rcu: Fix deadlock in TREE_PREEMPT_RCU CPU stall detection

Under TREE_PREEMPT_RCU, print_other_cpu_stall() invokes
rcu_print_task_stall() with the root rcu_node structure's ->lock
held, and rcu_print_task_stall() acquires that same lock for
self-deadlock. Fix this by removing the lock acquisition from
rcu_print_task_stall(), and making all callers acquire the lock
instead.

Tested-by: John Kacur <jkacur@redhat.com>
Tested-by: Thomas Gleixner <tglx@linutronix.de>
Located-by: Thomas Gleixner <tglx@linutronix.de>
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: <1266887105-1528-19-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 2 insertions, 3 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 65a807b4f58c..b07be37d2aa3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -476,7 +476,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
 
         printk(KERN_ERR "INFO: RCU detected CPU stalls:");
         rcu_for_each_leaf_node(rsp, rnp) {
+                raw_spin_lock_irqsave(&rnp->lock, flags);
                 rcu_print_task_stall(rnp);
+                raw_spin_unlock_irqrestore(&rnp->lock, flags);
                 if (rnp->qsmask == 0)
                         continue;
                 for (cpu = 0; cpu <= rnp->grphi - rnp->grplo; cpu++)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index a8b2e834fd3a..aecfe37e0117 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -318,18 +318,15 @@ EXPORT_SYMBOL_GPL(__rcu_read_unlock);
  */
 static void rcu_print_task_stall(struct rcu_node *rnp)
 {
-        unsigned long flags;
         struct list_head *lp;
         int phase;
         struct task_struct *t;
 
         if (rcu_preempted_readers(rnp)) {
-                raw_spin_lock_irqsave(&rnp->lock, flags);
                 phase = rnp->gpnum & 0x1;
                 lp = &rnp->blocked_tasks[phase];
                 list_for_each_entry(t, lp, rcu_node_entry)
                         printk(" P%d", t->pid);
-                raw_spin_unlock_irqrestore(&rnp->lock, flags);
         }
 }