rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access

In the Linux kernel, the following vulnerability has been resolved:

rcu: Protect rcu_print_task_exp_stall() ->exp_tasks access

For kernels built with CONFIG_PREEMPT_RCU=y, the following scenario can

result in a NULL-pointer dereference:

CPU1 CPU2

rcu_preempt_deferred_qs_irqrestore rcu_print_task_exp_stall

if (special.b.blocked) READ_ONCE(rnp->exp_tasks) != NULL

raw_spin_lock_rcu_node

np = rcu_next_node_entry(t, rnp)

if (&t->rcu_node_entry == rnp->exp_tasks)

WRITE_ONCE(rnp->exp_tasks, np)

....

raw_spin_unlock_irqrestore_rcu_node

raw_spin_lock_irqsave_rcu_node

t = list_entry(rnp->exp_tasks->prev,

struct task_struct, rcu_node_entry)

(if rnp->exp_tasks is NULL, this

will dereference a NULL pointer)

The problem is that CPU2 accesses the rcu_node structure's->exp_tasks

field without holding the rcu_node structure's ->lock and CPU2 did

not observe CPU1's change to rcu_node structure's ->exp_tasks in time.

Therefore, if CPU1 sets rcu_node structure's->exp_tasks pointer to NULL,

then CPU2 might dereference that NULL pointer.

This commit therefore holds the rcu_node structure's ->lock while

accessing that structure's->exp_tasks field.

[ paulmck: Apply Frederic Weisbecker feedback. ]