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". > Will
