• Zygo Blaxell's avatar
    btrfs: remove spurious WARN_ON(ref->count < 0) in find_parent_nodes · c8195a7b
    Zygo Blaxell authored
    Until v4.14, this warning was very infrequent:
    
    	WARNING: CPU: 3 PID: 18172 at fs/btrfs/backref.c:1391 find_parent_nodes+0xc41/0x14e0
    	Modules linked in: [...]
    	CPU: 3 PID: 18172 Comm: bees Tainted: G      D W    L  4.11.9-zb64+ #1
    	Hardware name: System manufacturer System Product Name/M5A78L-M/USB3, BIOS 2101    12/02/2014
    	Call Trace:
    	 dump_stack+0x85/0xc2
    	 __warn+0xd1/0xf0
    	 warn_slowpath_null+0x1d/0x20
    	 find_parent_nodes+0xc41/0x14e0
    	 __btrfs_find_all_roots+0xad/0x120
    	 ? extent_same_check_offsets+0x70/0x70
    	 iterate_extent_inodes+0x168/0x300
    	 iterate_inodes_from_logical+0x87/0xb0
    	 ? iterate_inodes_from_logical+0x87/0xb0
    	 ? extent_same_check_offsets+0x70/0x70
    	 btrfs_ioctl+0x8ac/0x2820
    	 ? lock_acquire+0xc2/0x200
    	 do_vfs_ioctl+0x91/0x700
    	 ? __fget+0x112/0x200
    	 SyS_ioctl+0x79/0x90
    	 entry_SYSCALL_64_fastpath+0x23/0xc6
    	 ? trace_hardirqs_off_caller+0x1f/0x140
    
    Starting with v4.14 (specifically 86d5f994 ("btrfs: convert prelimary
    reference tracking to use rbtrees")) the WARN_ON occurs three orders of
    magnitude more frequently--almost once per second while running workloads
    like bees.
    
    Replace the WARN_ON() with a comment rationale for its removal.
    The rationale is paraphrased from an explanation by Edmund Nadolski
    <enadolski@suse.de> on the linux-btrfs mailing list.
    
    Fixes: 8da6d581 ("Btrfs: added btrfs_find_all_roots()")
    Signed-off-by: default avatarZygo Blaxell <ce3g8jdj@umail.furryterror.org>
    Reviewed-by: default avatarLu Fengqi <lufq.fnst@cn.fujitsu.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    c8195a7b
backref.c 59.7 KB