• Takashi Iwai's avatar
    ALSA: usb-audio: Set callbacks via snd_usb_endpoint_set_callback() · 96e221f3
    Takashi Iwai authored
    
    
    The prepare_data_urb and retire_data_urb fields of the endpoint object
    are set dynamically at PCM trigger start/stop.  Those are evaluated in
    the endpoint handler, but there can be a race, especially if two
    different PCM substreams are handling the same endpoint for the
    implicit feedback case.  Also, the data_subs field of the endpoint is
    set and accessed dynamically, too, which has the same risk.
    
    As a slight improvement for the concurrency, this patch introduces the
    function to set the callbacks and the data in a shot with the memory
    barrier.  In the reader side, it's also fetched with the memory
    barrier.
    
    There is still a room of race if prepare and retire callbacks are set
    during executing the URB completion.  But such an inconsistency may
    happen only for the implicit fb source, i.e. it's only about the
    capture stream.  And luckily, the capture stream never sets the
    prepare callback, hence the problem doesn't happen practically.
    Tested-by: default avatarKeith Milner <kamilner@superlative.org>
    Tested-by: default avatarDylan Robinson <dylan_robinson@motu.com>
    Link: https://lore.kernel.org/r/20201123085347.19667-23-tiwai@suse.de
    
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    96e221f3