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

[Paul E. McKenney]

On Mon, Jul 10, 2006 at 09:09:35PM +0100, Esben Nielsen wrote:
> 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 :-(

;-)

One possibility would be a file in the Documentation directory.
Another would be something on the web, but a file in the Documentation
directory would have the advantage of being synched with the -rt version.

							Thanx, Paul
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