[2/2] powerpc/64: Fix race condition in setting lock bit in idle/wakeup code

[Michael Ellerman]

On Fri, 2016-21-10 at 09:04:17 UTC, Paul Mackerras wrote:
> 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: b32aadc1a8ed
> Signed-off-by: Paul Mackerras <paulus at ozlabs.org>

Applied to powerpc fixes, thanks.

I added: Cc: stable at vger.kernel.org # v4.2+

https://git.kernel.org/powerpc/c/09b7e37b18eecc1e347f4b1a3bc863

cheers