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Subject[006/151] rcu: Fix note_new_gpnum() uses of ->gpnum
2.6.32-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

commit 9160306e6f5b68bb64630c9031c517ca1cf463db upstream.

Impose a clear locking design on the note_new_gpnum()
function's use of the ->gpnum counter. This is done by updating
rdp->gpnum only from the corresponding leaf rcu_node structure's
rnp->gpnum field, and even then only under the protection of
that same rcu_node structure's ->lock field. Performance and
scalability are maintained using a form of double-checked
locking, and excessive spinning is avoided by use of the
spin_trylock() function. The use of spin_trylock() is safe due
to the fact that CPUs who fail to acquire this lock will try
again later. The hierarchical nature of the rcu_node data
structure limits contention (which could be limited further if
need be using the RCU_FANOUT kernel parameter).

Without this patch, obscure but quite possible races could
result in a quiescent state that occurred during one grace
period to be accounted to the following grace period, causing
this following grace period to end prematurely. Not good!

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12571987492350-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

---
kernel/rcutree.c | 34 ++++++++++++++++++++++++++++------
1 file changed, 28 insertions(+), 6 deletions(-)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -538,13 +538,33 @@ static void check_cpu_stall(struct rcu_s
/*
* Update CPU-local rcu_data state to record the newly noticed grace period.
* This is used both when we started the grace period and when we notice
- * that someone else started the grace period.
- */
+ * that someone else started the grace period. The caller must hold the
+ * ->lock of the leaf rcu_node structure corresponding to the current CPU,
+ * and must have irqs disabled.
+ */
+static void __note_new_gpnum(struct rcu_state *rsp, struct rcu_node *rnp, struct rcu_data *rdp)
+{
+ if (rdp->gpnum != rnp->gpnum) {
+ rdp->qs_pending = 1;
+ rdp->passed_quiesc = 0;
+ rdp->gpnum = rnp->gpnum;
+ }
+}
+
static void note_new_gpnum(struct rcu_state *rsp, struct rcu_data *rdp)
{
- rdp->qs_pending = 1;
- rdp->passed_quiesc = 0;
- rdp->gpnum = rsp->gpnum;
+ unsigned long flags;
+ struct rcu_node *rnp;
+
+ local_irq_save(flags);
+ rnp = rdp->mynode;
+ if (rdp->gpnum == ACCESS_ONCE(rnp->gpnum) || /* outside lock. */
+ !spin_trylock(&rnp->lock)) { /* irqs already off, retry later. */
+ local_irq_restore(flags);
+ return;
+ }
+ __note_new_gpnum(rsp, rnp, rdp);
+ spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
@@ -635,6 +655,9 @@ rcu_start_gp_per_cpu(struct rcu_state *r
*/
rdp->nxttail[RCU_NEXT_READY_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
rdp->nxttail[RCU_WAIT_TAIL] = rdp->nxttail[RCU_NEXT_TAIL];
+
+ /* Set state so that this CPU will detect the next quiescent state. */
+ __note_new_gpnum(rsp, rnp, rdp);
}

/*
@@ -662,7 +685,6 @@ rcu_start_gp(struct rcu_state *rsp, unsi
rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
record_gp_stall_check_time(rsp);
dyntick_record_completed(rsp, rsp->completed - 1);
- note_new_gpnum(rsp, rdp);

/* Special-case the common single-level case. */
if (NUM_RCU_NODES == 1) {



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