[Fwd: lost need_resched flag re-introduced?]

[Fwd: lost need_resched flag re-introduced?]

[Jun Sun]

I did not get reply from Linus.  Now try my luck with the kernel mailing
list.  Please cc your reply to my email account.  I stopped watching the
mailing list anymore.

Thanks.

Jun

Jun Sun wrote:
> 
> Linus,
> 
> A while back I reported the lost need_resched flag bug ( it happens if
> need_resched is set right before switch_to() is called).  Later on a one-line
> fix is added to __schedule_tail().
> 
>         current->need_resched |= prev->need_resched;
> 
> I looked at the latest kernel and found this one is gone.  Is the lost
> need_resched problem taken care of in some other way?  Or is it re-introduced?
> 
> In any case, I was going to propose a minor fix over the original one-line
> fix.  See the patch below.
> 
> On RISC machines, the original fix leaves a small window for setting the wrong
> flag.  A pseudo assembly code for the original fix is shown as follows:
> 
> 1. load current->need_resched to R1
> 2. load prev->need_resched to R2
> 3. R1 = R1 | R2
> 4. store R1 to current->need_resched
> 
> If at 1) both need_resched flags are 0, and an interrupt happens between 1)
> and 4), which sets current->resched to 1, we will have wrong result for
> current->need_resched after step 4).
> 
> Jun
> 
> inux/kernel/sched.c:   1.1 1.2 jsun 00/10/31 10:22:44 (modified, needs delta)
> 
> @@ -455,7 +455,9 @@
>   */
>  static inline void __schedule_tail(struct task_struct *prev)
>  {
> -       current->need_resched |= prev->need_resched;
> +       if (prev->need_resched) {
> +               current->need_resched = 1;
> +       }
>  #ifdef CONFIG_SMP
>         if ((prev->state == TASK_RUNNING) &&
>                         (prev != idle_task(smp_processor_id()))) {
-
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Re: [Fwd: lost need_resched flag re-introduced?]

[kuznet]

Hello!

> > A while back I reported the lost need_resched flag bug ( it happens if
> > need_resched is set right before switch_to() is called).  Later on a one-line
> > fix is added to __schedule_tail().
> > 
> >         current->need_resched |= prev->need_resched;
> > 
> > I looked at the latest kernel and found this one is gone.  Is the lost
> > need_resched problem taken care of in some other way?  Or is it re-introduced?

It is removed not only because it was wrong (which you have found too),
but because it was useless even if copied correctly.

current->need_resched is not changed in interrupt context outside
runqueue lock except for scheduler timer, where copying results
in nothing but spurious reschedule.

Alexey
-
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Re: [Fwd: lost need_resched flag re-introduced?]

[Jun Sun]

kuznet@ms2.inr.ac.ru wrote:
> 
> Hello!
> 
> > > A while back I reported the lost need_resched flag bug ( it happens if
> > > need_resched is set right before switch_to() is called).  Later on a one-line
> > > fix is added to __schedule_tail().
> > >
> > >         current->need_resched |= prev->need_resched;
> > >
> > > I looked at the latest kernel and found this one is gone.  Is the lost
> > > need_resched problem taken care of in some other way?  Or is it re-introduced?
> 
> It is removed not only because it was wrong (which you have found too),
> but because it was useless even if copied correctly.
> 
> current->need_resched is not changed in interrupt context outside
> runqueue lock except for scheduler timer, where copying results
> in nothing but spurious reschedule.
> 
> Alexey

Alexey,

I think wake_up_process() is called in interrupt routine quite often and it
will set need_resched flag (through reschedule_idle()).  ???

I did several run time tests and confirmed the missing need_resched flag was
happening.  Ether some new code takes care of it.  Or it is still there.

Jun
-
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Re: [Fwd: lost need_resched flag re-introduced?]

[kuznet]

Hello!

> I think wake_up_process() is called in interrupt routine quite often and it
> will set need_resched flag (through reschedule_idle()).  ???

It sets _right_ flag cpu_curr(this_cpu)->need_resched, rather than
current->need_resched.



> happening.  Ether some new code takes care of it.  Or it is still there.

?  8)

Alexey
-
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