Re: [PATCH tip/core/rcu 18/18] rcu: Better hotplug handling for synchronize_sched_expedited()

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Oct 08, 2015 at 11:01:28AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2015 at 09:26:53AM -0700, Paul E. McKenney wrote:
> > On Wed, Oct 07, 2015 at 04:26:27PM +0200, Peter Zijlstra wrote:
> 
> > Indeed, that approach looks better than moving rcu_note_context_switch(),
> > which probably results in deadlocks.  I will update my patch accordingly.
> 
> Yeah, calling rcu_note_context_switch() under the rq->lock is asking for
> trouble we don't need.

Please see below for the fixed version.  Thoughts?

(Queued for 4.5, want some serious testing on this.)

							Thanx, Paul

------------------------------------------------------------------------

commit 3ab3edf72a59a800e6e59ad3128f5b3b251b8962
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Wed Oct 7 09:10:48 2015 -0700

    rcu: Stop disabling interrupts in scheduler fastpaths
    
    We need the scheduler's fastpaths to be, well, fast, and unnecessarily
    disabling and re-enabling interrupts is not necessarily consistent with
    this goal.  Especially given that there are regions of the scheduler that
    already have interrupts disabled.
    
    This commit therefore moves the call to rcu_note_context_switch()
    to one of the interrupts-disabled regions of the scheduler, and
    removes the now-redundant disabling and re-enabling of interrupts from
    rcu_note_context_switch() and the functions it calls.
    
    Reported-by: Peter Zijlstra <peterz@infradead.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    [ paulmck: Shift rcu_note_context_switch() to avoid deadlock, as suggested
      by Peter Zijlstra. ]

diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
index 60d15a080d7c..9d3eda39bcd2 100644
--- a/include/linux/rcutree.h
+++ b/include/linux/rcutree.h
@@ -37,7 +37,7 @@ void rcu_cpu_stall_reset(void);
 /*
  * Note a virtualization-based context switch.  This is simply a
  * wrapper around rcu_note_context_switch(), which allows TINY_RCU
- * to save a few bytes.
+ * to save a few bytes. The caller must have disabled interrupts.
  */
 static inline void rcu_virt_note_context_switch(int cpu)
 {
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c9780fc47391..fbc9b5574e48 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -295,17 +295,16 @@ EXPORT_PER_CPU_SYMBOL_GPL(rcu_qs_ctr);
  * We inform the RCU core by emulating a zero-duration dyntick-idle
  * period, which we in turn do by incrementing the ->dynticks counter
  * by two.
+ *
+ * The caller must have disabled interrupts.
  */
 static void rcu_momentary_dyntick_idle(void)
 {
-	unsigned long flags;
 	struct rcu_data *rdp;
 	struct rcu_dynticks *rdtp;
 	int resched_mask;
 	struct rcu_state *rsp;
 
-	local_irq_save(flags);
-
 	/*
 	 * Yes, we can lose flag-setting operations.  This is OK, because
 	 * the flag will be set again after some delay.
@@ -335,13 +334,12 @@ static void rcu_momentary_dyntick_idle(void)
 		smp_mb__after_atomic(); /* Later stuff after QS. */
 		break;
 	}
-	local_irq_restore(flags);
 }
 
 /*
  * Note a context switch.  This is a quiescent state for RCU-sched,
  * and requires special handling for preemptible RCU.
- * The caller must have disabled preemption.
+ * The caller must have disabled interrupts.
  */
 void rcu_note_context_switch(void)
 {
@@ -371,9 +369,14 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
  */
 void rcu_all_qs(void)
 {
+	unsigned long flags;
+
 	barrier(); /* Avoid RCU read-side critical sections leaking down. */
-	if (unlikely(raw_cpu_read(rcu_sched_qs_mask)))
+	if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) {
+		local_irq_save(flags);
 		rcu_momentary_dyntick_idle();
+		local_irq_restore(flags);
+	}
 	this_cpu_inc(rcu_qs_ctr);
 	barrier(); /* Avoid RCU read-side critical sections leaking up. */
 }
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 97dfa7d57f79..7087fb047e2d 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -146,8 +146,8 @@ static void __init rcu_bootup_announce(void)
  * the corresponding expedited grace period will also be the end of the
  * normal grace period.
  */
-static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp,
-				   unsigned long flags) __releases(rnp->lock)
+static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
+	__releases(rnp->lock) /* But leaves rrupts disabled. */
 {
 	int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
 			 (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
@@ -235,7 +235,7 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp,
 		rnp->gp_tasks = &t->rcu_node_entry;
 	if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
 		rnp->exp_tasks = &t->rcu_node_entry;
-	raw_spin_unlock(&rnp->lock);
+	raw_spin_unlock(&rnp->lock); /* rrupts remain disabled. */
 
 	/*
 	 * Report the quiescent state for the expedited GP.  This expedited
@@ -250,7 +250,6 @@ static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp,
 	} else {
 		WARN_ON_ONCE(t->rcu_read_unlock_special.b.exp_need_qs);
 	}
-	local_irq_restore(flags);
 }
 
 /*
@@ -285,12 +284,11 @@ static void rcu_preempt_qs(void)
  * predating the current grace period drain, in other words, until
  * rnp->gp_tasks becomes NULL.
  *
- * Caller must disable preemption.
+ * Caller must disable interrupts.
  */
 static void rcu_preempt_note_context_switch(void)
 {
 	struct task_struct *t = current;
-	unsigned long flags;
 	struct rcu_data *rdp;
 	struct rcu_node *rnp;
 
@@ -300,7 +298,7 @@ static void rcu_preempt_note_context_switch(void)
 		/* Possibly blocking in an RCU read-side critical section. */
 		rdp = this_cpu_ptr(rcu_state_p->rda);
 		rnp = rdp->mynode;
-		raw_spin_lock_irqsave(&rnp->lock, flags);
+		raw_spin_lock(&rnp->lock); /* rrupts already disabled. */
 		smp_mb__after_unlock_lock();
 		t->rcu_read_unlock_special.b.blocked = true;
 		t->rcu_blocked_node = rnp;
@@ -317,7 +315,7 @@ static void rcu_preempt_note_context_switch(void)
 				       (rnp->qsmask & rdp->grpmask)
 				       ? rnp->gpnum
 				       : rnp->gpnum + 1);
-		rcu_preempt_ctxt_queue(rnp, rdp, flags);
+		rcu_preempt_ctxt_queue(rnp, rdp);
 	} else if (t->rcu_read_lock_nesting < 0 &&
 		   t->rcu_read_unlock_special.s) {
 
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c4e607873d6f..ac246b0b987a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3056,7 +3056,6 @@ static void __sched __schedule(void)
 
 	cpu = smp_processor_id();
 	rq = cpu_rq(cpu);
-	rcu_note_context_switch();
 	prev = rq->curr;
 
 	schedule_debug(prev);
@@ -3064,13 +3063,16 @@ static void __sched __schedule(void)
 	if (sched_feat(HRTICK))
 		hrtick_clear(rq);
 
+	local_irq_disable();
+	rcu_note_context_switch();
+
 	/*
 	 * Make sure that signal_pending_state()->signal_pending() below
 	 * can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
 	 * done by the caller to avoid the race with signal_wake_up().
 	 */
 	smp_mb__before_spinlock();
-	raw_spin_lock_irq(&rq->lock);
+	raw_spin_lock(&rq->lock);
 	lockdep_pin_lock(&rq->lock);
 
 	rq->clock_skip_update <<= 1; /* promote REQ to ACT */