PREEMPT/PREEMPT_RT question

Paul E. McKenney
2005-07-12T16:33:54Z

Hello!

OK, counter-flip RCU actually survives a pair of overnight runs on
CONFIG_PREEMPT running on 4-CPU machines, and also survives five
kernbenches and an LTP on another 4-CPU machine. (Overnight-run script
later in this message, FWIW.)

So, time to get serious about a bit of code cleanup:

o The heavyweight atomic operations in rcu_read_lock() and
rcu_read_unlock() are not needed in UP kernels, since
interrupts are disabled.

Is there already something like smp_atomic_inc() and
smp_atomic_dec() that generate atomic_inc()/atomic_dec() in
SMP kernels, but ++/-- in UP kernels? If not, any reasons
not to add them, for example, as follows?

#ifdef CONFIG_SMP
#define smp_atomic_inc(v) atomic_inc(v)
#define smp_atomic_dec(v) atomic_dec(v)
#else /* #ifdef CONFIG_SMP */
#define smp_atomic_inc(v) ((v)++)
#define smp_atomic_dec(v) ((v)++)
#endif /* #else #ifdef CONFIG_SMP */

Since interrupts must be disabled for these to be safe,
my guess is that I should define them locally in rcupdate.c.
If there turns out to be a general need for them, they can
be moved somewhere more public.

Objections?

o In order to get things to work in both CONFIG_PREEMPT and
CONFIG_PREEMPT_RT, I ended up using the following:

#ifdef CONFIG_PREEMPT_RT

#define rcu_spinlock_t _raw_spinlock_t
#define rcu_spin_lock(l, f) _raw_spin_lock(l)
#define rcu_spin_trylock(l, f) _raw_spin_trylock(l)
#define rcu_spin_unlock(l, f) _raw_spin_unlock(l)
#define RCU_SPIN_LOCK_UNLOCKED RAW_SPIN_LOCK_UNLOCKED

#else /* #ifdef CONFIG_PREEMPT_RT */

#define rcu_spinlock_t spinlock_t
#define rcu_spin_lock(l, f) spin_lock_irqsave(l, f)
#define rcu_spin_trylock(l, f) spin_trylock_irqsave(l, f)
#define rcu_spin_unlock(l, f) spin_unlock_irqrestore(l, f)
#define RCU_SPIN_LOCK_UNLOCKED SPIN_LOCK_UNLOCKED

#endif /* #else #ifdef CONFIG_PREEMPT_RT */

Then using rcu_spin_lock() &c everywhere. The problem is
that (as near as I can tell) the only way to prevent interrupts
from running on the current CPU in CONFIG_PREEMPT kernels is
to use the _irq spinlock primitives, but _raw_spin_lock() does
the job in CONFIG_PREEMPT_RT (since interrupts are run in process
context, right). I could use _irq in both, but that would
unnecessarily degrade interrupt latency in CONFIG_PREEMPT_RT.

Suggestions???

Some remaining shortcomings of the current code:

o Untested on CONFIG_PREEMPT_RT (working on this, suggestions
for 4-CPU-stable CONFIG_PREEMPT_RT versions most welcome).

o Uses atomic operations and memory barriers in the
rcu_read_lock() and rcu_read_unlock() fastpaths.
Getting rid of the atomic operations in UP is easy,
in SMP in the common case is reasonably straightforward,
getting rid of memory barriers is harder, but is likely
doable.

o Global callback management slows things down on SMP
(need to move to per-CPU callback management).

o Gross debug code needs pruning (working on this).

o Current implementation is probably too aggressive about
finishing out grace periods -- one of the machines
averaged one grace period every 29 milliseconds over a
15-hour time period. This likely understates the
aggressiveness, since the test is quite bursty.

o Given the small amount testing, there are undoubtably
more bugs.

All that aside, only a moderately severe degree of insanity is
required to run this code.

Improvements since last time:

o Boots and runs reasonably reliably.

o Got rid of the workqueue stuff, simplifying grace-period
detection code. Also allows synchronize_sched() to be
common code between "classic" and realtime RCU
implementations.

o Enabled preemption in RCU read-side critical sections
(even in CONFIG_PREEMPT kernels).

