Messages in this thread | | | Subject | Re: [PATCH 0/6] sched/cpusets fixes, more changes are needed | From | Peter Zijlstra <> | Date | Thu, 25 Mar 2010 11:22:11 +0100 |
| |
On Wed, 2010-03-24 at 19:09 +0100, Oleg Nesterov wrote: > On 03/24, Peter Zijlstra wrote: > > > > On Mon, 2010-03-15 at 10:09 +0100, Oleg Nesterov wrote: > > > > > > - do_fork() clears PF_STARTING and then calls wake_up_new_task() > > > which finally does s/WAKING/RUNNING. > > > > > > But. Nobody can take rq->lock in between. This means a signal > > > from irq (quite possible with CLONE_THREAD) or another rt > > > thread which preempts us can lockup. > > > > Hmm, the signal case might indeed be a problem, however I cannot see how > > the RT thread can be a problem because until we do wake_up_new_task() > > the child will not be runnable and can thus not be preempted. > > Indeed, but I meant the _parent_ can be preempted ;)
I still can't see how that would be a problem..
> In short. TASK_WAKING acts as a spinlock in fact. And since ttwu() can > be called from any context, it should be irq-safe: any owner must disable > inerrupts and preemption.
Agreed, and I think that's corrected with my patch.
> > The reason we have that TASK_WAKING stuff for fork is because > > wake_up_new_task() needs p->cpus_allowed to be stable > > Sure! But it is very easy to change wake_up_new_task() to set TASK_WAKING > like ttwu() does. Of course, this needs raw_spin_lock_irq(rq->lock) for > a moment, but afaics that is all?
My patch does that.
> > So the below patch makes select_task_rq_fair unlock the rq when needed, > > and then puts all ->select_task_rq() calls under rq->lock. This should > > allow us to remove the TASK_WAKING thing from fork which in turn allows > > us to remove the PF_STARTING check in task_is_waking. > > > > How does that look? > > I'll try to read this patch tomorrow. But could you please consider > the suggestion above?
I think I do all those :-)
I was still looking at removing the TASK_WAKING check from task_rq_lock() since now we do set_task_cpu() under rq->lock again so it should be good again.
Hmm, except for sched_fork() that still does set_task_cpu() without holding rq->lock, but that is before the child gets exposed so there should not be any concurrency.
| |