NFS: Fix a potential race between umount and nfs_access_cache_shrinker()

Thanks to Yawei Niu for spotting the race.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 5ca762de88bf..476cb0f837fd 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1694,13 +1694,19 @@ int nfs_access_cache_shrinker(int nr_to_scan, gfp_t gfp_mask)
 restart:
         spin_lock(&nfs_access_lru_lock);
         list_for_each_entry(nfsi, &nfs_access_lru_list, access_cache_inode_lru) {
+                struct rw_semaphore *s_umount;
                 struct inode *inode;
 
                 if (nr_to_scan-- == 0)
                         break;
+                s_umount = &nfsi->vfs_inode.i_sb->s_umount;
+                if (!down_read_trylock(s_umount))
+                        continue;
                 inode = igrab(&nfsi->vfs_inode);
-                if (inode == NULL)
+                if (inode == NULL) {
+                        up_read(s_umount);
                         continue;
+                }
                 spin_lock(&inode->i_lock);
                 if (list_empty(&nfsi->access_cache_entry_lru))
                         goto remove_lru_entry;
@@ -1719,6 +1725,7 @@ remove_lru_entry:
                 spin_unlock(&inode->i_lock);
                 spin_unlock(&nfs_access_lru_lock);
                 iput(inode);
+                up_read(s_umount);
                 goto restart;
         }
         spin_unlock(&nfs_access_lru_lock);