Test script (assumes that you have a linux-2.6.11.tar.bz2 file in
the current directory), relies on the fact that dcache's heavy use
of RCU:

#!/bin/sh

while :
do
i=0
while expr $i '<' 10
do
tar -xjf linux-2.6.11.tar.bz2
echo tar status $?
mv linux-2.6.11 argh$i
echo $i : `grep dentry_cache /proc/slabinfo`
i=`expr $i + 1`
done
for i in argh[0-9]
do
rm -rf $i &
done
wait
echo rm : `grep dentry_cache /proc/slabinfo`
done

Current (ugly) patch:

Not-signed-off-by: <paulmck@us.ibm.com>

diff -urpN -X dontdiff linux-2.6.12-rc6/arch/i386/Kconfig linux-2.6.12-rc6-ctrRCU/arch/i386/Kconfig
--- linux-2.6.12-rc6/arch/i386/Kconfig 2005-06-17 16:34:16.000000000 -0700
+++ linux-2.6.12-rc6-ctrRCU/arch/i386/Kconfig 2005-07-11 10:50:47.000000000 -0700
@@ -523,6 +523,31 @@ config PREEMPT
Say Y here if you are building a kernel for a desktop, embedded
or real-time system. Say N if you are unsure.

+config PREEMPT_RCU
+ bool "Preemptible RCU read-side critical sections"
+ depends on PREEMPT
+ default y
+ help
+ This option reduces the latency of the kernel when reacting to
+ real-time or interactive events by allowing a low priority process to
+ be preempted even if it is in an RCU read-side critical section.
+ This allows applications to run more reliably even when the system is
+ under load.
+
+ Say Y here if you enjoy debugging random oopses.
+ Say N if, for whatever reason, you want your kernel to actually work.
+
+config RCU_STATS
+ bool "/proc stats for preemptible RCU read-side critical sections"
+ depends on PREEMPT_RCU
+ default y
+ help
+ This option provides /proc stats to provide debugging info for
+ the preemptible realtime RCU implementation.
+
+ Say Y here if you want to see RCU stats in /proc
+ Say N if you are unsure.
+
config PREEMPT_BKL
bool "Preempt The Big Kernel Lock"
depends on PREEMPT
diff -urpN -X dontdiff linux-2.6.12-rc6/fs/proc/proc_misc.c linux-2.6.12-rc6-ctrRCU/fs/proc/proc_misc.c
--- linux-2.6.12-rc6/fs/proc/proc_misc.c 2005-06-17 16:35:03.000000000 -0700
+++ linux-2.6.12-rc6-ctrRCU/fs/proc/proc_misc.c 2005-07-11 11:46:04.000000000 -0700
@@ -549,6 +549,38 @@ void create_seq_entry(char *name, mode_t
entry->proc_fops = f;
}

+#ifdef CONFIG_RCU_STATS
+int rcu_read_proc(char *page, char **start, off_t off,
+ int count, int *eof, void *data)
+{
+ int len;
+ extern int rcu_read_proc_data(char *page);
+
+ len = rcu_read_proc_data(page);
+ return proc_calc_metrics(page, start, off, count, eof, len);
+}
+
+int rcu_read_proc_gp(char *page, char **start, off_t off,
+ int count, int *eof, void *data)
+{
+ int len;
+ extern int rcu_read_proc_gp_data(char *page);
+
+ len = rcu_read_proc_gp_data(page);
+ return proc_calc_metrics(page, start, off, count, eof, len);
+}
+
+int rcu_read_proc_ptrs(char *page, char **start, off_t off,
+ int count, int *eof, void *data)
+{
+ int len;
+ extern int rcu_read_proc_ptrs_data(char *page);
+
+ len = rcu_read_proc_ptrs_data(page);
+ return proc_calc_metrics(page, start, off, count, eof, len);
+}
+#endif /* #ifdef CONFIG_RCU_STATS */
+
void __init proc_misc_init(void)
{
struct proc_dir_entry *entry;
@@ -571,6 +603,11 @@ void __init proc_misc_init(void)
{"cmdline", cmdline_read_proc},
{"locks", locks_read_proc},
{"execdomains", execdomains_read_proc},
+#ifdef CONFIG_RCU_STATS
+ {"rcustats", rcu_read_proc},
+ {"rcugp", rcu_read_proc_gp},
+ {"rcuptrs", rcu_read_proc_ptrs},
+#endif /* #ifdef CONFIG_RCU_STATS */
{NULL,}
};
for (p = simple_ones; p->name; p++)
diff -urpN -X dontdiff linux-2.6.12-rc6/include/linux/rcupdate.h linux-2.6.12-rc6-ctrRCU/include/linux/rcupdate.h
--- linux-2.6.12-rc6/include/linux/rcupdate.h 2005-06-17 16:35:13.000000000 -0700
+++ linux-2.6.12-rc6-ctrRCU/include/linux/rcupdate.h 2005-07-01 14:55:35.000000000 -0700
@@ -59,6 +59,7 @@ struct rcu_head {
} while (0)

