btrfs: fix dead lock while running replace and defrag concurrently
This can be reproduced by fstests: btrfs/070
The scenario is like the following:
replace worker thread defrag thread
--------------------- -------------
copy_nocow_pages_worker btrfs_defrag_file
copy_nocow_pages_for_inode ...
btrfs_writepages
|A| lock_extent_bits extent_write_cache_pages
|B| lock_page
__extent_writepage
... writepage_delalloc
find_lock_delalloc_range
|B| lock_extent_bits
find_or_create_page
pagecache_get_page
|A| lock_page
This leads to an ABBA pattern deadlock. To fix it,
o we just change it to an AABB pattern which means to @unlock_extent_bits()
before we @lock_page(), and in this way the @extent_read_full_page_nolock()
is no longer in an locked context, so change it back to @extent_read_full_page()
to regain protection.
o Since we @unlock_extent_bits() earlier, then before @write_page_nocow(),
the extent may not really point at the physical block we want, so we
have to check it before write.
Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
Tested-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>
1 file changed