• Paul Mackerras's avatar
    powerpc/64: Fix race condition in setting lock bit in idle/wakeup code · 09b7e37b
    Paul Mackerras authored
    This fixes a race condition where one thread that is entering or
    leaving a power-saving state can inadvertently ignore the lock bit
    that was set by another thread, and potentially also clear it.
    The core_idle_lock_held function is called when the lock bit is
    seen to be set.  It polls the lock bit until it is clear, then
    does a lwarx to load the word containing the lock bit and thread
    idle bits so it can be updated.  However, it is possible that the
    value loaded with the lwarx has the lock bit set, even though an
    immediately preceding lwz loaded a value with the lock bit clear.
    If this happens then we go ahead and update the word despite the
    lock bit being set, and when called from pnv_enter_arch207_idle_mode,
    we will subsequently clear the lock bit.
    
    No identifiable misbehaviour has been attributed to this race.
    
    This fixes it by checking the lock bit in the value loaded by the
    lwarx.  If it is set then we just go back and keep on polling.
    
    Fixes: b32aadc1 ("powerpc/powernv: Fix race in updating core_idle_state")
    Cc: stable@vger.kernel.org # v4.2+
    Signed-off-by: default avatarPaul Mackerras <paulus@ozlabs.org>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    09b7e37b
idle_book3s.S 16.4 KB