Re: [PATCH -rt] catch put_task_struct RCU handling up to mainline

[Esben Nielsen <nielsen.esben@googlemail.com>]

On Mon, 10 Jul 2006, Paul E. McKenney wrote:

> On Mon, Jul 10, 2006 at 07:10:49PM +0100, Esben Nielsen wrote:
> > On Mon, 10 Jul 2006, Paul E. McKenney wrote:
> > > On Sat, Jul 08, 2006 at 02:59:37PM +0100, Esben Nielsen wrote:
>
> [ . . . ]
>
> > > > The work should be defered to a low priority task. Using rcu is
> > > > probably overkill because it also introduces other delays. A tasklet
> > > > or a dedicated task would be better.
> > >
> > > Agreed -- if there is in fact a legitimate non-error code path, then
> > > a patch that used some deferral mechanism would be good.  But RCU is
> > > overkill, and misleading overkill at that!
> >
> > I think this is a legitimate situation. lock 1 is owned by B which is
> > blocked on lock 2 which is owned by C
> >
> >  CPU1:                                      CPU2
> >     RT task A locks lock 1                C runs something
> >     A boosts B to RT
> >     A does get_task_struct B
> >     A enables interrupts                  C unlocks lock 2
> >     An very long interrupt is running     B unlocks lock 2
> >                                           B unlocks lock 1
> >                                           B is deboosted
> >                                           B exits
> >     A gets CPU1 again
> >     A does put_task_struct B
> >
> > I don't know if the timing is realistic, but theoretically it is possible.
> > It might also be possible the B exits on another CPU even without the long
> > interrupt handler. If A has cpu affinity to CPU1 it is enough if a higher
> > priority task preempts it on CPU1.
>
> For this to happen, either A has to be at a lower priority than the irq
> tasks or the interrupt has to be a hard irq (e.g., scheduling clock
> interrupt).  In the first case, the added cleanup processing seems
> inconsequential compared to (say) an interrupt doing network protocol
> processing.  In the second case, B does not do its put_task_struct()
> until after the hard irq returns (because the put_task_struct() is invoked
> from a call_rcu() callback), which makes the above scenario unlikely,
> though perhaps not impossible.
>
> If the second scenario is in fact possible, would you be willing to
> supply the appropriate deferral code?  I believe we both agree that RCU
> is not really the right deferral mechanism in this situation.

Let your patch go through. I'll stop complaining :-)
Is there anywhere where we can make a list of known issues like this?
I can't promise I will get time to fix this one :-(

Esben


> 							Thanx, Paul
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