, contexts = {exp = 0x557fa0b30590, count = 1, base_allocation = true, allocation_depth = false, bitmaps = 0x0}, opt = 7, optlen = 0} > (gdb) p co_tls_current > $3 = (Coroutine *) 0x7fdaf00061d0 This one isn't easy to debug... The first problem here is that two nbd_trip() coroutines are scheduled in the same iothread, and creating the second one overwrites client->recv_coroutine, which triggers the assertion in the first one. This can be fixed by introducing a new mutex in NBDClient and taking it in nbd_client_receive_next_request() so that there is no race between checking client->recv_coroutine != NULL and setting it to a new coroutine. (Not entirely sure why two different threads are doing this, maybe the main thread reentering in drained_end and the iothread waiting for the next request?) However, I'm seeing new assertion failures when I do that: client->quiescing isn't set in the -EAGAIN case in nbd_trip(). I haven't really figured out yet where this comes from. Taking the new NBDClient lock in the drain functions and in nbd_trip() doesn't seem to be enough to fix it anyway. Or maybe I didn't quite find the right places to take it. I'm not sure if the list of NBD clients needs a lock, too, or if it is only ever accessed in the main thread, but that should be unrelated to the assertion failures I'm seeing. Kevin