Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753184Ab3CJSZv (ORCPT ); Sun, 10 Mar 2013 14:25:51 -0400 Received: from mx1.redhat.com ([209.132.183.28]:61164 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752893Ab3CJSZu (ORCPT ); Sun, 10 Mar 2013 14:25:50 -0400 Date: Sun, 10 Mar 2013 19:23:58 +0100 From: Oleg Nesterov To: Eric Wong Cc: Al Viro , Andrew Morton , Eric Dumazet , linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Davide Libenzi , "Paul E. McKenney" Subject: Re: [PATCH] epoll: fix sparse error on RCU assignment Message-ID: <20130310182358.GA686@redhat.com> References: <20130310113559.GA16551@dcvr.yhbt.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130310113559.GA16551@dcvr.yhbt.net> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2247 Lines: 59 On 03/10, Eric Wong wrote: > > This fixes the following sparse error when using > CONFIG_SPARSE_RCU_POINTER=y and "make C=2 fs/eventpoll.o" > > fs/eventpoll.c:514:17: error: incompatible types in comparison expression (different address spaces) ep_remove_wait_queue() does rcu_dereference(pwq->whead) and rcu_dereference_sparse(__rcu) complains, I guess. > --- a/fs/eventpoll.c > +++ b/fs/eventpoll.c > @@ -228,7 +228,7 @@ struct eppoll_entry { > wait_queue_t wait; > > /* The wait queue head that linked the "wait" wait queue item */ > - wait_queue_head_t *whead; > + wait_queue_head_t __rcu *whead; Well, perhaps this change is fine... but otoh this this a bit misleading. It is not actually __rcu. The special case is sighand->signalfd_wqh, and the commemt in ep_remove_wait_queue() means: if ->whead is not stable then we can only race with signalfd_cleanup(), and rcu_read_lock() ensures this memory can't go away. We do not even need smp_read_barrier_depends() here, ACCESS_ONCE() should be enough. Perhaps it would be better to simply shut up this warning somehow... > }; > > /* Wrapper struct used by poll queueing */ > @@ -929,7 +929,7 @@ static int ep_poll_callback(wait_queue_t *wait, unsigned mode, int sync, void *k > struct eventpoll *ep = epi->ep; > > if ((unsigned long)key & POLLFREE) { > - ep_pwq_from_wait(wait)->whead = NULL; > + RCU_INIT_POINTER(ep_pwq_from_wait(wait)->whead, NULL); > /* > * whead = NULL above can race with ep_remove_wait_queue() > * which can do another remove_wait_queue() after us, so we > @@ -1018,7 +1018,7 @@ static void ep_ptable_queue_proc(struct file *file, wait_queue_head_t *whead, > > if (epi->nwait >= 0 && (pwq = kmem_cache_alloc(pwq_cache, GFP_KERNEL))) { > init_waitqueue_func_entry(&pwq->wait, ep_poll_callback); > - pwq->whead = whead; > + RCU_INIT_POINTER(pwq->whead, whead); > pwq->base = epi; > add_wait_queue(whead, &pwq->wait); > list_add_tail(&pwq->llink, &epi->pwqlist); > -- > Eric Wong -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/