[syzbot] [afs?] INFO: task hung in afs_cell_purge (2)

[David Howells]

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git v7.1

    afs: Fix hang in afs_cell_purge()
    
    Fix a hang in afs_cell_purge() due to net->cells_outstanding being updated
    before the check for idr_alloc_cyclic() failing.
    
    Reported-by: syzbot+750f21d691e244b473b1 at syzkaller.appspotmail.com
    Suggested-by: Hillf Danton <hdanton at sina.com>
    Signed-off-by: David Howells <dhowells at redhat.com>
    cc: Marc Dionne <marc.dionne at auristor.com>
    cc: linux-afs at lists.infradead.org

diff --git a/fs/afs/cell.c b/fs/afs/cell.c
index 9738684dbdd2..e0fab1609f27 100644
--- a/fs/afs/cell.c
+++ b/fs/afs/cell.c
@@ -205,11 +205,11 @@ static struct afs_cell *afs_alloc_cell(struct afs_net *net,
 	cell->dns_source = vllist->source;
 	cell->dns_status = vllist->status;
 	smp_store_release(&cell->dns_lookup_count, 1); /* vs source/status */
-	atomic_inc(&net->cells_outstanding);
 	ret = idr_alloc_cyclic(&net->cells_dyn_ino, cell,
 			       2, INT_MAX / 2, GFP_KERNEL);
 	if (ret < 0)
 		goto error;
+	atomic_inc(&net->cells_outstanding);
 	cell->dynroot_ino = ret;
 	cell->debug_id = atomic_inc_return(&cell_debug_id);