linux-kernel - [PATCH tip/core/rcu 4/6] rcu: Remove unused ->rcu_read_unlock_special.b.deferred

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Date:   Wed, 15 Apr 2020 10:56:12 -0700
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 4/6] rcu: Remove unused ->rcu_read_unlock_special.b.deferred_qs field

From: Lai Jiangshan <laijs@...ux.alibaba.com>

The ->rcu_read_unlock_special.b.deferred_qs field is set to true in
rcu_read_unlock_special() but never set to false.  This is not
particularly useful, so this commit removes this field.

The only possible justification for this field is to ease debugging
of RCU deferred quiscent states, but the combination of the other
->rcu_read_unlock_special fields plus ->rcu_blocked_node and of course
->rcu_read_lock_nesting should cover debugging needs.  And if this last
proves incorrect, this patch can always be reverted, along with the
required setting of ->rcu_read_unlock_special.b.deferred_qs to false
in rcu_preempt_deferred_qs_irqrestore().

Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 include/linux/sched.h    | 2 +-
 kernel/rcu/tree_plugin.h | 1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4418f5c..a4b727f5 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -613,7 +613,7 @@ union rcu_special {
 		u8			blocked;
 		u8			need_qs;
 		u8			exp_hint; /* Hint for performance. */
-		u8			deferred_qs;
+		u8			pad; /* No garbage from compiler! */
 	} b; /* Bits. */
 	u32 s; /* Set of bits. */
 };
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 263c766..f31c599 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -634,7 +634,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
 				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
 			}
 		}
-		t->rcu_read_unlock_special.b.deferred_qs = true;
 		local_irq_restore(flags);
 		return;
 	}
-- 
2.9.5