afs: Mark afs_net::ws_cell as __rcu and set using rcu functions

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 <lkp@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>

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);
 

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;

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();