Commit c12172c0 authored Jan 04, 2009 by Paul E. McKenney Committed by Ingo Molnar Jan 05, 2009

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>

parent 52942b6b

kernel/rcutree.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -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;
		}

Admin message