+#ifndef CONFIG_PREEMPT_RCU

/* Global control variables for rcupdate callback mechanism. */
struct rcu_ctrlblk {
@@ -192,6 +193,18 @@ static inline int rcu_pending(int cpu)
*/
#define rcu_read_unlock() preempt_enable()

+#else /* #ifndef CONFIG_PREEMPT_RCU */
+
+#define rcu_qsctr_inc(cpu)
+#define rcu_bh_qsctr_inc(cpu)
+#define call_rcu_bh(head, rcu) call_rcu(head, rcu)
+
+extern void rcu_read_lock(void);
+extern void rcu_read_unlock(void);
+extern int rcu_pending(int cpu);
+
+#endif /* #else #ifndef CONFIG_PREEMPT_RCU */
+
/*
* So where is rcu_write_lock()? It does not exist, as there is no
* way for writers to lock out RCU readers. This is a feature, not
@@ -213,14 +226,22 @@ static inline int rcu_pending(int cpu)
* can use just rcu_read_lock().
*
*/
+#ifndef CONFIG_PREEMPT_RCU
#define rcu_read_lock_bh() local_bh_disable()
+#else /* #ifndef CONFIG_PREEMPT_RCU */
+#define rcu_read_lock_bh() { rcu_read_lock(); local_bh_disable(); }
+#endif /* #else #ifndef CONFIG_PREEMPT_RCU */

/*
* rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section
*
* See rcu_read_lock_bh() for more information.
*/
+#ifndef CONFIG_PREEMPT_RCU
#define rcu_read_unlock_bh() local_bh_enable()
+#else /* #ifndef CONFIG_PREEMPT_RCU */
+#define rcu_read_unlock_bh() { local_bh_enable(); rcu_read_unlock(); }
+#endif /* #else #ifndef CONFIG_PREEMPT_RCU */

/**
* rcu_dereference - fetch an RCU-protected pointer in an
@@ -269,7 +290,11 @@ static inline int rcu_pending(int cpu)
* synchronize_kernel() API. In contrast, synchronize_rcu() only
* guarantees that rcu_read_lock() sections will have completed.
*/
+#ifndef CONFIG_PREEMPT_RCU
#define synchronize_sched() synchronize_rcu()
+#else /* #ifndef CONFIG_PREEMPT_RCU */
+extern void synchronize_sched(void);
+#endif /* #else #ifndef CONFIG_PREEMPT_RCU */

extern void rcu_init(void);
extern void rcu_check_callbacks(int cpu, int user);
diff -urpN -X dontdiff linux-2.6.12-rc6/include/linux/sched.h linux-2.6.12-rc6-ctrRCU/include/linux/sched.h
--- linux-2.6.12-rc6/include/linux/sched.h 2005-06-17 16:35:13.000000000 -0700
+++ linux-2.6.12-rc6-ctrRCU/include/linux/sched.h 2005-06-22 17:25:49.000000000 -0700
@@ -740,6 +740,11 @@ struct task_struct {
nodemask_t mems_allowed;
int cpuset_mems_generation;
#endif
+#ifdef CONFIG_PREEMPT_RCU
+ int rcu_read_lock_nesting;
+ atomic_t *rcu_flipctr1;
+ atomic_t *rcu_flipctr2;
+#endif
};

static inline pid_t process_group(struct task_struct *tsk)
diff -urpN -X dontdiff linux-2.6.12-rc6/kernel/rcupdate.c linux-2.6.12-rc6-ctrRCU/kernel/rcupdate.c
--- linux-2.6.12-rc6/kernel/rcupdate.c 2005-06-17 16:35:15.000000000 -0700
+++ linux-2.6.12-rc6-ctrRCU/kernel/rcupdate.c 2005-07-11 16:27:10.000000000 -0700
@@ -47,6 +47,46 @@
#include <linux/rcupdate.h>
#include <linux/cpu.h>

