[PATCH v2 -rcu dev 1/2] Revert b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter")

* [PATCH v2 -rcu dev 1/2] Revert b8c17e6664c4 ("rcu: Maintain special bits at bottom of ->dynticks counter")
@ 2019-08-30 16:26 Joel Fernandes (Google)
  2019-08-30 16:26 ` [PATCH v2 -rcu dev 2/2] rcu/dyntick-idle: Add better tracing Joel Fernandes (Google)
  0 siblings, 1 reply; 2+ messages in thread
From: Joel Fernandes (Google) @ 2019-08-30 16:26 UTC (permalink / raw)
  To: linux-kernel
  Cc: Joel Fernandes (Google),
	Andy Lutomirski, Ingo Molnar, Josh Triplett, Lai Jiangshan,
	Mathieu Desnoyers, Mike Rapoport, Paul E. McKenney, Petr Mladek,
	rcu, Sakari Ailus, Steven Rostedt, Yafang Shao

This code is unused and can be removed now. Revert was straightforward.

Tested with light rcutorture.

Link: http://lore.kernel.org/r/CALCETrWNPOOdTrFabTDd=H7+wc6xJ9rJceg6OL1S0rTV5pfSsA@mail.gmail.com
Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>


---
Only made some commit message changes in this since v1.

 include/linux/rcutiny.h |  3 --
 kernel/rcu/tree.c       | 82 ++++++++++-------------------------------
 2 files changed, 19 insertions(+), 66 deletions(-)

diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
index b7607e2667ae..b3f689711289 100644
--- a/include/linux/rcutiny.h
+++ b/include/linux/rcutiny.h
@@ -14,9 +14,6 @@
 
 #include <asm/param.h> /* for HZ */
 
-/* Never flag non-existent other CPUs! */
-static inline bool rcu_eqs_special_set(int cpu) { return false; }
-
 static inline unsigned long get_state_synchronize_rcu(void)
 {
 	return 0;
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 68ebf0eb64c8..417dd00b9e87 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -69,20 +69,10 @@
 
 /* Data structures. */
 
-/*
- * Steal a bit from the bottom of ->dynticks for idle entry/exit
- * control.  Initially this is for TLB flushing.
- */
-#define RCU_DYNTICK_CTRL_MASK 0x1
-#define RCU_DYNTICK_CTRL_CTR  (RCU_DYNTICK_CTRL_MASK + 1)
-#ifndef rcu_eqs_special_exit
-#define rcu_eqs_special_exit() do { } while (0)
-#endif
-
 static DEFINE_PER_CPU_SHARED_ALIGNED(struct rcu_data, rcu_data) = {
 	.dynticks_nesting = 1,
 	.dynticks_nmi_nesting = DYNTICK_IRQ_NONIDLE,
-	.dynticks = ATOMIC_INIT(RCU_DYNTICK_CTRL_CTR),
+	.dynticks = ATOMIC_INIT(1),
 };
 struct rcu_state rcu_state = {
 	.level = { &rcu_state.node[0] },
@@ -229,20 +219,15 @@ void rcu_softirq_qs(void)
 static void rcu_dynticks_eqs_enter(void)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
-	int seq;
+	int special;
 
 	/*
-	 * CPUs seeing atomic_add_return() must see prior RCU read-side
+	 * CPUs seeing atomic_inc_return() must see prior RCU read-side
 	 * critical sections, and we also must force ordering with the
 	 * next idle sojourn.
 	 */
-	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
-	/* Better be in an extended quiescent state! */
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
-		     (seq & RCU_DYNTICK_CTRL_CTR));
-	/* Better not have special action (TLB flush) pending! */
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
-		     (seq & RCU_DYNTICK_CTRL_MASK));
+	special = atomic_inc_return(&rdp->dynticks);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && special & 0x1);
 }
 
 /*
@@ -252,22 +237,15 @@ static void rcu_dynticks_eqs_enter(void)
 static void rcu_dynticks_eqs_exit(void)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
-	int seq;
+	int special;
 
