From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from nautica.notk.org ([91.121.71.147]:33391 "EHLO nautica.notk.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S966279AbeF1NsG (ORCPT ); Thu, 28 Jun 2018 09:48:06 -0400 Date: Thu, 28 Jun 2018 15:40:29 +0200 From: Dominique Martinet To: Matthew Wilcox Cc: v9fs-developer@lists.sourceforge.net, Latchesar Ionkov , Eric Van Hensbergen , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Ron Minnich Subject: Re: [V9fs-developer] [PATCH 4/6] 9p: Remove an unnecessary memory barrier Message-ID: <20180628134029.GA24673@nautica> References: <20180628132629.3148-1-willy@infradead.org> <20180628132629.3148-5-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20180628132629.3148-5-willy@infradead.org> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Matthew Wilcox wrote on Thu, Jun 28, 2018: > --- a/net/9p/client.c > +++ b/net/9p/client.c > @@ -436,13 +436,9 @@ void p9_client_cb(struct p9_client *c, struct p9_req_t *req, int status) > { > p9_debug(P9_DEBUG_MUX, " tag %d\n", req->tc->tag); > > - /* > - * This barrier is needed to make sure any change made to req before > - * the other thread wakes up will indeed be seen by the waiting side. > - */ > - smp_wmb(); > req->status = status; > > + /* wake_up is an implicit write memory barrier */ Nope. Please note the wmb is _before_ setting status, basically it protects from cpu optimizations where status could be set before other fields, then other core opportunistically checking and finding status is good so other thread continuing. I could only reproduce this bug with infiniband network, but it is very definitely needed. Here is the commit message of when I added that barrier: ----- 9P: Add memory barriers to protect request fields over cb/rpc threads handoff We need barriers to guarantee this pattern works as intended: [w] req->rc, 1 [r] req->status, 1 wmb rmb [w] req->status, 1 [r] req->rc Where the wmb ensures that rc gets written before status, and the rmb ensures that if you observe status == 1, rc is the new value. ----- It might need an update to the comment though, if you thought about removing it... -- Dominique Martinet | Asmadeus