Revert "rcu: Decrease memory-barrier usage based on semi-formal proof"

This reverts commit e59fb3120becfb36b22ddb8bd27d065d3cdca499.

This reversion was due to (extreme) boot-time slowdowns on SPARC seen by
Yinghai Lu and on x86 by Ingo
.
This is a non-trivial reversion due to intervening commits.

Conflicts:

	Documentation/RCU/trace.txt
	kernel/rcutree.c

Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 74 insertions, 56 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 5616b17e4a22..e486f7c3ffb8 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -162,7 +162,7 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 #ifdef CONFIG_NO_HZ
 DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
         .dynticks_nesting = 1,
-        .dynticks = ATOMIC_INIT(1),
+        .dynticks = 1,
 };
 #endif /* #ifdef CONFIG_NO_HZ */
 
@@ -321,25 +321,13 @@ void rcu_enter_nohz(void)
         unsigned long flags;
         struct rcu_dynticks *rdtp;
 
+        smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
         local_irq_save(flags);
         rdtp = &__get_cpu_var(rcu_dynticks);
-        if (--rdtp->dynticks_nesting) {
-                local_irq_restore(flags);
-                return;
-        }
-        /* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
-        smp_mb__before_atomic_inc();  /* See above. */
-        atomic_inc(&rdtp->dynticks);
-        smp_mb__after_atomic_inc();  /* Force ordering with next sojourn. */
-        WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
+        rdtp->dynticks++;
+        rdtp->dynticks_nesting--;
+        WARN_ON_ONCE(rdtp->dynticks & 0x1);
         local_irq_restore(flags);
-
-        /* If the interrupt queued a callback, get out of dyntick mode. */
-        if (in_irq() &&
-            (__get_cpu_var(rcu_sched_data).nxtlist ||
-             __get_cpu_var(rcu_bh_data).nxtlist ||
-             rcu_preempt_needs_cpu(smp_processor_id())))
-                set_need_resched();
 }
 
 /*
@@ -355,16 +343,11 @@ void rcu_exit_nohz(void)
 
         local_irq_save(flags);
         rdtp = &__get_cpu_var(rcu_dynticks);
-        if (rdtp->dynticks_nesting++) {
-                local_irq_restore(flags);
-                return;
-        }
-        smp_mb__before_atomic_inc();  /* Force ordering w/previous sojourn. */
-        atomic_inc(&rdtp->dynticks);
-        /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
-        smp_mb__after_atomic_inc();  /* See above. */
-        WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
+        rdtp->dynticks++;
+        rdtp->dynticks_nesting++;
+        WARN_ON_ONCE(!(rdtp->dynticks & 0x1));
         local_irq_restore(flags);
+        smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
 }
 
 /**
@@ -378,15 +361,11 @@ void rcu_nmi_enter(void)
 {
         struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
 
-        if (rdtp->dynticks_nmi_nesting == 0 &&
-            (atomic_read(&rdtp->dynticks) & 0x1))
+        if (rdtp->dynticks & 0x1)
                 return;
-        rdtp->dynticks_nmi_nesting++;
-        smp_mb__before_atomic_inc();  /* Force delay from prior write. */
-        atomic_inc(&rdtp->dynticks);
-        /* CPUs seeing atomic_inc() must see later RCU read-side crit sects */
-        smp_mb__after_atomic_inc();  /* See above. */
-        WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks) & 0x1));
+        rdtp->dynticks_nmi++;
+        WARN_ON_ONCE(!(rdtp->dynticks_nmi & 0x1));
+        smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
 }
 
 /**
@@ -400,14 +379,11 @@ void rcu_nmi_exit(void)
 {
         struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
 
-        if (rdtp->dynticks_nmi_nesting == 0 ||
-            --rdtp->dynticks_nmi_nesting != 0)
+        if (rdtp->dynticks & 0x1)
                 return;
-        /* CPUs seeing atomic_inc() must see prior RCU read-side crit sects */
-        smp_mb__before_atomic_inc();  /* See above. */
-        atomic_inc(&rdtp->dynticks);
-        smp_mb__after_atomic_inc();  /* Force delay to next write. */
-        WARN_ON_ONCE(atomic_read(&rdtp->dynticks) & 0x1);
+        smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
+        rdtp->dynticks_nmi++;
+        WARN_ON_ONCE(rdtp->dynticks_nmi & 0x1);
 }
 