+struct rcu_synchronize {
+ struct rcu_head head;
+ struct completion completion;
+};
+
+/* Because of FASTCALL declaration of complete, we use this wrapper */
+static void wakeme_after_rcu(struct rcu_head *head)
+{
+ struct rcu_synchronize *rcu;
+
+ rcu = container_of(head, struct rcu_synchronize, head);
+ complete(&rcu->completion);
+}
+
+/**
+ * synchronize_rcu - wait until a grace period has elapsed.
+ *
+ * Control will return to the caller some time after a full grace
+ * period has elapsed, in other words after all currently executing RCU
+ * read-side critical sections have completed. RCU read-side critical
+ * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
+ * and may be nested.
+ *
+ * If your read-side code is not protected by rcu_read_lock(), do -not-
+ * use synchronize_rcu().
+ */
+void synchronize_rcu(void)
+{
+ struct rcu_synchronize rcu;
+
+ init_completion(&rcu.completion);
+ /* Will wake me after RCU finished */
+ call_rcu(&rcu.head, wakeme_after_rcu);
+
+ /* Wait for it */
+ wait_for_completion(&rcu.completion);
+}
+
+#ifndef CONFIG_PREEMPT_RCU
+
/* Definition for rcupdate control block. */
struct rcu_ctrlblk rcu_ctrlblk =
{ .cur = -300, .completed = -300 };
@@ -429,42 +469,392 @@ void __init rcu_init(void)
register_cpu_notifier(&rcu_nb);
}

-struct rcu_synchronize {
- struct rcu_head head;
- struct completion completion;
+/*
+ * Deprecated, use synchronize_rcu() or synchronize_sched() instead.
+ */
+void synchronize_kernel(void)
+{
+ synchronize_rcu();
+}
+
+module_param(maxbatch, int, 0);
+EXPORT_SYMBOL(call_rcu); /* WARNING: GPL-only in April 2006. */
+EXPORT_SYMBOL(call_rcu_bh); /* WARNING: GPL-only in April 2006. */
+EXPORT_SYMBOL_GPL(synchronize_rcu);
+EXPORT_SYMBOL(synchronize_kernel); /* WARNING: GPL-only in April 2006. */
+
+#else /* #ifndef CONFIG_PREEMPT_RCU */
+
+#ifdef CONFIG_PREEMPT_RT
+
+#error "Hey!!! This fails miserably even in straightforward testing!!!"
+
+#define rcu_spinlock_t _raw_spinlock_t
+#define rcu_spin_lock(l, f) _raw_spin_lock(l)
+#define rcu_spin_trylock(l, f) _raw_spin_trylock(l)
+#define rcu_spin_unlock(l, f) _raw_spin_unlock(l)
+#define RCU_SPIN_LOCK_UNLOCKED RAW_SPIN_LOCK_UNLOCKED
+
+#else /* #ifdef CONFIG_PREEMPT_RT */
+
+#define rcu_spinlock_t spinlock_t
+#define rcu_spin_lock(l, f) spin_lock_irqsave(l, f)
+#define rcu_spin_trylock(l, f) spin_trylock_irqsave(l, f)
+#define rcu_spin_unlock(l, f) spin_unlock_irqrestore(l, f)
+#define RCU_SPIN_LOCK_UNLOCKED SPIN_LOCK_UNLOCKED
+
+#endif /* #else #ifdef CONFIG_PREEMPT_RT */
+
+struct rcu_data {
+ rcu_spinlock_t lock;
+ long batch;
+ struct tasklet_struct rcu_tasklet;
+ struct rcu_head *nextlist;
+ struct rcu_head **nexttail;
+ struct rcu_head *waitlist;
+ struct rcu_head **waittail;
+ struct rcu_head *donelist;
+ struct rcu_head **donetail;
+ long n_next_length;
+#ifdef CONFIG_RCU_STATS
+ long n_next_add;
+ long n_wait_length;
+ long n_wait_add;
+ long n_done_length;
+ long n_done_add;
+ long n_done_remove;
+ atomic_t n_done_invoked;
+ long n_rcu_process_callbacks;
+ long n_rcu_check_callbacks;
+ long n_rcu_advance_callbacks1;
+ long n_rcu_advance_callbacks2;
+ long n_rcu_advance_callbacks3;
+ long n_rcu_advance_callbacks4;
+ long n_rcu_advance_callbacks5;
+ atomic_t n_synchronize_rcu;
+ atomic_t n_synchronize_rcu_flip1; /*&&&&*/
+ atomic_t n_synchronize_rcu_flip2; /*&&&&*/
+ atomic_t n_synchronize_rcu_flip3; /*&&&&*/
+ atomic_t n_synchronize_rcu_flip4; /*&&&&*/
+ long n_synchronize_rcu_flip_early1;
+#endif /* #ifdef CONFIG_RCU_STATS */
+};
+struct rcu_ctrlblk {
+ spinlock_t fliplock;
+ long batch;
};
+static struct rcu_data rcu_data;
+static struct rcu_ctrlblk rcu_ctrlblk = {
+ .fliplock = RCU_SPIN_LOCK_UNLOCKED,
+ .batch = 0,
+};
+static DEFINE_PER_CPU(atomic_t [2], rcu_flipctr) =
+ { ATOMIC_INIT(0), ATOMIC_INIT(0) };

