• Josef Bacik's avatar
    Btrfs: kill trans_mutex · a4abeea4
    Josef Bacik authored
    We use trans_mutex for lots of things, here's a basic list
    
    1) To serialize trans_handles joining the currently running transaction
    2) To make sure that no new trans handles are started while we are committing
    3) To protect the dead_roots list and the transaction lists
    
    Really the serializing trans_handles joining is not too hard, and can really get
    bogged down in acquiring a reference to the transaction.  So replace the
    trans_mutex with a trans_lock spinlock and use it to do the following
    
    1) Protect fs_info->running_transaction.  All trans handles have to do is check
    this, and then take a reference of the transaction and keep on going.
    2) Protect the fs_info->trans_list.  This doesn't get used too much, basically
    it just holds the current transactions, which will usually just be the currently
    committing transaction and the currently running transaction at most.
    3) Protect the dead roots list.  This is only ever processed by splicing the
    list so this is relatively simple.
    4) Protect the fs_info->reloc_ctl stuff.  This is very lightweight and was using
    the trans_mutex before, so this is a pretty straightforward change.
    5) Protect fs_info->no_trans_join.  Because we don't hold the trans_lock over
    the entirety of the commit we need to have a way to block new people from
    creating a new transaction while we're doing our work.  So we set no_trans_join
    and in join_transaction we test to see if that is set, and if it is we do a
    wait_on_commit.
    6) Make the transaction use count atomic so we don't need to take locks to
    modify it when we're dropping references.
    7) Add a commit_lock to the transaction to make sure multiple people trying to
    commit the same transaction don't race and commit at the same time.
    8) Make open_ioctl_trans an atomic so we don't have to take any locks for ioctl
    trans.
    
    I have tested this with xfstests, but obviously it is a pretty hairy change so
    lots of testing is greatly appreciated.  Thanks,
    Signed-off-by: default avatarJosef Bacik <josef@redhat.com>
    a4abeea4
file.c 37.1 KB