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Subject[PATCH tip/core/urgent 1/7] rcu: decrease rcu_report_exp_rnp coupling with scheduler
Date
PREEMPT_RCU read-side critical sections blocking an expedited grace
period invoke rcu_report_exp_rnp(). When the last such critical section
has completed, rcu_report_exp_rnp() invokes the scheduler to wake up the
task that invoked synchronize_rcu_expedited() -- needlessly holding the
root rcu_node structure's lock while doing so, thus needlessly providing
a way for RCU and the scheduler to deadlock.

This commit therefore releases the root rcu_node structure's lock before
calling wake_up().

Reported-by: Ed Tomlinson <edt@aei.ca>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcutree_plugin.h | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 75113cb..94a674e 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -696,8 +696,10 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
raw_spin_lock_irqsave(&rnp->lock, flags);
for (;;) {
if (!sync_rcu_preempt_exp_done(rnp))
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
break;
if (rnp->parent == NULL) {
+ raw_spin_unlock_irqrestore(&rnp->lock, flags);
wake_up(&sync_rcu_preempt_exp_wq);
break;
}
@@ -707,7 +709,6 @@ static void rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp)
raw_spin_lock(&rnp->lock); /* irqs already disabled */
rnp->expmask &= ~mask;
}
- raw_spin_unlock_irqrestore(&rnp->lock, flags);
}

/*
--
1.7.3.2


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