• Chris Mason's avatar
    Btrfs: fix deadlock on async thread startup · 61d92c32
    Chris Mason authored
    
    
    The btrfs async worker threads are used for a wide variety of things,
    including processing bio end_io functions.  This means that when
    the endio threads aren't running, the rest of the FS isn't
    able to do the final processing required to clear PageWriteback.
    
    The endio threads also try to exit as they become idle and
    start more as the work piles up.  The problem is that starting more
    threads means kthreadd may need to allocate ram, and that allocation
    may wait until the global number of writeback pages on the system is
    below a certain limit.
    
    The result of that throttling is that end IO threads wait on
    kthreadd, who is waiting on IO to end, which will never happen.
    
    This commit fixes the deadlock by handing off thread startup to a
    dedicated thread.  It also fixes a bug where the on-demand thread
    creation was creating far too many threads because it didn't take into
    account threads being started by other procs.
    Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
    61d92c32
ctree.h 75.6 KB