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

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Date:	Fri, 15 Mar 2013 18:42:40 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Eric Wong <normalperson@...t.net>
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

On 03/14, Eric Wong wrote:
>
> 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.

Me too ;)

OK, probably we should use your patch, although personally I'd prefer
to simply shut up the warning, say

	- whead = rcu_dereference(pwq->whead);
	+ whead = rcu_dereference((void __rcu*)pwq->whead);

> Without my proposed patch, sparse _errors_ out on me,

it is only sparse...

But OK, I won't argue with you patch.

Oleg.

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