[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@xxxxxxxxxxxxxx>
Tested-by: Andi Kleen <andi@xxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---

 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;
 	}

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