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>

diff --git a/Documentation/RCU/trace.txt b/Documentation/RCU/trace.txt
index 8173cec473aa0099a185d694bad1b1b6ee48743d..c078ad48f7a187aa8d76e9885d1776a46a4255e7 100644 (file)
--- a/Documentation/RCU/trace.txt
+++ b/Documentation/RCU/trace.txt
@@ -99,11 +99,18 @@ o   "qp" indicates that RCU still expects a quiescent state from
 
 o      "dt" is the current value of the dyntick counter that is incremented
        when entering or leaving dynticks idle state, either by the
-       scheduler or by irq.  This number is even if the CPU is in
-       dyntick idle mode and odd otherwise.  The number after the first
-       "/" is the interrupt nesting depth when in dyntick-idle state,
-       or one greater than the interrupt-nesting depth otherwise.
-       The number after the second "/" is the NMI nesting depth.
+       scheduler or by irq.  The number after the "/" is the interrupt
+       nesting depth when in dyntick-idle state, or one greater than
+       the interrupt-nesting depth otherwise.
+
+       This field is displayed only for CONFIG_NO_HZ kernels.
+
+o      "dn" is the current value of the dyntick counter that is incremented
+       when entering or leaving dynticks idle state via NMI.  If both
+       the "dt" and "dn" values are even, then this CPU is in dynticks
+       idle mode and may be ignored by RCU.  If either of these two
+       counters is odd, then RCU must be alert to the possibility of
+       an RCU read-side critical section running on this CPU.
 
        This field is displayed only for CONFIG_NO_HZ kernels.
 

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 5616b17e4a225d5c61a1f847ee0405149446e0ab..e486f7c3ffb8b239a31eddb37f9ed0251629bdeb 100644 (file)
--- 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();
 }

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 93d4a1c2e88b2a46c9f0daae78063c953cc49fa6..257664815d5d9cdfd645d08f6bd2e873e10a2cff 100644 (file)
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -84,9 +84,11 @@
  * Dynticks per-CPU state.
  */
 struct rcu_dynticks {
-       int dynticks_nesting;   /* Track irq/process nesting level. */
-       int dynticks_nmi_nesting; /* Track NMI nesting level. */
-       atomic_t dynticks;      /* Even value for dynticks-idle, else odd. */
+       int dynticks_nesting;   /* Track nesting level, sort of. */
+       int dynticks;           /* Even value for dynticks-idle, else odd. */
+       int dynticks_nmi;       /* Even value for either dynticks-idle or */
+                               /*  not in nmi handler, else odd.  So this */
+                               /*  remains even for nmi from irq handler. */
 };
 
 /* RCU's kthread states for tracing. */
@@ -282,6 +284,7 @@ struct rcu_data {
        /* 3) dynticks interface. */
        struct rcu_dynticks *dynticks;  /* Shared per-CPU dynticks state. */
        int dynticks_snap;              /* Per-GP tracking for dynticks. */
+       int dynticks_nmi_snap;          /* Per-GP tracking for dynticks_nmi. */
 #endif /* #ifdef CONFIG_NO_HZ */
 
        /* 4) reasons this CPU needed to be kicked by force_quiescent_state */

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index ed339702481dfc3a7746a10a38cc3309fa82e238..3f6559a5f5cd7911fac44ac500b670e125e73157 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1520,6 +1520,7 @@ int rcu_needs_cpu(int cpu)
 {
        int c = 0;
        int snap;
+       int snap_nmi;
        int thatcpu;
 
        /* Check for being in the holdoff period. */
@@ -1530,10 +1531,10 @@ int rcu_needs_cpu(int cpu)
        for_each_online_cpu(thatcpu) {
                if (thatcpu == cpu)
                        continue;
-               snap = atomic_add_return(0, &per_cpu(rcu_dynticks,
-                                                    thatcpu).dynticks);
+               snap = per_cpu(rcu_dynticks, thatcpu).dynticks;
+               snap_nmi = per_cpu(rcu_dynticks, thatcpu).dynticks_nmi;
                smp_mb(); /* Order sampling of snap with end of grace period. */
-               if ((snap & 0x1) != 0) {
+               if (((snap & 0x1) != 0) || ((snap_nmi & 0x1) != 0)) {
                        per_cpu(rcu_dyntick_drain, cpu) = 0;
                        per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
                        return rcu_needs_cpu_quick_check(cpu);

diff --git a/kernel/rcutree_trace.c b/kernel/rcutree_trace.c
index 9678cc3650f5e9c8f4e3eb409d26e9c4e784e7d5..aa0fd72b4bc714ae3b8f4c41f48f712513e8d6f3 100644 (file)
--- a/kernel/rcutree_trace.c
+++ b/kernel/rcutree_trace.c
@@ -69,10 +69,10 @@ static void print_one_rcu_data(struct seq_file *m, struct rcu_data *rdp)
                   rdp->passed_quiesc, rdp->passed_quiesc_completed,
                   rdp->qs_pending);
 #ifdef CONFIG_NO_HZ
-       seq_printf(m, " dt=%d/%d/%d df=%lu",
-                  atomic_read(&rdp->dynticks->dynticks),
+       seq_printf(m, " dt=%d/%d dn=%d df=%lu",
+                  rdp->dynticks->dynticks,
                   rdp->dynticks->dynticks_nesting,
-                  rdp->dynticks->dynticks_nmi_nesting,
+                  rdp->dynticks->dynticks_nmi,
                   rdp->dynticks_fqs);
 #endif /* #ifdef CONFIG_NO_HZ */
        seq_printf(m, " of=%lu ri=%lu", rdp->offline_fqs, rdp->resched_ipi);
@@ -141,9 +141,9 @@ static void print_one_rcu_data_csv(struct seq_file *m, struct rcu_data *rdp)
                   rdp->qs_pending);
 #ifdef CONFIG_NO_HZ
        seq_printf(m, ",%d,%d,%d,%lu",
-                  atomic_read(&rdp->dynticks->dynticks),
+                  rdp->dynticks->dynticks,
                   rdp->dynticks->dynticks_nesting,
-                  rdp->dynticks->dynticks_nmi_nesting,
+                  rdp->dynticks->dynticks_nmi,
                   rdp->dynticks_fqs);
 #endif /* #ifdef CONFIG_NO_HZ */
        seq_printf(m, ",%lu,%lu", rdp->offline_fqs, rdp->resched_ipi);
@@ -167,7 +167,7 @@ static int show_rcudata_csv(struct seq_file *m, void *unused)
 {
        seq_puts(m, "\"CPU\",\"Online?\",\"c\",\"g\",\"pq\",\"pqc\",\"pq\",");
 #ifdef CONFIG_NO_HZ
-       seq_puts(m, "\"dt\",\"dt nesting\",\"dt NMI nesting\",\"df\",");
+       seq_puts(m, "\"dt\",\"dt nesting\",\"dn\",\"df\",");
 #endif /* #ifdef CONFIG_NO_HZ */
        seq_puts(m, "\"of\",\"ri\",\"ql\",\"b\",\"ci\",\"co\",\"ca\"\n");
 #ifdef CONFIG_TREE_PREEMPT_RCU