rcu: Mark callbacks not currently participating in barrier operation

RCU keeps a count of the number of callbacks that the current
rcu_barrier() is waiting on, but there is currently no easy way to
work out which callback is stuck.  One way to do this is to mark idle
RCU-barrier callbacks by making the ->next pointer point to the callback
itself, and this commit does just that.

Later commits will use this for debug output.

Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e641cc681901a5e5c7a7e1791a836ba0db89d7d3..f931171daecdf83ee17967eaf13d2c94786af948 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4403,6 +4403,7 @@ static void rcu_barrier_callback(struct rcu_head *rhp)
 {
        unsigned long __maybe_unused s = rcu_state.barrier_sequence;
 
+       rhp->next = rhp; // Mark the callback as having been invoked.
        if (atomic_dec_and_test(&rcu_state.barrier_cpu_count)) {
                rcu_barrier_trace(TPS("LastCB"), -1, s);
                complete(&rcu_state.barrier_completion);
@@ -5424,6 +5425,8 @@ static void __init rcu_init_one(void)
                while (i > rnp->grphi)
                        rnp++;
                per_cpu_ptr(&rcu_data, i)->mynode = rnp;
+               per_cpu_ptr(&rcu_data, i)->barrier_head.next =
+                       &per_cpu_ptr(&rcu_data, i)->barrier_head;
                rcu_boot_init_percpu_data(i);
        }
 }