On Wed, Sep 04, 2019 at 07:33:58PM +0300, Nikolay Borisov wrote:
> When doing any form of incremental send the parent and the child trees
> need to be compared via btrfs_compare_trees. This can result in long
> loop chains without ever relinquishing the CPU. This causes softlockup
> detector to trigger when comparing trees with a lot of items. Example
> report:
>
> watchdog: BUG: soft lockup - CPU#0 stuck for 24s! [snapperd:16153]
> CPU: 0 PID: 16153 Comm: snapperd Not tainted 5.2.9-1-default #1 openSUSE
> Tumbleweed (unreleased)
> Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
> pstate: 40000005 (nZcv daif -PAN -UAO)
> pc : __ll_sc_arch_atomic_sub_return+0x14/0x20
> lr : btrfs_release_extent_buffer_pages+0xe0/0x1e8 [btrfs]
> sp : ffff00001273b7e0
> Call trace:
> __ll_sc_arch_atomic_sub_return+0x14/0x20
> release_extent_buffer+0xdc/0x120 [btrfs]
> free_extent_buffer.part.0+0xb0/0x118 [btrfs]
> free_extent_buffer+0x24/0x30 [btrfs]
> btrfs_release_path+0x4c/0xa0 [btrfs]
> btrfs_free_path.part.0+0x20/0x40 [btrfs]
> btrfs_free_path+0x24/0x30 [btrfs]
> get_inode_info+0xa8/0xf8 [btrfs]
> finish_inode_if_needed+0xe0/0x6d8 [btrfs]
> changed_cb+0x9c/0x410 [btrfs]
> btrfs_compare_trees+0x284/0x648 [btrfs]
> send_subvol+0x33c/0x520 [btrfs]
> btrfs_ioctl_send+0x8a0/0xaf0 [btrfs]
> btrfs_ioctl+0x199c/0x2288 [btrfs]
> do_vfs_ioctl+0x4b0/0x820
> ksys_ioctl+0x84/0xb8
> __arm64_sys_ioctl+0x28/0x38
> el0_svc_common.constprop.0+0x7c/0x188
> el0_svc_handler+0x34/0x90
> el0_svc+0x8/0xc
>
> Fix this by adding a call to cond_resched at the beginning of the main
> loop in btrfs_compare_trees.
>
> Fixes: 7069830a9e38 ("Btrfs: add btrfs_compare_trees function")
> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
Reviewed-by: David Sterba <dste...@suse.com>
