linux-kernel - Re: [PATCH] nvme: fix lockdep warning in nvme_mpath_clear_current

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Date:   Mon, 14 May 2018 06:34:03 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Linux Kernel Mailinglist <linux-kernel@...r.kernel.org>,
        Linux NVMe Mailinglist <linux-nvme@...ts.infradead.org>,
        Hannes Reinecke <hare@...e.de>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH] nvme: fix lockdep warning in
 nvme_mpath_clear_current_path

On Mon, May 14, 2018 at 06:31:05AM -0700, Paul E. McKenney wrote:
> > > +	if (head &&
> > > +	    ns == rcu_dereference_protected(head->current_path,
> > > +				lockdep_is_held(&ns->ctrl->subsys->lock)))
> > >  		rcu_assign_pointer(head->current_path, NULL);
> > >  }
> > >  struct nvme_ns *nvme_find_path(struct nvme_ns_head *head);
> > 
> > We don't really dereference it at all in fact, but just check the
> > pointers for equality.  I wonder if there is a better way to do this,
> > as my ANA patches add a caller without the lock (and withou SRU
> > protection either now that I think of it) - for a pure pointer compare
> > we really should not need any sort of protection.
> 
> If you are just looking at the value of an RCU-protected pointer, then
> using rcu_access_pointer() will cause RCU to just read out the value
> and otherwise keeps its mouth shut.

That is exactly the function I was looking for.  And given that srcu
and rcu use the same annotations I should have through of being able
to use it of course.  As you see above we only use the return value
to do a comparison, so we should be perfectly fine.

Johannes, can you respin the patch to use rcu_access_pointer?