linux-kernel - Re: [PATCH] epoll: fix sparse error on RCU assignment

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Message-ID: <20130314024559.GA19856@dcvr.yhbt.net>
Date:	Thu, 14 Mar 2013 02:45:59 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...IV.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] epoll: fix sparse error on RCU assignment

Oleg Nesterov <oleg@...hat.com> wrote:
> 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...

Hi, I've been hoping others would give a reply and offer a better
solution than min.

Without my proposed patch, sparse _errors_ out on me, so it prevent sparse
from reporting the many other warnings I create in my patches.
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