When RCU-TASKS-TRACE pre-gp takes a snapshot of the current task running
on all online CPUs, no explicit ordering synchronizes properly with a
context switch.  This lack of ordering can permit the new task to miss
pre-grace-period update-side accesses.  The following diagram, courtesy
of Paul, shows the possible bad scenario:

        CPU 0                                           CPU 1
        -----                                           -----

        // Pre-GP update side access
        WRITE_ONCE(*X, 1);
        smp_mb();
        r0 = rq->curr;
                                                        RCU_INIT_POINTER(rq->curr, TASK_B)
                                                        spin_unlock(rq)
                                                        rcu_read_lock_trace()
                                                        r1 = X;
        /* ignore TASK_B */

Either r0==TASK_B or r1==1 is needed but neither is guaranteed.

One possible solution to solve this is to wait for an RCU grace period
at the beginning of the RCU-tasks-trace grace period before taking the
current tasks snaphot. However this would introduce large additional
latencies to RCU-tasks-trace grace periods.

Another solution is to lock the target runqueue while taking the current
task snapshot. This ensures that the update side sees the latest context
switch and subsequent context switches will see the pre-grace-period
update side accesses.

This commit therefore adds runqueue locking to cpu_curr_snapshot().

Fixes: e386b672 ("rcu-tasks: Eliminate RCU Tasks Trace IPIs to online CPUs")
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>