linux-kernel - Re: [PATCH 2/2] fs: nfs: dir.c: Fix sparse error

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Message-ID: <20191206161201.GF2889@paulmck-ThinkPad-P72>
Date:   Fri, 6 Dec 2019 08:12:01 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     madhuparnabhowmik04@...il.com, trond.myklebust@...merspace.com,
        anna.schumaker@...app.com, linux-nfs@...r.kernel.org,
        rcu@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] fs: nfs: dir.c: Fix sparse error

On Fri, Dec 06, 2019 at 11:00:02AM -0500, Joel Fernandes wrote:
> +Paul, here is the dependent patch for the list_prev_rcu() patch Madhuparna
> posted.

Got it, thank you!

And however this turns out, it does illustrate the value of the sparse
address-space checks!

							Thanx, Paul

> On Fri, Dec 06, 2019 at 08:46:40PM +0530, madhuparnabhowmik04@...il.com wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
> > 
> > This patch fixes the following errors:
> > fs/nfs/dir.c:2353:14: error: incompatible types in comparison expression (different address spaces):
> > fs/nfs/dir.c:2353:14:    struct list_head [noderef] <asn:4> *
> > fs/nfs/dir.c:2353:14:    struct list_head *
> > 
> > caused due to directly accessing the prev pointer of
> > a RCU protected list.
> > Accessing the pointer using the macro list_prev_rcu() fixes this error.
> > 
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
> > ---
> >  fs/nfs/dir.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
> > index e180033e35cf..2035254cc283 100644
> > --- a/fs/nfs/dir.c
> > +++ b/fs/nfs/dir.c
> > @@ -2350,7 +2350,7 @@ static int nfs_access_get_cached_rcu(struct inode *inode, const struct cred *cre
> >  	rcu_read_lock();
> >  	if (nfsi->cache_validity & NFS_INO_INVALID_ACCESS)
> >  		goto out;
> > -	lh = rcu_dereference(nfsi->access_cache_entry_lru.prev);
> > +	lh = rcu_dereference(list_prev_rcu(&nfsi->access_cache_entry_lru));
> >  	cache = list_entry(lh, struct nfs_access_entry, lru);
> >  	if (lh == &nfsi->access_cache_entry_lru ||
> >  	    cred != cache->cred)
> > -- 
> > 2.17.1
> >