• David S. Miller's avatar
    [NET]: Close NETIF_F_LLTX race conditions. · 4debb9ea
    David S. Miller authored
    When drivers other than loopback were using the LLTX
    feature a race window was present.  While sending
    queued packets, the packet scheduler layer drops the
    queue lock then calls directly into the drivers xmit
    handler.  The driver then grabs it's private TX lock
    and goes to work.
    
    However, as soon as we've dropped the queue lock another
    thread doing TX processing for that card can execute
    a netif_stop_queue() due to the TX queue filling up.
    This race window causes problems because a properly coded
    driver should never end up in it's ->hard_start_xmit()
    handler if the queue on the device has been stopped and
    we even BUG() trap for this condition in all of the device
    drivers.  That is how this race window was discovered
    by Roland and the Infiniband folks.
    
    Various suggestions were made to close this race.  One
    of which involved holding onto the queue lock all the
    way into the ->hard_start_xmit() routine.  Then having
    the driver drop that lock only after taking it's private
    TX lock.  This solution was deemed grotty because it is
    not wise to put queueing discipline internals into the
    device drivers.
    
    The solution taken here, which is based upon ideas from
    Stephen Hemminger, is twofold:
    
    1) Leave LLTX around for purely software devices that
       need no locking at all for TX processing.  The existing
       example is loopback, although all tunnel devices could
       be converted in this way too.
    
    2) Stop trying to use LLTX for the other devices.  Instead
       achieve the same goal using a different mechanism.
    
    For #2, the thing we were trying to achieve with LLTX
    was to eliminate excess locking.  We accomplish that
    now by letting the device driver use dev->xmit_lock directly
    instead of a seperate priv->tx_lock of some sort.
    
    In order to allow that, we had to turn dev->xmit_lock into
    a hardware IRQ disabling lock instead of a BH disabling one.
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    4debb9ea
pktgen.c 85.2 KB