[PATCH] Fix rcutree grace-period-latency bug on small systems

[PATCH] Fix rcutree grace-period-latency bug on small systems

[Paul E. McKenney]

Kudos to Andi Kleen for finding a grace-period-latency problem!  The
problem was that the special-case code for small machines never updated
the ->signaled field to indicate that grace-period initialization had
completed, which prevented force_quiescent_state() from ever expediting
grace periods.  This problem resulted in grace periods extending for more
than 20 seconds.  Not subtle.  I introduced this bug during my inspection
process when I fixed a race between grace-period initialization and
force_quiescent_state() execution.

The following patch properly updates the ->signaled field for the
"small"-system case (no more than 32 CPUs for 32-bit kernels and no more
than 64 CPUs for 64-bit kernels).

I believe that this patch should be included for 2.6.29.

Located-by: Andi Kleen <andi@firstfloor.org>
Tested-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 rcutree.c |    1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index a342b03..88d921c 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -572,6 +572,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
 	/* Special-case the common single-level case. */
 	if (NUM_RCU_NODES == 1) {
 		rnp->qsmask = rnp->qsmaskinit;
+		rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
 		spin_unlock_irqrestore(&rnp->lock, flags);
 		return;
 	}

Re: [PATCH] Fix rcutree grace-period-latency bug on small systems

[Andi Kleen]

> The following patch properly updates the ->signaled field for the
> "small"-system case (no more than 32 CPUs for 32-bit kernels and no more
> than 64 CPUs for 64-bit kernels).
> 
> I believe that this patch should be included for 2.6.29.

Yes, it's really needed. One of my test system takes forever to boot
without it when tree rcu is enabled.

Thanks Paul for the debugging work.

-Andi

Re: [PATCH] Fix rcutree grace-period-latency bug on small systems

[Ingo Molnar]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> Kudos to Andi Kleen for finding a grace-period-latency problem!  The
> problem was that the special-case code for small machines never updated
> the ->signaled field to indicate that grace-period initialization had
> completed, which prevented force_quiescent_state() from ever expediting
> grace periods.  This problem resulted in grace periods extending for more
> than 20 seconds.  Not subtle.  I introduced this bug during my inspection
> process when I fixed a race between grace-period initialization and
> force_quiescent_state() execution.
> 
> The following patch properly updates the ->signaled field for the
> "small"-system case (no more than 32 CPUs for 32-bit kernels and no more
> than 64 CPUs for 64-bit kernels).
> 
> I believe that this patch should be included for 2.6.29.
> 
> Located-by: Andi Kleen <andi@firstfloor.org>
> Tested-by: Andi Kleen <andi@firstfloor.org>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> 
>  rcutree.c |    1 +
>  1 file changed, 1 insertion(+)

Applied to tip/core/urgent, thanks guys!

	Ingo