On Mon, Sep 02, 2019 at 11:16:23AM +0200, Peter Zijlstra wrote: > On Sat, Aug 31, 2019 at 03:41:17PM +0100, Alessio Balsini wrote: > > Right! > > > > Verified that sysctl_sched_dl_period_max and sysctl_sched_dl_period_min > > values > > are now always consistent. > > > > I spent some time in trying to figure out if not having any mutex in > > __checkparam_dl() is safe. There can surely happen that "max < min", e.g.:
> > Sharing my thoughts, a "BUG_ON(max < min)" in __checkparam_dl() is then a > > guaranteed source of explosions, but the good news is that "if (period < > > min || > > period > max" in __checkparam_dl() surely fails if "max < min". Also the > > fact > > that, when we are writing the new sysctl_sched_dl_* values, only one is > > actually changed at a time, that surely helps to preserve the consistency. > > > > But is that enough? > > Strictly speaking, no, I suppose it is not. We can have two changes in > between the two READ_ONCE()s and then we'd be able to observe a > violation. > > The easy way to fix that is do something like: > > + synchronize_rcu(); > mutex_unlock(&mutex); > > in sched_dl_period_handler(). And do: > > + preempt_disable(); > max = (u64)READ_ONCE(sysctl_sched_dl_period_max) * NSEC_PER_USEC; > min = (u64)READ_ONCE(sysctl_sched_dl_period_min) * NSEC_PER_USEC; > + preempt_enable(); > > in __checkparam_dl(). > > That would prohibit we see two changes, and seeing only the single > change is safe. I pushed out a new version; and added patch to sched_rt_handler() on top. Please have a look at: git://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git sched/wip-deadline I'll move these two patches to sched/core if everything looks good.
