Btrfs: only use the existing eb if it's count isn't 0
We can run into a problem where we find an eb for our existing page already on
the radix tree but it has a ref count of 0. It hasn't yet been removed by RCU
yet so this can cause issues where we will use the EB after free. So do
atomic_inc_not_zero on the exists->refs and if it is zero just do
synchronize_rcu() and try again. We won't have to worry about new allocators
coming in since they will block on the page lock at this point. Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
index 0381b60..0f74262 100644
--- a/fs/btrfs/extent_io.c
+++ b/fs/btrfs/extent_io.c
@@ -3750,7 +3750,7 @@
}
if (uptodate)
set_bit(EXTENT_BUFFER_UPTODATE, &eb->bflags);
-
+again:
ret = radix_tree_preload(GFP_NOFS & ~__GFP_HIGHMEM);
if (ret)
goto free_eb;
@@ -3760,7 +3760,13 @@
if (ret == -EEXIST) {
exists = radix_tree_lookup(&tree->buffer,
start >> PAGE_CACHE_SHIFT);
- atomic_inc(&exists->refs);
+ if (!atomic_inc_not_zero(&exists->refs)) {
+ spin_unlock(&tree->buffer_lock);
+ radix_tree_preload_end();
+ synchronize_rcu();
+ exists = NULL;
+ goto again;
+ }
spin_unlock(&tree->buffer_lock);
radix_tree_preload_end();
goto free_eb;