 	/*
-	 * CPUs seeing atomic_add_return() must see prior idle sojourns,
+	 * CPUs seeing atomic_inc_return() must see prior idle sojourns,
 	 * and we also must force ordering with the next RCU read-side
 	 * critical section.
 	 */
-	seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
-	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
-		     !(seq & RCU_DYNTICK_CTRL_CTR));
-	if (seq & RCU_DYNTICK_CTRL_MASK) {
-		atomic_andnot(RCU_DYNTICK_CTRL_MASK, &rdp->dynticks);
-		smp_mb__after_atomic(); /* _exit after clearing mask. */
-		/* Prefer duplicate flushes to losing a flush. */
-		rcu_eqs_special_exit();
-	}
+	special = atomic_inc_return(&rdp->dynticks);
+	WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(special & 0x1));
 }
 
 /*
@@ -284,9 +262,9 @@ static void rcu_dynticks_eqs_online(void)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
-	if (atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR)
+	if (atomic_read(&rdp->dynticks) & 0x1)
 		return;
-	atomic_add(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks);
+	atomic_add(0x1, &rdp->dynticks);
 }
 
 /*
@@ -298,7 +276,7 @@ bool rcu_dynticks_curr_cpu_in_eqs(void)
 {
 	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
-	return !(atomic_read(&rdp->dynticks) & RCU_DYNTICK_CTRL_CTR);
+	return !(atomic_read(&rdp->dynticks) & 0x1);
 }
 
 /*
@@ -309,7 +287,7 @@ int rcu_dynticks_snap(struct rcu_data *rdp)
 {
 	int snap = atomic_add_return(0, &rdp->dynticks);
 
-	return snap & ~RCU_DYNTICK_CTRL_MASK;
+	return snap;
 }
 
 /*
@@ -318,7 +296,7 @@ int rcu_dynticks_snap(struct rcu_data *rdp)
  */
 static bool rcu_dynticks_in_eqs(int snap)
 {
-	return !(snap & RCU_DYNTICK_CTRL_CTR);
+	return !(snap & 0x1);
 }
 
 /*
@@ -331,28 +309,6 @@ static bool rcu_dynticks_in_eqs_since(struct rcu_data *rdp, int snap)
 	return snap != rcu_dynticks_snap(rdp);
 }
 
-/*
- * Set the special (bottom) bit of the specified CPU so that it
- * will take special action (such as flushing its TLB) on the
- * next exit from an extended quiescent state.  Returns true if
- * the bit was successfully set, or false if the CPU was not in
- * an extended quiescent state.
- */
-bool rcu_eqs_special_set(int cpu)
-{
-	int old;
-	int new;
-	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
-
-	do {
-		old = atomic_read(&rdp->dynticks);
-		if (old & RCU_DYNTICK_CTRL_CTR)
-			return false;
-		new = old | RCU_DYNTICK_CTRL_MASK;
-	} while (atomic_cmpxchg(&rdp->dynticks, old, new) != old);
-	return true;
-}
-
 /*
  * Let the RCU core know that this CPU has gone through the scheduler,
  * which is a quiescent state.  This is called when the need for a
@@ -366,13 +322,13 @@ bool rcu_eqs_special_set(int cpu)
  */
 void rcu_momentary_dyntick_idle(void)
 {
-	int special;
+	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
+	int special = atomic_add_return(2, &rdp->dynticks);
 
-	raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
-	special = atomic_add_return(2 * RCU_DYNTICK_CTRL_CTR,
-				    &this_cpu_ptr(&rcu_data)->dynticks);
 	/* It is illegal to call this from idle state. */
-	WARN_ON_ONCE(!(special & RCU_DYNTICK_CTRL_CTR));
+	WARN_ON_ONCE(!(special & 0x1));
+
+	raw_cpu_write(rcu_data.rcu_need_heavy_qs, false);
 	rcu_preempt_deferred_qs(current);
 }
 EXPORT_SYMBOL_GPL(rcu_momentary_dyntick_idle);
-- 
2.23.0.187.g17f5b7556c-goog

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