-/* Because of FASTCALL declaration of complete, we use this wrapper */
-static void wakeme_after_rcu(struct rcu_head *head)
+void
+rcu_read_lock(void)
{
- struct rcu_synchronize *rcu;
+ int flipctr;
+ unsigned long oldirq;

- rcu = container_of(head, struct rcu_synchronize, head);
- complete(&rcu->completion);
+ local_irq_save(oldirq); /* @@@ inside "if" for PREEMPT_RT... */
+ if (current->rcu_read_lock_nesting++ == 0) {
+
+ /*
+ * Outermost nesting of rcu_read_lock(), so atomically
+ * increment the current counter for the current CPU.
+ */
+
+ flipctr = rcu_ctrlblk.batch & 0x1;
+ smp_read_barrier_depends();
+ current->rcu_flipctr1 = &(__get_cpu_var(rcu_flipctr)[flipctr]);
+ /* Can optimize to non-atomic on fastpath, but start simple. */
+ atomic_inc(current->rcu_flipctr1);
+ smp_mb__after_atomic_inc(); /* might optimize out... */
+ if (unlikely(flipctr != (rcu_ctrlblk.batch & 0x1))) {
+
+ /*
+ * We raced with grace-period processing (flip).
+ * Although we cannot be preempted here, there
+ * could be interrupts, ECC errors and the like,
+ * so just nail down both sides of the rcu_flipctr
+ * array for the duration of our RCU read-side
+ * critical section, preventing a second flip
+ * from racing with us. At some point, it would
+ * be safe to decrement one of the counters, but
+ * we have no way of knowing when that would be.
+ * So just decrement them both in rcu_read_unlock().
+ */
+
+ current->rcu_flipctr2 =
+ &(__get_cpu_var(rcu_flipctr)[!flipctr]);
+ /* Can again optimize to non-atomic on fastpath. */
+ atomic_inc(current->rcu_flipctr2);
+ smp_mb__after_atomic_inc(); /* might optimize out... */
+ }
+ }
+#ifndef CONFIG_PREEMPT_RT
+ /* preempt_disable(); */
+#endif /* #ifndef CONFIG_PREEMPT_RT */
+ local_irq_restore(oldirq); /*@@@ should be able to precede. */
+}
+
+void
+rcu_read_unlock(void)
+{
+#ifndef CONFIG_PREEMPT_RT
+ unsigned long oldirq;
+
+ local_irq_save(oldirq); /* @@@ should be able to reverse... */
+ /* preempt_enable(); */
+#endif /* #ifndef CONFIG_PREEMPT_RT */
+ if (--current->rcu_read_lock_nesting == 0) {
+
+ /*
+ * Just atomically decrement whatever we incremented.
+ * Might later want to awaken some task waiting for the
+ * grace period to complete, but keep it simple for the
+ * moment.
+ */
+
+ smp_mb__before_atomic_dec();
+ atomic_dec(current->rcu_flipctr1);
+ current->rcu_flipctr1 = NULL;
+ if (unlikely(current->rcu_flipctr2 != NULL)) {
+ atomic_dec(current->rcu_flipctr2);
+ current->rcu_flipctr2 = NULL;
+ }
+ }
+#ifndef CONFIG_PREEMPT_RT
+ local_irq_restore(oldirq);
+#endif /* #ifndef CONFIG_PREEMPT_RT */
+}
+
+static void
+__rcu_advance_callbacks(void)
+{
+
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_rcu_advance_callbacks1++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ if (rcu_data.batch != rcu_ctrlblk.batch) {
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_rcu_advance_callbacks2++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ if (rcu_data.waitlist != NULL) {
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_rcu_advance_callbacks3++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ *rcu_data.donetail = rcu_data.waitlist;
+ rcu_data.donetail = rcu_data.waittail;
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_done_length += rcu_data.n_wait_length;
+ rcu_data.n_done_add += rcu_data.n_wait_length;
+ rcu_data.n_wait_length = 0;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ }
+ if (rcu_data.nextlist != NULL) {
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_rcu_advance_callbacks4++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ rcu_data.waitlist = rcu_data.nextlist;
+ rcu_data.waittail = rcu_data.nexttail;
+ rcu_data.nextlist = NULL;
+ rcu_data.nexttail = &rcu_data.nextlist;
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_wait_length += rcu_data.n_next_length;
+ rcu_data.n_wait_add += rcu_data.n_next_length;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ rcu_data.n_next_length = 0;
+ } else {
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_rcu_advance_callbacks5++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ rcu_data.waitlist = NULL;
+ rcu_data.waittail = &rcu_data.waitlist;
+ }
+ rcu_data.batch = rcu_ctrlblk.batch;
+ }
}

