fs: assign sb->s_bdi to default_backing_dev_info if the bdi is going away

We don't have proper reference counting for this yet, so we run into
cases where the device is pulled and we OOPS on flushing the fs data.
This happens even though the dirty inodes have already been
migrated to the default_backing_dev_info.

Reported-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Tested-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

3 files changed, 5 insertions, 3 deletions

diff --git a/fs/super.c b/fs/super.c
index 7e9dd4cc2c01..0d89e93f654e 100644
--- a/fs/super.c
+++ b/fs/super.c
@@ -71,6 +71,7 @@ static struct super_block *alloc_super(struct file_system_type *type)
 #else
                 INIT_LIST_HEAD(&s->s_files);
 #endif
+                s->s_bdi = &default_backing_dev_info;
                 INIT_LIST_HEAD(&s->s_instances);
                 INIT_HLIST_BL_HEAD(&s->s_anon);
                 INIT_LIST_HEAD(&s->s_inodes);
@@ -1003,6 +1004,7 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
         }
         BUG_ON(!mnt->mnt_sb);
         WARN_ON(!mnt->mnt_sb->s_bdi);
+        WARN_ON(mnt->mnt_sb->s_bdi == &default_backing_dev_info);
         mnt->mnt_sb->s_flags |= MS_BORN;
 
         error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);
diff --git a/fs/sync.c b/fs/sync.c
index ba76b9623e7e..412dc89163d3 100644
--- a/fs/sync.c
+++ b/fs/sync.c
@@ -33,7 +33,7 @@ static int __sync_filesystem(struct super_block *sb, int wait)
          * This should be safe, as we require bdi backing to actually
          * write out data in the first place
          */
-        if (!sb->s_bdi || sb->s_bdi == &noop_backing_dev_info)
+        if (sb->s_bdi == &noop_backing_dev_info)
                 return 0;
 
         if (sb->s_qcop && sb->s_qcop->quota_sync)
@@ -79,7 +79,7 @@ EXPORT_SYMBOL_GPL(sync_filesystem);
 
 static void sync_one_sb(struct super_block *sb, void *arg)
 {
-        if (!(sb->s_flags & MS_RDONLY) && sb->s_bdi)
+        if (!(sb->s_flags & MS_RDONLY))
                 __sync_filesystem(sb, *(int *)arg);
 }
 /*
diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index c91e139a652e..8fe9d3407921 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -598,7 +598,7 @@ static void bdi_prune_sb(struct backing_dev_info *bdi)
         spin_lock(&sb_lock);
         list_for_each_entry(sb, &super_blocks, s_list) {
                 if (sb->s_bdi == bdi)
-                        sb->s_bdi = NULL;
+                        sb->s_bdi = &default_backing_dev_info;
         }
         spin_unlock(&sb_lock);
 }