[PATCH 3/8] srcu: Use local_lock() for per-CPU struct srcu_data access

[Sebastian Andrzej Siewior]

SRCU disables interrupts to get a stable per-CPU pointer and then
acquires the spinlock which is in the per-CPU data structure. The
release uses spin_unlock_irqrestore(). While this is correct on a non-RT
kernel, this conflicts with the RT semantics because the spinlock is
converted to a 'sleeping' spinlock. Sleeping locks can obviously not be
acquired with interrupts disabled.

Add a local lock and use the corresponding local lock operations.  Split
the restore into unlock and local_lock_irqrestore(). The local lock
operations map to local_irq_disable/enable() on a non-RT kernel. On a RT
kernel the local lock is substituted with a real per CPU lock which
serializes the access and guarantees CPU locality, but keeps the code
section preemptible. No functional change.

Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: rcu@xxxxxxxxxxxxxxx
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 kernel/rcu/srcutree.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 0c71505f0e19c..8d2b5f75145d7 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -25,6 +25,7 @@
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/srcu.h>
+#include <linux/locallock.h>
 
 #include "rcu.h"
 #include "rcu_segcblist.h"
@@ -735,6 +736,7 @@ static void srcu_flip(struct srcu_struct *ssp)
 	smp_mb(); /* D */  /* Pairs with C. */
 }
 
+static DEFINE_LOCAL_LOCK(sda_lock);
 /*
  * If SRCU is likely idle, return true, otherwise return false.
  *
@@ -765,13 +767,13 @@ static bool srcu_might_be_idle(struct srcu_struct *ssp)
 	unsigned long tlast;
 
 	/* If the local srcu_data structure has callbacks, not idle.  */
-	local_irq_save(flags);
+	local_lock_irqsave(sda_lock, flags);
 	sdp = this_cpu_ptr(ssp->sda);
 	if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
-		local_irq_restore(flags);
+		local_unlock_irqrestore(sda_lock, flags);
 		return false; /* Callbacks already present, so not idle. */
 	}
-	local_irq_restore(flags);
+	local_unlock_irqrestore(sda_lock, flags);
 
 	/*
 	 * No local callbacks, so probabalistically probe global state.
@@ -851,7 +853,7 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 	}
 	rhp->func = func;
 	idx = srcu_read_lock(ssp);
-	local_irq_save(flags);
+	local_lock_irqsave(sda_lock, flags);
 	sdp = this_cpu_ptr(ssp->sda);
 	spin_lock_rcu_node(sdp);
 	rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp);
@@ -867,7 +869,8 @@ static void __call_srcu(struct srcu_struct *ssp, struct rcu_head *rhp,
 		sdp->srcu_gp_seq_needed_exp = s;
 		needexp = true;
 	}
-	spin_unlock_irqrestore_rcu_node(sdp, flags);
+	spin_unlock_rcu_node(sdp);
+	local_unlock_irqrestore(sda_lock, flags);
 	if (needgp)
 		srcu_funnel_gp_start(ssp, sdp, s, do_norm);
 	else if (needexp)
-- 
2.26.2