linux-kernel - [RFC 2/3] rcu: Simplify rcu_note_context

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Date:   Mon,  1 Jul 2019 00:04:14 -0400
From:   "Joel Fernandes (Google)" <joel@...lfernandes.org>
To:     linux-kernel@...r.kernel.org
Cc:     "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        rcu@...r.kernel.org, kernel-team@...roid.com,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kselftest@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Shuah Khan <shuah@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: [RFC 2/3] rcu: Simplify rcu_note_context_switch exit from critical section

The rcu_preempt_note_context_switch() tries to handle cases where
__rcu_read_unlock() got preempted and then the context switch path does
the reporting of the quiscent state along with clearing any bits in the
rcu_read_unlock_special union.

This can be handled by just calling rcu_deferred_qs() which was added
during the RCU consolidation work and already does these checks.

Tested RCU config TREE03 for an hour which succeeds.

Cc: rcu@...r.kernel.org
Cc: kernel-team@...roid.com
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
---
 kernel/rcu/tree_plugin.h | 9 ---------
 1 file changed, 9 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index bff6410fac06..ebb4d46a6267 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -313,15 +313,6 @@ void rcu_note_context_switch(bool preempt)
 				       ? rnp->gp_seq
 				       : rcu_seq_snap(&rnp->gp_seq));
 		rcu_preempt_ctxt_queue(rnp, rdp);
-	} else if (t->rcu_read_lock_nesting < 0 &&
-		   t->rcu_read_unlock_special.s) {
-
-		/*
-		 * Complete exit from RCU read-side critical section on
-		 * behalf of preempted instance of __rcu_read_unlock().
-		 */
-		rcu_read_unlock_special(t);
-		rcu_preempt_deferred_qs(t);
 	} else {
 		rcu_preempt_deferred_qs(t);
 	}
-- 
2.22.0.410.gd8fdbe21b5-goog