Messages in this thread | | | Date | Mon, 10 Jul 2006 21:09:35 +0100 (BST) | Subject | Re: [PATCH -rt] catch put_task_struct RCU handling up to mainline | From | Esben Nielsen <> |
| |
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 > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |