On 07/07/20 10:34, Qais Yousef wrote: > On 07/06/20 16:49, Valentin Schneider wrote: >> >> On 06/07/20 15:28, Qais Yousef wrote: >> > CC: linux-fsdevel@xxxxxxxxxxxxxxx >> > --- >> > >> > Peter >> > >> > I didn't do the >> > >> > read_lock(&taslist_lock); >> > smp_mb__after_spinlock(); >> > read_unlock(&tasklist_lock); >> > >> > dance you suggested on IRC as it didn't seem necessary. But maybe I missed >> > something. >> > >> >> So the annoying bit with just uclamp_fork() is that it happens *before* the >> task is appended to the tasklist. This means without too much care we >> would have (if we'd do a sync at uclamp_fork()): >> >> CPU0 (sysctl write) CPU1 (concurrent forker) >> >> copy_process() >> uclamp_fork() >> p.uclamp_min = state >> state = foo >> >> for_each_process_thread(p, t) >> update_state(t); >> list_add(p) >> >> i.e. that newly forked process would entirely sidestep the update. Now, >> with Peter's suggested approach we can be in a much better situation. If we >> have this in the sysctl update: >> >> state = foo; >> >> read_lock(&taslist_lock); >> smp_mb__after_spinlock(); >> read_unlock(&tasklist_lock); >> >> for_each_process_thread(p, t) >> update_state(t); >> >> While having this in the fork: >> >> write_lock(&tasklist_lock); >> list_add(p); >> write_unlock(&tasklist_lock); >> >> sched_post_fork(p); // state re-read here; probably wants an mb first >> >> Then we can no longer miss an update. If the forked p doesn't see the new >> value, it *must* have been added to the tasklist before the updater loops >> over it, so the loop will catch it. If it sees the new value, we're done. > > uclamp_fork() has nothing to do with the race. If copy_process() duplicates the > task_struct of an RT task, it'll copy the old value. > Quite so; my point was if we were to use uclamp_fork() as to re-read the value. > I'd expect the newly introduced sched_post_fork() (also in copy_process() after > the list update) to prevent this race altogether. > > Now we could end up with a problem if for_each_process_thread() doesn't see the > newly forked task _after_ sched_post_fork(). Hence my question to Peter. > >> >> AIUI, the above strategy doesn't require any use of RCU. The update_state() >> and sched_post_fork() can race, but as per the above they should both be >> writing the same value. > > for_each_process_thread() must be protected by either tasklist_lock or > rcu_read_lock(). > Right > The other RCU logic I added is not to protect against the race above. I > describe the other race condition in a comment. I take it that's the one in uclamp_sync_util_min_rt_default()? __setscheduler_uclamp() can't be preempted as we hold task_rq_lock(). It can indeed race with the sync though, but again with the above suggested setup it would either: - see the old value, but be guaranteed to be iterated over later by the updater - see the new value sched_post_fork() being preempted out is a bit more annoying, but what prevents us from making that bit preempt-disabled? I have to point out I'm assuming here updaters are serialized, which does seem to be see the case (cf. uclamp_mutex). > Basically another updater on a > different cpu via fork() and sched_setattr() might read an old value and get > preempted. The rcu synchronization will ensure concurrent updaters have > finished before iterating the list. > > Thanks
