Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,UNPARSEABLE_RELAY,URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1D8B4C43387 for ; Fri, 11 Jan 2019 21:54:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D4E402183F for ; Fri, 11 Jan 2019 21:54:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=oracle.com header.i=@oracle.com header.b="qoW4KO/H" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725767AbfAKVyJ (ORCPT ); Fri, 11 Jan 2019 16:54:09 -0500 Received: from userp2120.oracle.com ([156.151.31.85]:54750 "EHLO userp2120.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725536AbfAKVyI (ORCPT ); Fri, 11 Jan 2019 16:54:08 -0500 Received: from pps.filterd (userp2120.oracle.com [127.0.0.1]) by userp2120.oracle.com (8.16.0.22/8.16.0.22) with SMTP id x0BLrcpe124875; Fri, 11 Jan 2019 21:54:05 GMT DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=oracle.com; h=content-type : mime-version : subject : from : in-reply-to : date : cc : content-transfer-encoding : message-id : references : to; s=corp-2018-07-02; bh=zWZ/Qs2MAyWNWMi9kIilWppaMDfxqBhdAdyyW2ts4YI=; b=qoW4KO/H46JsuAJ4CDdZbSZZo/4XoX7U9FA7XTCuCCG5BtU8VIjXqCiahNV60SDRPcCk 1h12yqRORQdpwpfo6nygdsj9a+o+b/1+A0l9kfZpoSvXfXCzDox3YBzJZtPxryjClrJD CYBMsgPSnV989nPfRfJo1oGOwnfKD7Q38dobDlUy7Zp8pmF8EVJhXPLTc1opKtwCRT9S azWhj+10kni5Hh1VQZ0YHCTvOv1DLzjqhipjxJoFxDjnnUCtsM4xWg+prcgkBCITWkG0 T/UJgQS6gi6bBqTLnJhTewrTBpLW4oR2x2LC1TNxZPC3uW8PP7dv+aAUl725VRUEsx0/ kw== Received: from aserv0021.oracle.com (aserv0021.oracle.com [141.146.126.233]) by userp2120.oracle.com with ESMTP id 2ptn7rf33y-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 11 Jan 2019 21:54:05 +0000 Received: from userv0122.oracle.com (userv0122.oracle.com [156.151.31.75]) by aserv0021.oracle.com (8.14.4/8.14.4) with ESMTP id x0BLs4jx024498 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Fri, 11 Jan 2019 21:54:04 GMT Received: from abhmp0018.oracle.com (abhmp0018.oracle.com [141.146.116.24]) by userv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x0BLs3k6030101; Fri, 11 Jan 2019 21:54:03 GMT Received: from anon-dhcp-171.1015granger.net (/68.61.232.219) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Fri, 11 Jan 2019 13:54:03 -0800 Content-Type: text/plain; charset=us-ascii Mime-Version: 1.0 (Mac OS X Mail 11.5 \(3445.9.1\)) Subject: Re: [PATCH] SUNRPC: Don't allow compiler optimisation of svc_xprt_release_slot() From: Chuck Lever In-Reply-To: Date: Fri, 11 Jan 2019 16:54:01 -0500 Cc: Trond Myklebust , Linux NFS Mailing List Content-Transfer-Encoding: quoted-printable Message-Id: <6F5B73B7-E9F8-4FDB-8381-E5C02772C6A5@oracle.com> References: <20190103141712.24381-1-trond.myklebust@hammerspace.com> <20190103224529.GA6907@fieldses.org> <20190104173912.GC11787@fieldses.org> <20190107213218.GD7753@fieldses.org> <20190108150107.GA15921@fieldses.org> <4077991d3d3acee4c37c7c8c6dc2b76930c9584e.camel@hammerspace.com> <20190109165142.GB32189@fieldses.org> <300445038b75d5efafe9391eb4b8e83d9d6e3633.camel@hammerspace.com> <20190111211235.GA27206@fieldses.org> To: Bruce Fields X-Mailer: Apple Mail (2.3445.9.1) X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9133 signatures=668680 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 suspectscore=0 malwarescore=0 phishscore=0 bulkscore=0 spamscore=0 mlxscore=0 mlxlogscore=976 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1901110173 Sender: linux-nfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-nfs@vger.kernel.org > On Jan 11, 2019, at 4:52 PM, Chuck Lever = wrote: >=20 >=20 >=20 >> On Jan 11, 2019, at 4:12 PM, bfields@fieldses.org wrote: >>=20 >> On Wed, Jan 09, 2019 at 05:41:36PM +0000, Trond Myklebust wrote: >>> The above is stating that >>>=20 >>> smp_rmb(); >>> smp_read_barrier_depends(); >>> if (xprt->xpt_flags & ....) >>>=20 >>> is redundant and can be replaced with just >>>=20 >>> smp_rmb(); >>> if (xprt->xpt_flags & ....) >>>=20 >>> However that's not the case for smp_rmb() followed by READ_ONCE(). = That >>> would expand to >>>=20 >>> smp_rmb(); >>> if (xprt->xpt_flags & ...) { >>> smp_read_barrier_depends(); >>> } else >>> smp_read_barrier_depends(); >>>=20 >>> which is not redundant. It is ensuring (on alpha only) that the read = of >>> xprt->xpt_flags is also not re-ordered w.r.t. other data reads that >>> follow. >>>=20 >>> See, for instance, kernel/events/core.c which has several examples, = or >>> kernel/exit.c. >>=20 >> You're right, I was confused. >>=20 >> So, I think we need your patch plus something like this. >>=20 >> Chuck, maybe you could help me with the "XXX: Chuck:" parts? >=20 > I haven't been following. Why