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Subject[PATCH tip/core/rcu 07/11] rcu: Clear .exp_hint only when deferred quiescent state has been reported
Date
From: Lai Jiangshan <laijs@linux.alibaba.com>

Currently, the .exp_hint flag is cleared in rcu_read_unlock_special(),
which works, but which can also prevent subsequent rcu_read_unlock() calls
from helping expedite the quiescent state needed by an ongoing expedited
RCU grace period. This commit therefore defers clearing of .exp_hint
from rcu_read_unlock_special() to rcu_preempt_deferred_qs_irqrestore(),
thus ensuring that intervening calls to rcu_read_unlock() have a chance
to help end the expedited grace period.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree_plugin.h | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 8cdce11..7487c79 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -444,6 +444,7 @@ rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
local_irq_restore(flags);
return;
}
+ t->rcu_read_unlock_special.b.exp_hint = false;
t->rcu_read_unlock_special.b.deferred_qs = false;
if (special.b.need_qs) {
rcu_qs();
@@ -610,7 +611,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
struct rcu_node *rnp = rdp->mynode;

- t->rcu_read_unlock_special.b.exp_hint = false;
exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
(rdp->grpmask & rnp->expmask) ||
tick_nohz_full_cpu(rdp->cpu);
@@ -640,7 +640,6 @@ static void rcu_read_unlock_special(struct task_struct *t)
local_irq_restore(flags);
return;
}
- WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false);
rcu_preempt_deferred_qs_irqrestore(t, flags);
}

--
2.9.5
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