The patch below does not apply to the 5.4-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.
thanks,
greg k-h
------------------ original commit in Linus's tree ------------------
From 3c17916510428dbccdf657de050c34e208347089 Mon Sep 17 00:00:00 2001
From: Nikolay Borisov nborisov@suse.com Date: Mon, 8 Feb 2021 10:26:54 +0200 Subject: [PATCH] btrfs: fix race between extent freeing/allocation when using bitmaps
During allocation the allocator will try to allocate an extent using cluster policy. Once the current cluster is exhausted it will remove the entry under btrfs_free_cluster::lock and subsequently acquire btrfs_free_space_ctl::tree_lock to dispose of the already-deleted entry and adjust btrfs_free_space_ctl::total_bitmap. This poses a problem because there exists a race condition between removing the entry under one lock and doing the necessary accounting holding a different lock since extent freeing only uses the 2nd lock. This can result in the following situation:
T1: T2: btrfs_alloc_from_cluster insert_into_bitmap <holds tree_lock> if (entry->bytes == 0) if (block_group && !list_empty(&block_group->cluster_list)) { rb_erase(entry)
spin_unlock(&cluster->lock); (total_bitmaps is still 4) spin_lock(&cluster->lock); <doesn't find entry in cluster->root> spin_lock(&ctl->tree_lock); <goes to new_bitmap label, adds <blocked since T2 holds tree_lock> <a new entry and calls add_new_bitmap> recalculate_thresholds <crashes, due to total_bitmaps becoming 5 and triggering an ASSERT>
To fix this ensure that once depleted, the cluster entry is deleted when both cluster lock and tree locks are held in the allocator (T1), this ensures that even if there is a race with a concurrent insert_into_bitmap call it will correctly find the entry in the cluster and add the new space to it.
CC: stable@vger.kernel.org # 4.4+ Reviewed-by: Josef Bacik josef@toxicpanda.com Signed-off-by: Nikolay Borisov nborisov@suse.com Signed-off-by: David Sterba dsterba@suse.com
diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c index 5400294bd271..abcf951e6b44 100644 --- a/fs/btrfs/free-space-cache.c +++ b/fs/btrfs/free-space-cache.c @@ -3125,8 +3125,6 @@ u64 btrfs_alloc_from_cluster(struct btrfs_block_group *block_group, entry->bytes -= bytes; }
- if (entry->bytes == 0) - rb_erase(&entry->offset_index, &cluster->root); break; } out: @@ -3143,7 +3141,10 @@ u64 btrfs_alloc_from_cluster(struct btrfs_block_group *block_group, ctl->free_space -= bytes; if (!entry->bitmap && !btrfs_free_space_trimmed(entry)) ctl->discardable_bytes[BTRFS_STAT_CURR] -= bytes; + + spin_lock(&cluster->lock); if (entry->bytes == 0) { + rb_erase(&entry->offset_index, &cluster->root); ctl->free_extents--; if (entry->bitmap) { kmem_cache_free(btrfs_free_space_bitmap_cachep, @@ -3156,6 +3157,7 @@ u64 btrfs_alloc_from_cluster(struct btrfs_block_group *block_group, kmem_cache_free(btrfs_free_space_cachep, entry); }
+ spin_unlock(&cluster->lock); spin_unlock(&ctl->tree_lock);
return ret;