On 2019-08-21 15:50:33 [+0200], Thomas Gleixner wrote: > On Wed, 21 Aug 2019, Sebastian Andrzej Siewior wrote: > > > On 2019-08-21 10:24:07 [+0100], Julien Grall wrote: > > > The update to timer->base is protected by the base->cpu_base->lock(). > > > However, hrtimer_grab_expirty_lock() does not access it with the lock. > > > > > > So it would theorically be possible to have timer->base changed under > > > our feet. We need to prevent the compiler to refetch timer->base so the > > > check and the access is performed on the same base. > > > > It is not a problem if the timer's bases changes. We get here because we > > want to help the timer to complete its callback. > > The base can only change if the timer gets re-armed on another CPU which > > means is completed callback. In every case we can cancel the timer on > > the next iteration. > > It _IS_ a problem when the base changes and the compiler reloads > > CPU0 CPU1 > base = timer->base; > > lock(base->....); > switch base > > reload > base = timer->base; > > unlock(base->....); > > See? so read_once() it is then. Sebastian