-/**
- * synchronize_rcu - wait until a grace period has elapsed.
- *
- * Control will return to the caller some time after a full grace
- * period has elapsed, in other words after all currently executing RCU
- * read-side critical sections have completed. RCU read-side critical
- * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
- * and may be nested.
+/*
+ * Attempt a single flip of the counters. Remember, a single flip does
+ * -not- constitute a grace period. Instead, the interval between
+ * a pair of consecutive flips is a grace period.
*
- * If your read-side code is not protected by rcu_read_lock(), do -not-
- * use synchronize_rcu().
+ * If anyone is nuts enough to run this CONFIG_PREEMPT_RCU implementation
+ * on a large SMP, they might want to use a hierarchical organization of
+ * the per-CPU-counter pairs.
+ */
+static void
+rcu_try_flip(void)
+{
+ int cpu;
+ long flipctr;
+
+#ifdef CONFIG_RCU_STATS
+ atomic_inc(&rcu_data.n_synchronize_rcu_flip1);
+#endif /* #ifdef CONFIG_RCU_STATS */
+ flipctr = rcu_data.batch;
+ if (unlikely(!spin_trylock(&rcu_ctrlblk.fliplock)))
+ return;
+#if 0
+ spin_lock(&rcu_ctrlblk.fliplock);
+#endif
+ if (unlikely(flipctr != rcu_data.batch)) {
+
+ /* Our work is done! ;-) */
+
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_synchronize_rcu_flip_early1++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ spin_unlock(&rcu_ctrlblk.fliplock);
+ return;
+ }
+ flipctr &= 0x1;
+
+ /*
+ * Check for completion of all RCU read-side critical sections
+ * that started prior to the previous flip.
+ */
+
+#ifdef CONFIG_RCU_STATS
+ atomic_inc(&rcu_data.n_synchronize_rcu_flip2);
+#endif /* #ifdef CONFIG_RCU_STATS */
+ for_each_cpu(cpu) {
+ if (atomic_read(&per_cpu(rcu_flipctr, cpu)[!flipctr]) != 0) {
+ spin_unlock(&rcu_ctrlblk.fliplock);
+ return;
+ }
+ }
+
+ /* Do the flip. */
+
+#ifdef CONFIG_RCU_STATS
+ atomic_inc(&rcu_data.n_synchronize_rcu_flip3);
+#endif /* #ifdef CONFIG_RCU_STATS */
+ smp_mb();
+ rcu_ctrlblk.batch++;
+
+#ifdef CONFIG_RCU_STATS
+ atomic_inc(&rcu_data.n_synchronize_rcu_flip4);
+#endif /* #ifdef CONFIG_RCU_STATS */
+
+ spin_unlock(&rcu_ctrlblk.fliplock);
+}
+
+void
+rcu_check_callbacks(int cpu, int user)
+{
+ unsigned long flags;
+
+ rcu_try_flip();
+ rcu_spin_lock(&rcu_data.lock, flags);
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_rcu_check_callbacks++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ __rcu_advance_callbacks();
+ if (rcu_data.donelist == NULL) {
+ rcu_spin_unlock(&rcu_data.lock, flags);
+ } else {
+ rcu_spin_unlock(&rcu_data.lock, flags);
+ tasklet_schedule(&rcu_data.rcu_tasklet);
+ }
+}
+
+static
+void rcu_process_callbacks(unsigned long data)
+{
+ unsigned long flags;
+ struct rcu_head *next, *list;
+
+ rcu_spin_lock(&rcu_data.lock, flags);
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_rcu_process_callbacks++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ list = rcu_data.donelist;
+ if (list == NULL) {
+ rcu_spin_unlock(&rcu_data.lock, flags);
+ return;
+ }
+ rcu_data.donelist = NULL;
+ rcu_data.donetail = &rcu_data.donelist;
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_done_remove += rcu_data.n_done_length;
+ rcu_data.n_done_length = 0;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ rcu_spin_unlock(&rcu_data.lock, flags);
+ while (list) {
+ next = list->next;
+ list->func(list);
+ list = next;
+#ifdef CONFIG_RCU_STATS
+ atomic_inc(&rcu_data.n_done_invoked);
+#endif /* #ifdef CONFIG_RCU_STATS */
+ }
+}
+
+void fastcall
+call_rcu(struct rcu_head *head,
+ void (*func)(struct rcu_head *rcu))
+{
+ unsigned long flags;
+
+ head->func = func;
+ head->next = NULL;
+ rcu_spin_lock(&rcu_data.lock, flags);
+ __rcu_advance_callbacks();
+ *rcu_data.nexttail = head;
+ rcu_data.nexttail = &head->next;
+#ifdef CONFIG_RCU_STATS
+ rcu_data.n_next_add++;
+#endif /* #ifdef CONFIG_RCU_STATS */
+ rcu_data.n_next_length++;
+ rcu_spin_unlock(&rcu_data.lock, flags);
+}
+
+/*
+ * Crude hack, reduces but does not eliminate possibility of failure.
+ * Needs to wait for all CPUs to pass through a -voluntary- context
+ * switch to eliminate possibility of failure. (Maybe just crank
+ * priority down...)
*/
-void synchronize_rcu(void)
+void
+synchronize_sched(void)
{
- struct rcu_synchronize rcu;
+ cpumask_t oldmask;
+ int cpu;

