Assuming thread 1 (may be VFS want to release the page) wants to
release a CLEAN page and thus the eb attaching the page.
and thread 2 wants to access the eb and cow the eb.
Thread 1
Thread 2
btree_releasepage
try_release_extent_buffer
release_extent_buffer
->if (atomic_dec_and_test(&eb->refs)) {
->spin_unlock(&eb->refs_lock);
-------switch to thread 2 here------
read_block_for_search
btrfs_find_tree_block
radix_tree_lookup && atomic_inc_not_zero
................
btrfs_mark_buffer_dirty(eb)
---------switch to thread 1 here ------
- >spin_lock(&tree->buffer_lock);
->radix_tree_delete(&tree->buffer,
eb->start >> PAGE_CACHE_SHIFT);
->spin_unlock(&tree->buffer_lock);
-->btrfs_release_extent_buffer_page
-->trigger BUG_ON(extent_buffer_under_io(eb))!!!!
This is because Thread 2 can still find the eb while thread 1 has dec
the eb->refs to 0, so eventually trigger the BUG_ON.
I have triggered the BUG_ON while testing btrfs, but I am not sure if
the bug-on is rise by my guess above.
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