Messages in this thread | | | Date | Fri, 15 Mar 2013 18:42:40 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH] epoll: fix sparse error on RCU assignment |
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On 03/14, Eric Wong wrote: > > Oleg Nesterov <oleg@redhat.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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