• Nikolay Borisov's avatar
    btrfs: convert snapshot/nocow exlcusion to drew lock · dcc3eb96
    Nikolay Borisov authored
    This patch removes all haphazard code implementing nocow writers
    exclusion from pending snapshot creation and switches to using the drew
    lock to ensure this invariant still holds.
    
    'Readers' are snapshot creators from create_snapshot and 'writers' are
    nocow writers from buffered write path or btrfs_setsize. This locking
    scheme allows for multiple snapshots to happen while any nocow writers
    are blocked, since writes to page cache in the nocow path will make
    snapshots inconsistent.
    
    So for performance reasons we'd like to have the ability to run multiple
    concurrent snapshots and also favors readers in this case. And in case
    there aren't pending snapshots (which will be the majority of the cases)
    we rely on the percpu's writers counter to avoid cacheline contention.
    
    The main gain from using the drew lock is it's now a lot easier to
    reason about the guarantees of the locking scheme and whether there is
    some silent breakage lurking.
    Signed-off-by: default avatarNikolay Borisov <nborisov@suse.com>
    Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
    Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
    dcc3eb96
extent-tree.c 154 KB