- init_completion(&rcu.completion);
- /* Will wake me after RCU finished */
- call_rcu(&rcu.head, wakeme_after_rcu);
+ if (sched_getaffinity(0, &oldmask) < 0) {
+ oldmask = cpu_possible_map;
+ }
+ for_each_cpu(cpu) {
+ sched_setaffinity(0, cpumask_of_cpu(cpu));
+ schedule();
+ }
+ sched_setaffinity(0, oldmask);
+}

- /* Wait for it */
- wait_for_completion(&rcu.completion);
+int
+rcu_pending(int cpu)
+{
+ return (rcu_data.donelist != NULL ||
+ rcu_data.waitlist != NULL ||
+ rcu_data.nextlist != NULL);
+}
+
+void __init rcu_init(void)
+{
+/*&&&&*/printk("WARNING: experimental RCU implementation.\n");
+ rcu_data.lock = RCU_SPIN_LOCK_UNLOCKED;
+ rcu_data.batch = 0;
+ rcu_data.nextlist = NULL;
+ rcu_data.nexttail = &rcu_data.nextlist;
+ rcu_data.waitlist = NULL;
+ rcu_data.waittail = &rcu_data.waitlist;
+ rcu_data.donelist = NULL;
+ rcu_data.donetail = &rcu_data.donelist;
+ tasklet_init(&rcu_data.rcu_tasklet, rcu_process_callbacks, 0UL);
}

