On Thu, Nov 19, 2020 at 11:27:55AM +0000, Valentin Schneider wrote: > > On 19/11/20 11:05, Will Deacon wrote: > > On Thu, Nov 19, 2020 at 09:18:20AM +0000, Quentin Perret wrote: > >> > @@ -1937,20 +1931,69 @@ static int __set_cpus_allowed_ptr(struct task_struct *p, > >> > * OK, since we're going to drop the lock immediately > >> > * afterwards anyway. > >> > */ > >> > - rq = move_queued_task(rq, &rf, p, dest_cpu); > >> > + rq = move_queued_task(rq, rf, p, dest_cpu); > >> > } > >> > out: > >> > - task_rq_unlock(rq, p, &rf); > >> > + task_rq_unlock(rq, p, rf); > >> > >> And that's a little odd to have here no? Can we move it back on the > >> caller's side? > > > > I don't think so, unfortunately. __set_cpus_allowed_ptr_locked() can trigger > > migration, so it can drop the rq lock as part of that and end up relocking a > > new rq, which it also unlocks before returning. Doing the unlock in the > > caller is therfore even weirder, because you'd have to return the lock > > pointer or something horrible like that. > > > > I did add a comment about this right before the function and it's an > > internal function to the scheduler so I think it's ok. > > > > An alternative here would be to add a new SCA_RESTRICT flag for > __set_cpus_allowed_ptr() (see migrate_disable() faff in > tip/sched/core). Not fond of either approaches, but the flag thing would > avoid this "quirk". I tried this when I read about the migrate_disable() stuff on lwn, but I didn't really find it any better to work with tbh. It also doesn't help with the locking that Quentin was mentioning, does it? (i.e. you still have to allocate). Will
