• Johannes Berg's avatar
    mac80211: fix scan channel race · 58905ca5
    Johannes Berg authored
    When a software scan starts, it first sets sw_scanning, but
    leaves the scan_channel "unset" (it currently actually gets
    initialised to a default). Now, when something else tries
    to (re)configure the hardware in the window between these two
    events (after sw_scanning = true, but before scan_channel is
    set), the current code switches to the (unset!) scan_channel.
    This causes trouble, especially when switching bands and
    sending frames on the wrong channel.
    To work around this, leave scan_channel initialised to NULL
    and use it to determine whether or not a switch to a different
    channel should occur (and also use the same condition to check
    whether to adjust power for scan or not).
    Additionally, avoid reconfiguring the hardware completely when
    recalculating idle resulted in no changes, this was the problem
    that originally led us to discover the race condition in the
    first place, which was helpfully bisected by Pavel. This part
    of the patch should not be necessary with the other fixes, but
    not calling the ieee80211_hw_config function when we know it to
    be unnecessary is certainly a correct thing to do.
    Unfortunately, this patch cannot and does not fix the race
    condition completely, but due to the way the scan code is
    structured it makes the particular problem Pavel discovered
    (race while changing channel at the same time as transmitting
    frames) go away. To fix it completely, more work especially
    with locking configuration is needed.
    Bisected-by: default avatarPavel Roskin <proski@gnu.org>
    Signed-off-by: default avatarJohannes Berg <johannes@sipsolutions.net>
    Signed-off-by: default avatarJohn W. Linville <linville@tuxdriver.com>
iface.c 26 KB