Btrfs: Fix locking around adding new space_info

Storage allocated to different raid levels in btrfs is tracked by
a btrfs_space_info structure, and all of the current space_infos are
collected into a list_head.

Most filesystems have 3 or 4 of these structs total, and the list is
only changed when new raid levels are added or at unmount time.

This commit adds rcu locking on the list head, and properly frees
things at unmount time.  It also clears the space_info->full flag
whenever new space is added to the FS.

The locking for the space info list goes like this:

reads: protected by rcu_read_lock()
writes: protected by the chunk_mutex

At unmount time we don't need special locking because all the readers
are gone.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 42 insertions, 3 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 9abf81f71c46..fefe83ad2059 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -20,6 +20,7 @@
 #include <linux/writeback.h>
 #include <linux/blkdev.h>
 #include <linux/sort.h>
+#include <linux/rcupdate.h>
 #include "compat.h"
 #include "hash.h"
 #include "crc32c.h"
@@ -330,13 +331,33 @@ static struct btrfs_space_info *__find_space_info(struct btrfs_fs_info *info,
 {
         struct list_head *head = &info->space_info;
         struct btrfs_space_info *found;
-        list_for_each_entry(found, head, list) {
-                if (found->flags == flags)
+
+        rcu_read_lock();
+        list_for_each_entry_rcu(found, head, list) {
+                if (found->flags == flags) {
+                        rcu_read_unlock();
                         return found;
+                }
         }
+        rcu_read_unlock();
         return NULL;
 }
 
+/*
+ * after adding space to the filesystem, we need to clear the full flags
+ * on all the space infos.
+ */
+void btrfs_clear_space_info_full(struct btrfs_fs_info *info)
+{
+        struct list_head *head = &info->space_info;
+        struct btrfs_space_info *found;
+
+        rcu_read_lock();
+        list_for_each_entry_rcu(found, head, list)
+                found->full = 0;
+        rcu_read_unlock();
+}
+
 static u64 div_factor(u64 num, int factor)
 {
         if (factor == 10)
@@ -1903,7 +1924,6 @@ static int update_space_info(struct btrfs_fs_info *info, u64 flags,
         if (!found)
                 return -ENOMEM;
 
-        list_add(&found->list, &info->space_info);
         INIT_LIST_HEAD(&found->block_groups);
         init_rwsem(&found->groups_sem);
         spin_lock_init(&found->lock);
@@ -1917,6 +1937,7 @@ static int update_space_info(struct btrfs_fs_info *info, u64 flags,
         found->full = 0;
         found->force_alloc = 0;
         *space_info = found;
+        list_add_rcu(&found->list, &info->space_info);
         return 0;
 }
 
@@ -6320,6 +6341,7 @@ out:
 int btrfs_free_block_groups(struct btrfs_fs_info *info)
 {
         struct btrfs_block_group_cache *block_group;
+        struct btrfs_space_info *space_info;
         struct rb_node *n;
 
         spin_lock(&info->block_group_cache_lock);
@@ -6341,6 +6363,23 @@ int btrfs_free_block_groups(struct btrfs_fs_info *info)
                 spin_lock(&info->block_group_cache_lock);
         }
         spin_unlock(&info->block_group_cache_lock);
+
+        /* now that all the block groups are freed, go through and
+         * free all the space_info structs.  This is only called during
+         * the final stages of unmount, and so we know nobody is
+         * using them.  We call synchronize_rcu() once before we start,
+         * just to be on the safe side.
+         */
+        synchronize_rcu();
+
+        while(!list_empty(&info->space_info)) {
+                space_info = list_entry(info->space_info.next,
+                                        struct btrfs_space_info,
+                                        list);
+
+                list_del(&space_info->list);
+                kfree(space_info);
+        }
         return 0;
 }