/*
@@ -472,11 +862,71 @@ void synchronize_rcu(void)
*/
void synchronize_kernel(void)
{
+ synchronize_sched();
+}
+
+#ifdef CONFIG_RCU_STATS
+int rcu_read_proc_data(char *page)
+{
+ return sprintf(page,
+ "ggp=%ld lgp=%ld\n"
+ "sr=%d srf1=%d srf2=%d srf3=%d srf4=%d\n"
+ "ac1=%ld ac2=%ld ac3=%ld ac4=%ld ac5=%ld\n"
+ "sre1=%ld\n"
+ "na=%ld nl=%ld wa=%ld wl=%ld rcc=%ld rpc=%ld dl=%ld dr=%ld di=%d\n",
+ rcu_ctrlblk.batch,
+ rcu_data.batch,
+
+ atomic_read(&rcu_data.n_synchronize_rcu),
+ atomic_read(&rcu_data.n_synchronize_rcu_flip1),
+ atomic_read(&rcu_data.n_synchronize_rcu_flip2),
+ atomic_read(&rcu_data.n_synchronize_rcu_flip3),
+ atomic_read(&rcu_data.n_synchronize_rcu_flip4),
+
+ rcu_data.n_rcu_advance_callbacks1,
+ rcu_data.n_rcu_advance_callbacks2,
+ rcu_data.n_rcu_advance_callbacks3,
+ rcu_data.n_rcu_advance_callbacks4,
+ rcu_data.n_rcu_advance_callbacks5,
+
+ rcu_data.n_synchronize_rcu_flip_early1,
+
+ rcu_data.n_next_add,
+ rcu_data.n_next_length,
+ rcu_data.n_wait_add,
+ rcu_data.n_wait_length,
+ rcu_data.n_rcu_check_callbacks,
+ rcu_data.n_rcu_process_callbacks,
+ rcu_data.n_done_length,
+ rcu_data.n_done_remove,
+ atomic_read(&rcu_data.n_done_invoked));
+}
+
+int rcu_read_proc_gp_data(char *page)
+{
+ long oldgp = rcu_ctrlblk.batch;
+
synchronize_rcu();
+ return sprintf(page, "oldggp=%ld newggp=%ld\n",
+ oldgp, rcu_ctrlblk.batch);
}

-module_param(maxbatch, int, 0);
-EXPORT_SYMBOL(call_rcu); /* WARNING: GPL-only in April 2006. */
-EXPORT_SYMBOL(call_rcu_bh); /* WARNING: GPL-only in April 2006. */
+int rcu_read_proc_ptrs_data(char *page)
+{
+ return sprintf(page,
+ "nl=%p/%p nt=%p wl=%p/%p wt=%p dl=%p/%p dt=%p\n",
+ &rcu_data.nextlist, rcu_data.nextlist, rcu_data.nexttail,
+ &rcu_data.waitlist, rcu_data.waitlist, rcu_data.waittail,
+ &rcu_data.donelist, rcu_data.donelist, rcu_data.donetail
+ );
+}
+#endif /* #ifdef CONFIG_RCU_STATS */
+
+EXPORT_SYMBOL(call_rcu); /* WARNING: GPL-only in April 2006. */
EXPORT_SYMBOL_GPL(synchronize_rcu);
-EXPORT_SYMBOL(synchronize_kernel); /* WARNING: GPL-only in April 2006. */
+EXPORT_SYMBOL_GPL(synchronize_sched);
+EXPORT_SYMBOL(rcu_read_lock); /* WARNING: GPL-only in April 2006. */
+EXPORT_SYMBOL(rcu_read_unlock); /* WARNING: GPL-only in April 2006. */
+EXPORT_SYMBOL(synchronize_kernel); /* WARNING: Removal in April 2006. */
+
+#endif /* #else #ifndef CONFIG_PREEMPT_RCU */