powerpc/64: Fix race condition in setting lock bit in idle/wakeup code

powerpc/64: Fix race condition in setting lock bit in idle/wakeup code

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: 58c701ba391d ("powerpc/powernv: Fix race in updating core_idle_state")
Cc: stable@vger.kernel.org # v4.2+
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>