Commit efd88b02 authored Oct 19, 2017 by Paul E. McKenney

rcu: Add comment giving debug strategy for double call_rcu()

The following statement has for some reason proven non-intuitive:

WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));

This commit therefore adds a comment that states that this warning
usually triggers in response to a double call_rcu(), which is sort
of like a double free. The comment also suggests building with
CONFIG_DEBUG_OBJECTS_RCU_HEAD=y to track down the double call_rcu().

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

parent 9122caf9

kernel/rcu/tree.c

+5 −0

Original line number	Diff line number	Diff line
		@@ -2789,6 +2789,11 @@ static void rcu_do_batch(struct rcu_state rsp, struct rcu_data rdp)
		rdp->n_force_qs_snap = rsp->n_force_qs;
		} else if (count < rdp->qlen_last_fqs_check - qhimark)
		rdp->qlen_last_fqs_check = count;

		/*
		* The following usually indicates a double call_rcu(). To track
		* this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.
		*/
		WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));

		local_irq_restore(flags);