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Subject[PATCH tip/core/rcu 11/12] rcu: Execute RCU reader shortly after rcu_core for strict GPs
Date
From: "Paul E. McKenney" <paulmck@kernel.org>

A kernel built with CONFIG_RCU_STRICT_GRACE_PERIOD=y needs a quiescent
state to appear very shortly after a CPU has noticed a new grace period.
Placing an RCU reader immediately after this point is ineffective because
this normally happens in softirq context, which acts as a big RCU reader.
This commit therefore introduces a new per-CPU work_struct, which is
used at the end of rcu_core() processing to schedule an RCU read-side
critical section from within a clean environment.

Reported-by Jann Horn <jannh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tree.c | 13 +++++++++++++
kernel/rcu/tree.h | 1 +
2 files changed, 14 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index dd7af40..ac37343 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2634,6 +2634,14 @@ void rcu_force_quiescent_state(void)
}
EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);

+// Workqueue handler for an RCU reader for kernels enforcing struct RCU
+// grace periods.
+static void strict_work_handler(struct work_struct *work)
+{
+ rcu_read_lock();
+ rcu_read_unlock();
+}
+
/* Perform RCU core processing work for the current CPU. */
static __latent_entropy void rcu_core(void)
{
@@ -2678,6 +2686,10 @@ static __latent_entropy void rcu_core(void)
/* Do any needed deferred wakeups of rcuo kthreads. */
do_nocb_deferred_wakeup(rdp);
trace_rcu_utilization(TPS("End RCU core"));
+
+ // If strict GPs, schedule an RCU reader in a clean environment.
+ if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD))
+ queue_work_on(rdp->cpu, rcu_gp_wq, &rdp->strict_work);
}

static void rcu_core_si(struct softirq_action *h)
@@ -3874,6 +3886,7 @@ rcu_boot_init_percpu_data(int cpu)

/* Set up local state, ensuring consistent view of global state. */
rdp->grpmask = leaf_node_cpu_bit(rdp->mynode, cpu);
+ INIT_WORK(&rdp->strict_work, strict_work_handler);
WARN_ON_ONCE(rdp->dynticks_nesting != 1);
WARN_ON_ONCE(rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp)));
rdp->rcu_ofl_gp_seq = rcu_state.gp_seq;
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 309bc7f..e4f66b8 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -165,6 +165,7 @@ struct rcu_data {
/* period it is aware of. */
struct irq_work defer_qs_iw; /* Obtain later scheduler attention. */
bool defer_qs_iw_pending; /* Scheduler attention pending? */
+ struct work_struct strict_work; /* Schedule readers for strict GPs. */

/* 2) batch handling */
struct rcu_segcblist cblist; /* Segmented callback list, with */
--
2.9.5
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