 /**
@@ -418,7 +394,13 @@ void rcu_nmi_exit(void)
  */
 void rcu_irq_enter(void)
 {
-        rcu_exit_nohz();
+        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
+
+        if (rdtp->dynticks_nesting++)
+                return;
+        rdtp->dynticks++;
+        WARN_ON_ONCE(!(rdtp->dynticks & 0x1));
+        smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
 }
 
 /**
@@ -430,7 +412,18 @@ void rcu_irq_enter(void)
  */
 void rcu_irq_exit(void)
 {
-        rcu_enter_nohz();
+        struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
+
+        if (--rdtp->dynticks_nesting)
+                return;
+        smp_mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
+        rdtp->dynticks++;
+        WARN_ON_ONCE(rdtp->dynticks & 0x1);
+
+        /* If the interrupt queued a callback, get out of dyntick mode. */
+        if (__this_cpu_read(rcu_sched_data.nxtlist) ||
+            __this_cpu_read(rcu_bh_data.nxtlist))
+                set_need_resched();
 }
 
 #ifdef CONFIG_SMP
@@ -442,8 +435,19 @@ void rcu_irq_exit(void)
  */
 static int dyntick_save_progress_counter(struct rcu_data *rdp)
 {
-        rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
-        return 0;
+        int ret;
+        int snap;
+        int snap_nmi;
+
+        snap = rdp->dynticks->dynticks;
+        snap_nmi = rdp->dynticks->dynticks_nmi;
+        smp_mb();       /* Order sampling of snap with end of grace period. */
+        rdp->dynticks_snap = snap;
+        rdp->dynticks_nmi_snap = snap_nmi;
+        ret = ((snap & 0x1) == 0) && ((snap_nmi & 0x1) == 0);
+        if (ret)
+                rdp->dynticks_fqs++;
+        return ret;
 }
 
 /*
@@ -454,11 +458,16 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
  */
 static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
 {
-        unsigned long curr;
-        unsigned long snap;
+        long curr;
+        long curr_nmi;
+        long snap;
+        long snap_nmi;
 
-        curr = (unsigned long)atomic_add_return(0, &rdp->dynticks->dynticks);
-        snap = (unsigned long)rdp->dynticks_snap;
+        curr = rdp->dynticks->dynticks;
+        snap = rdp->dynticks_snap;
+        curr_nmi = rdp->dynticks->dynticks_nmi;
+        snap_nmi = rdp->dynticks_nmi_snap;
+        smp_mb(); /* force ordering with cpu entering/leaving dynticks. */
 
         /*
          * If the CPU passed through or entered a dynticks idle phase with
@@ -468,7 +477,8 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
          * read-side critical section that started before the beginning
          * of the current RCU grace period.
          */
-        if ((curr & 0x1) == 0 || ULONG_CMP_GE(curr, snap + 2)) {
+        if ((curr != snap || (curr & 0x1) == 0) &&
+            (curr_nmi != snap_nmi || (curr_nmi & 0x1) == 0)) {
                 rdp->dynticks_fqs++;
                 return 1;
         }
@@ -897,12 +907,6 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
         unsigned long gp_duration;
 
         WARN_ON_ONCE(!rcu_gp_in_progress(rsp));
-
-        /*
-         * Ensure that all grace-period and pre-grace-period activity
-         * is seen before the assignment to rsp->completed.
-         */
-        smp_mb(); /* See above block comment. */
         gp_duration = jiffies - rsp->gp_start;
         if (gp_duration > rsp->gp_max)
                 rsp->gp_max = gp_duration;
@@ -1450,11 +1454,25 @@ __rcu_process_callbacks(struct rcu_state *rsp, struct rcu_data *rdp)
  */
 static void rcu_process_callbacks(void)
 {
+        /*
+         * Memory references from any prior RCU read-side critical sections
+         * executed by the interrupted code must be seen before any RCU
+         * grace-period manipulations below.
+         */
+        smp_mb(); /* See above block comment. */
+
         __rcu_process_callbacks(&rcu_sched_state,
                                 &__get_cpu_var(rcu_sched_data));
         __rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
         rcu_preempt_process_callbacks();
 
+        /*
+         * Memory references from any later RCU read-side critical sections
+         * executed by the interrupted code must be seen after any RCU
+         * grace-period manipulations above.
+         */
+        smp_mb(); /* See above block comment. */
+
         /* If we are last CPU on way to dyntick-idle mode, accelerate it. */
         rcu_needs_cpu_flush();
 }