do you think those are necessary? > We've had set_bit and atomic_{inc,dec} in this code for ages, > and I've never noticed a problem. >=20 > Rather than adding another CPU pipeline bubble in the RDMA code, > though, could you simply move the set_bit() call site inside the > critical sections? er, inside the preceding critical section. Just reverse the order of the spin_unlock and the set_bit. >=20 >=20 >> (This applies on top of your patch plus another that just renames the >> stupidly long svc_xprt_has_something_to_do() to svc_xprt_read().) >>=20 >> --b. >>=20 >> commit d7356c3250d4 >> Author: J. Bruce Fields >> Date: Fri Jan 11 15:36:40 2019 -0500 >>=20 >> svcrpc: fix unlikely races preventing queueing of sockets >>=20 >> In the rpc server, When something happens that might be reason to = wake >> up a thread to do something, what we do is >>=20 >> - modify xpt_flags, sk_sock->flags, xpt_reserved, or >> xpt_nr_rqsts to indicate the new situation >> - call svc_xprt_enqueue() to decide whether to wake up a = thread. >>=20 >> svc_xprt_enqueue may require multiple conditions to be true before >> queueing up a thread to handle the xprt. In the SMP case, one of = the >> other CPU's may have set another required condition, and in that = case, >> although both CPUs run svc_xprt_enqueue(), it's possible that = neither >> call sees the writes done by the other CPU in time, and neither one >> recognizes that all the required conditions have been set. A = socket >> could therefore be ignored indefinitely. >>=20 >> Add memory barries to ensure that any svc_xprt_enqueue() call will >> always see the conditions changed by other CPUs before deciding to >> ignore a socket. >>=20 >> I've never seen this race reported. In the unlikely event it = happens, >> another event will usually come along and the problem will fix = itself. >> So I don't think this is worth backporting to stable. >>=20 >> Signed-off-by: J. Bruce Fields >>=20 >> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c >> index d410ae512b02..2af21b84b3b6 100644 >> --- a/net/sunrpc/svc_xprt.c >> +++ b/net/sunrpc/svc_xprt.c >> @@ -357,6 +357,7 @@ static void svc_xprt_release_slot(struct svc_rqst = *rqstp) >> struct svc_xprt *xprt =3D rqstp->rq_xprt; >> if (test_and_clear_bit(RQ_DATA, &rqstp->rq_flags)) { >> atomic_dec(&xprt->xpt_nr_rqsts); >> + smp_wmb(); /* See smp_rmb() in svc_xprt_ready() */ >> svc_xprt_enqueue(xprt); >> } >> } >> @@ -365,6 +366,15 @@ static bool svc_xprt_ready(struct svc_xprt = *xprt) >> { >> unsigned long xpt_flags; >>=20 >> + /* >> + * If another cpu has recently updated xpt_flags, >> + * sk_sock->flags, xpt_reserved, or xpt_nr_rqsts, we need to >> + * know about it; otherwise it's possible that both that cpu and >> + * this one could call svc_xprt_enqueue() without either >> + * svc_xprt_enqueue() recognizing that the conditions below >> + * are satisfied, and we could stall indefinitely: >> + */ >> + smp_rmb(); >> READ_ONCE(xprt->xpt_flags); >>=20 >> if (xpt_flags & (BIT(XPT_CONN) | BIT(XPT_CLOSE))) >> @@ -479,7 +489,7 @@ void svc_reserve(struct svc_rqst *rqstp, int = space) >> if (xprt && space < rqstp->rq_reserved) { >> atomic_sub((rqstp->rq_reserved - space), = &xprt->xpt_reserved); >> rqstp->rq_reserved =3D space; >> - >> + smp_wmb(); /* See smp_rmb() in svc_xprt_ready() */ >> svc_xprt_enqueue(xprt); >> } >> } >> diff --git a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c = b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c >> index 828b149eaaef..377244992ae8 100644 >> --- a/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c >> +++ b/net/sunrpc/xprtrdma/svc_rdma_recvfrom.c >> @@ -316,6 +316,7 @@ static void svc_rdma_wc_receive(struct ib_cq *cq, = struct ib_wc *wc) >> list_add_tail(&ctxt->rc_list, &rdma->sc_rq_dto_q); >> spin_unlock(&rdma->sc_rq_dto_lock); >> set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags); >> + /* XXX: Chuck: do we need an smp_mb__after_atomic() here? */ >> if (!test_bit(RDMAXPRT_CONN_PENDING, &rdma->sc_flags)) >> svc_xprt_enqueue(&rdma->sc_xprt); >> goto out; >> diff --git a/net/sunrpc/xprtrdma/svc_rdma_rw.c = b/net/sunrpc/xprtrdma/svc_rdma_rw.c >> index dc1951759a8e..e1a790487d69 100644 >> --- a/net/sunrpc/xprtrdma/svc_rdma_rw.c >> +++ b/net/sunrpc/xprtrdma/svc_rdma_rw.c >> @@ -290,6 +290,7 @@ static void svc_rdma_wc_read_done(struct ib_cq = *cq, struct ib_wc *wc) >> spin_unlock(&rdma->sc_rq_dto_lock); >>=20 >> set_bit(XPT_DATA, &rdma->sc_xprt.xpt_flags); >> + /* XXX: Chuck: do we need a smp_mb__after_atomic() here? = */ >> svc_xprt_enqueue(&rdma->sc_xprt); >> } >>=20 >=20 > -- > Chuck Lever -- Chuck Lever