rcu: fix rcutree grace-period-latency bug on small systems
Impact: fix delays during bootup
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).
Reported-by: Andi Kleen <andi@firstfloor.org>
Tested-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
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 @@
/* 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;
}
