[tip/core/rcu,27/30] rcu: Make nocb_gp_wait() double-check unexpected-callback warning
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Message ID 20200214235607.13749-27-paulmck@kernel.org
State In Next
Commit a91ec8e7fff00864935c8ff048ea8d8313a0b7b9
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Series
  • Miscellaneous fixes for v5.7
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Commit Message

Paul E. McKenney Feb. 14, 2020, 11:56 p.m. UTC
From: "Paul E. McKenney" <paulmck@kernel.org>

Currently, nocb_gp_wait() unconditionally complains if there is a
callback not already associated with a grace period.  This assumes that
either there was no such callback initially on the one hand, or that
the rcu_advance_cbs() function assigned all such callbacks to a grace
period on the other.  However, in theory there are some situations that
would prevent rcu_advance_cbs() from assigning all of the callbacks.

This commit therefore checks for unassociated callbacks immediately after
rcu_advance_cbs() returns, while the corresponding rcu_node structure's
->lock is still held.  If there are unassociated callbacks at that point,
the subsequent WARN_ON_ONCE() is disabled.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tree_plugin.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Patch
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diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 70b3c0f..36e71c9 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1931,6 +1931,7 @@  static void nocb_gp_wait(struct rcu_data *my_rdp)
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
 	unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.
+	bool wasempty = false;
 
 	/*
 	 * Each pass through the following loop checks for CBs and for the
@@ -1970,10 +1971,13 @@  static void nocb_gp_wait(struct rcu_data *my_rdp)
 		     rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
 			raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
 			needwake_gp = rcu_advance_cbs(rnp, rdp);
+			wasempty = rcu_segcblist_restempty(&rdp->cblist,
+							   RCU_NEXT_READY_TAIL);
 			raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
 		}
 		// Need to wait on some grace period?
-		WARN_ON_ONCE(!rcu_segcblist_restempty(&rdp->cblist,
+		WARN_ON_ONCE(wasempty &&
+			     !rcu_segcblist_restempty(&rdp->cblist,
 						      RCU_NEXT_READY_TAIL));
 		if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
 			if (!needwait_gp ||