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    Subject[PATCH 3.16 29/72] btrfs: Relinquish CPUs in btrfs_compare_trees
    3.16.79-rc1 review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Nikolay Borisov <nborisov@suse.com>

    commit 6af112b11a4bc1b560f60a618ac9c1dcefe9836e upstream.

    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")
    Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
    Signed-off-by: Nikolay Borisov <nborisov@suse.com>
    Reviewed-by: David Sterba <dsterba@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    [bwh: Backported to 3.16: adjust filename]
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    ---
    fs/btrfs/ctree.c | 1 +
    1 file changed, 1 insertion(+)

    --- a/fs/btrfs/ctree.c
    +++ b/fs/btrfs/ctree.c
    @@ -5446,6 +5446,7 @@ int btrfs_compare_trees(struct btrfs_roo
    advance_left = advance_right = 0;

    while (1) {
    + cond_resched();
    if (advance_left && !left_end_reached) {
    ret = tree_advance(left_root, left_path, &left_level,
    left_root_level,
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