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    Subject[PATCH 4.4 171/268] btrfs: Fix possible softlock on single core machines
    Date
    4.4-stable review patch.  If anyone has any objections, please let me know.

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

    From: Nikolay Borisov <nborisov@suse.com>

    [ Upstream commit 1e1c50a929bc9e49bc3f9935b92450d9e69f8158 ]

    do_chunk_alloc implements a loop checking whether there is a pending
    chunk allocation and if so causes the caller do loop. Generally this
    loop is executed only once, however testing with btrfs/072 on a single
    core vm machines uncovered an extreme case where the system could loop
    indefinitely. This is due to a missing cond_resched when loop which
    doesn't give a chance to the previous chunk allocator finish its job.

    The fix is to simply add the missing cond_resched.

    Fixes: 6d74119f1a3e ("Btrfs: avoid taking the chunk_mutex in do_chunk_alloc")
    Signed-off-by: Nikolay Borisov <nborisov@suse.com>
    Reviewed-by: David Sterba <dsterba@suse.com>
    Signed-off-by: David Sterba <dsterba@suse.com>
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/btrfs/extent-tree.c | 1 +
    1 file changed, 1 insertion(+)

    --- a/fs/btrfs/extent-tree.c
    +++ b/fs/btrfs/extent-tree.c
    @@ -4392,6 +4392,7 @@ again:
    if (wait_for_alloc) {
    mutex_unlock(&fs_info->chunk_mutex);
    wait_for_alloc = 0;
    + cond_resched();
    goto again;
    }


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