[PATCH v4 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity

[Qais Yousef]

On 12/01/20 16:56, Will Deacon wrote:
> On Fri, Nov 27, 2020 at 01:19:16PM +0000, Qais Yousef wrote:
> > On 11/24/20 15:50, Will Deacon wrote:
> > 
> > [...]
> > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index d2003a7d5ab5..818c8f7bdf2a 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1860,24 +1860,18 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
> > >  }
> > >  
> > >  /*
> > > - * Change a given task's CPU affinity. Migrate the thread to a
> > > - * proper CPU and schedule it away if the CPU it's executing on
> > > - * is removed from the allowed bitmask.
> > > - *
> > > - * NOTE: the caller must have a valid reference to the task, the
> > > - * task must not exit() & deallocate itself prematurely. The
> > > - * call is not atomic; no spinlocks may be held.
> > > + * Called with both p->pi_lock and rq->lock held; drops both before returning.
> > 
> > nit: wouldn't it be better for the caller to acquire and release the locks?
> > Not a big deal but it's always confusing when half of the work done outside the
> > function and the other half done inside.
> 
> That came up in the last version of the patches iirc, but the problem is
> that __set_cpus_allowed_ptr_locked() can trigger migration, which can
> drop the lock and take another one for the new runqueue.
> 
> Given that this function is internal to the scheduler, I think we can
> probably live with it.

I guess task_rq_lock() always entails be prepared for surprises!

Works for me.

Thanks

--
Qais Yousef