[tip: core/rcu] rcu: Remove unused ->rcu_read_unlock_special.b.deferred_qs field

[tip-bot2 for Lai Jiangshan]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     f0bdf6d473cf12a488a78422e15aafdfe77cf853
Gitweb:        https://git.kernel.org/tip/f0bdf6d473cf12a488a78422e15aafdfe77cf853
Author:        Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
AuthorDate:    Sat, 15 Feb 2020 14:52:32 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 27 Apr 2020 11:03:50 -07:00

rcu: Remove unused ->rcu_read_unlock_special.b.deferred_qs field

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@xxxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 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..a4b727f 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;
 	}