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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>
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@ -2789,6 +2789,11 @@ static void rcu_do_batch(struct rcu_state *rsp, struct rcu_data *rdp)
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rdp->n_force_qs_snap = rsp->n_force_qs;
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} else if (count < rdp->qlen_last_fqs_check - qhimark)
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rdp->qlen_last_fqs_check = count;
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/*
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* The following usually indicates a double call_rcu(). To track
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* this down, try building with CONFIG_DEBUG_OBJECTS_RCU_HEAD=y.
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*/
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WARN_ON_ONCE(rcu_segcblist_empty(&rdp->cblist) != (count == 0));
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local_irq_restore(flags);
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