[PATCH 7/8] afs: Mark afs_net::ws_cell as __rcu and set using rcu functions

[David Howells]

The afs_net::ws_cell member is sometimes used under RCU conditions from
within an seq-readlock.  It isn't, however, marked __rcu and it isn't set
using the proper RCU barrier-imposing functions.

Fix this by annotating it with __rcu and using appropriate barriers to
make sure accesses are correctly ordered.

Without this, the code can produce the following warning:

>> fs/afs/proc.c:151:24: sparse: incompatible types in comparison expression 
>> (different address spaces)

Fixes: f044c8847bb6 ("afs: Lay the groundwork for supporting network 
namespaces")
Reported-by: kbuild test robot <l...@intel.com>
Signed-off-by: David Howells <dhowe...@redhat.com>
---

 fs/afs/cell.c     |    8 ++++----
 fs/afs/internal.h |    2 +-
 fs/afs/proc.c     |    2 +-
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index fdf4c36cff79..80fd127239ce 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -341,8 +341,8 @@ int afs_cell_init(struct afs_net *net, const char *rootcell)
 
        /* install the new cell */
        write_seqlock(&net->cells_lock);
-       old_root = net->ws_cell;
-       net->ws_cell = new_root;
+       old_root = rcu_access_pointer(net->ws_cell);
+       rcu_assign_pointer(net->ws_cell, new_root);
        write_sequnlock(&net->cells_lock);
 
        afs_put_cell(net, old_root);
@@ -755,8 +755,8 @@ void afs_cell_purge(struct afs_net *net)
        _enter("");
 
        write_seqlock(&net->cells_lock);
-       ws = net->ws_cell;
-       net->ws_cell = NULL;
+       ws = rcu_access_pointer(net->ws_cell);
+       RCU_INIT_POINTER(net->ws_cell, NULL);
        write_sequnlock(&net->cells_lock);
        afs_put_cell(net, ws);
 
diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index f8086ec95e24..5d922ad148a8 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -231,7 +231,7 @@ struct afs_net {
 
        /* Cell database */
        struct rb_root          cells;
-       struct afs_cell         *ws_cell;
+       struct afs_cell __rcu   *ws_cell;
        struct work_struct      cells_manager;
        struct timer_list       cells_timer;
        atomic_t                cells_outstanding;
diff --git a/fs/afs/proc.c b/fs/afs/proc.c
index b45ee7576aa8..362f281b1b16 100644
--- a/fs/afs/proc.c
+++ b/fs/afs/proc.c
@@ -173,7 +173,7 @@ static ssize_t afs_proc_rootcell_read(struct file *file, 
char __user *buf,
 
        if (*_pos > 0)
                return 0;
-       if (!net->ws_cell)
+       if (!rcu_access_pointer(net->ws_cell))
                return 0;
 
        rcu_read_lock();