• Jacob Keller's avatar
    igb: fix race condition with PTP_TX_IN_PROGRESS bits · 4ccdc013
    Jacob Keller authored
    Hardware related to the igb driver has a limitation of only handling one
    Tx timestamp at a time. Thus, the driver uses a state bit lock to
    enforce that only one timestamp request is honored at a time.
    Unfortunately this suffers from a simple race condition. The bit lock is
    not cleared until after skb_tstamp_tx() is called notifying the stack of
    a new Tx timestamp. Even a well behaved application which sends only one
    timestamp request at once and waits for a response might wake up and
    send a new packet before the bit lock is cleared. This results in
    needlessly dropping some Tx timestamp requests.
    We can fix this by unlocking the state bit as soon as we read the
    Timestamp register, as this is the first point at which it is safe to
    To avoid issues with the skb pointer, we'll use a copy of the pointer
    and set the global variable in the driver structure to NULL first. This
    ensures that the next timestamp request does not modify our local copy
    of the skb pointer.
    This ensures that well behaved applications do not accidentally race
    with the unlock bit. Obviously an application which sends multiple Tx
    timestamp requests at once will still only timestamp one packet at
    a time. Unfortunately there is nothing we can do about this.
    Reported-by: default avatarDavid Mirabito <davidm@metamako.com>
    Signed-off-by: default avatarJacob Keller <jacob.e.keller@intel.com>
    Tested-by: default avatarAaron Brown <aaron.f.brown@intel.com>
    Signed-off-by: default avatarJeff Kirsher <jeffrey.t.kirsher@intel.com>