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SubjectRe: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
Paul:

Here's my version of your patch from yesterday. It's basically the same,
but I cleaned up the code in a few places and fixed a bug (the sign of idx
in srcu_read_unlock). Also I changed the init routine back to void, since
it's no longer an error if the per-cpu allocation fails.

More importantly, I added a static initializer and included the fast-path
in synchronize_srcu. It's protected by the new symbol
SMP__STORE_MB_LOAD_WORKS, which should be defined in arch-specific headers
for those architectures where the store-mb-load pattern is safe.

Alan

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>

---
Index: usb-2.6/include/linux/srcu.h
===================================================================
--- usb-2.6.orig/include/linux/srcu.h
+++ usb-2.6/include/linux/srcu.h
@@ -35,19 +35,19 @@ struct srcu_struct {
int completed;
struct srcu_struct_array *per_cpu_ref;
struct mutex mutex;
+ atomic_t hardluckref[2];
};

-#ifndef CONFIG_PREEMPT
-#define srcu_barrier() barrier()
-#else /* #ifndef CONFIG_PREEMPT */
-#define srcu_barrier()
-#endif /* #else #ifndef CONFIG_PREEMPT */
+#define SRCU_INITIALIZER(srcu_name) { \
+ .mutex = __MUTEX_INITIALIZER(srcu_name.mutex), \
+ .hardluckref = {ATOMIC_INIT(0), ATOMIC_INIT(0)} }

-int init_srcu_struct(struct srcu_struct *sp);
-void cleanup_srcu_struct(struct srcu_struct *sp);
-int srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
-void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
-void synchronize_srcu(struct srcu_struct *sp);
-long srcu_batches_completed(struct srcu_struct *sp);
+extern void init_srcu_struct(struct srcu_struct *sp);
+extern void cleanup_srcu_struct(struct srcu_struct *sp);
+extern int srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
+extern void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
+extern void synchronize_srcu(struct srcu_struct *sp);
+extern long srcu_batches_completed(struct srcu_struct *sp);
+extern int srcu_readers_active(struct srcu_struct *sp);

#endif
Index: usb-2.6/kernel/srcu.c
===================================================================
--- usb-2.6.orig/kernel/srcu.c
+++ usb-2.6/kernel/srcu.c
@@ -34,6 +34,18 @@
#include <linux/smp.h>
#include <linux/srcu.h>

+/*
+ * Initialize the per-CPU array, returning the pointer.
+ */
+static inline struct srcu_struct_array *alloc_srcu_struct_percpu(void)
+{
+ struct srcu_struct_array *sap;
+
+ sap = alloc_percpu(struct srcu_struct_array);
+ smp_wmb();
+ return sap;
+}
+
/**
* init_srcu_struct - initialize a sleep-RCU structure
* @sp: structure to initialize.
@@ -42,12 +54,13 @@
* to any other function. Each srcu_struct represents a separate domain
* of SRCU protection.
*/
-int init_srcu_struct(struct srcu_struct *sp)
+void init_srcu_struct(struct srcu_struct *sp)
{
sp->completed = 0;
mutex_init(&sp->mutex);
- sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
- return (sp->per_cpu_ref ? 0 : -ENOMEM);
+ sp->per_cpu_ref = alloc_srcu_struct_percpu();
+ atomic_set(&sp->hardluckref[0], 0);
+ atomic_set(&sp->hardluckref[1], 0);
}

/*
@@ -58,11 +71,14 @@ int init_srcu_struct(struct srcu_struct
static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
{
int cpu;
+ struct srcu_struct_array *sap;
int sum;

- sum = 0;
- for_each_possible_cpu(cpu)
- sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
+ sum = atomic_read(&sp->hardluckref[idx]);
+ sap = rcu_dereference(sp->per_cpu_ref);
+ if (likely(sap != NULL))
+ for_each_possible_cpu(cpu)
+ sum += per_cpu_ptr(sap, cpu)->c[idx];
return sum;
}

@@ -94,7 +110,8 @@ void cleanup_srcu_struct(struct srcu_str
WARN_ON(sum); /* Leakage unless caller handles error. */
if (sum != 0)
return;
- free_percpu(sp->per_cpu_ref);
+ if (sp->per_cpu_ref != NULL)
+ free_percpu(sp->per_cpu_ref);
sp->per_cpu_ref = NULL;
}

@@ -102,19 +119,37 @@ void cleanup_srcu_struct(struct srcu_str
* srcu_read_lock - register a new reader for an SRCU-protected structure.
* @sp: srcu_struct in which to register the new reader.
*
- * Counts the new reader in the appropriate per-CPU element of the
- * srcu_struct. Must be called from process context.
+ * Counts the new reader in the appropriate per-CPU element of @sp.
+ * Must be called from process context.
+ *
* Returns an index that must be passed to the matching srcu_read_unlock().
+ * The index is mapped to negative numbers if the per-cpu array in @sp
+ * is not and cannot be allocated.
*/
int srcu_read_lock(struct srcu_struct *sp)
{
int idx;
+ struct srcu_struct_array *sap;
+
+ if (unlikely(sp->per_cpu_ref == NULL &&
+ mutex_trylock(&sp->mutex))) {
+ if (sp->per_cpu_ref == NULL)
+ sp->per_cpu_ref = alloc_srcu_struct_percpu();
+ mutex_unlock(&sp->mutex);
+ }

preempt_disable();
idx = sp->completed & 0x1;
barrier(); /* ensure compiler looks -once- at sp->completed. */
- per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
- srcu_barrier(); /* ensure compiler won't misorder critical section. */
+ sap = rcu_dereference(sp->per_cpu_ref);
+ if (likely(sap != NULL)) {
+ per_cpu_ptr(sap, smp_processor_id())->c[idx]++;
+ smp_mb();
+ } else {
+ atomic_inc(&sp->hardluckref[idx]);
+ smp_mb__after_atomic_inc();
+ idx = -1 - idx;
+ }
preempt_enable();
return idx;
}
@@ -131,10 +166,16 @@ int srcu_read_lock(struct srcu_struct *s
*/
void srcu_read_unlock(struct srcu_struct *sp, int idx)
{
- preempt_disable();
- srcu_barrier(); /* ensure compiler won't misorder critical section. */
- per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
- preempt_enable();
+ if (likely(idx >= 0)) {
+ smp_mb();
+ preempt_disable();
+ per_cpu_ptr(rcu_dereference(sp->per_cpu_ref),
+ smp_processor_id())->c[idx]--;
+ preempt_enable();
+ } else {
+ smp_mb__before_atomic_dec();
+ atomic_dec(&sp->hardluckref[-1 - idx]);
+ }
}

/**
@@ -158,6 +199,11 @@ void synchronize_srcu(struct srcu_struct
idx = sp->completed;
mutex_lock(&sp->mutex);

+ /* Initialize if not already initialized. */
+
+ if (sp->per_cpu_ref == NULL)
+ sp->per_cpu_ref = alloc_srcu_struct_percpu();
+
/*
* Check to see if someone else did the work for us while we were
* waiting to acquire the lock. We need -two- advances of
@@ -168,71 +214,35 @@ void synchronize_srcu(struct srcu_struct
* either (1) wait for two or (2) supply the second ourselves.
*/

- if ((sp->completed - idx) >= 2) {
- mutex_unlock(&sp->mutex);
- return;
- }
+ if ((sp->completed - idx) >= 2)
+ goto done;

- synchronize_sched(); /* Force memory barrier on all CPUs. */
+ smp_mb(); /* ensure srcu_read_lock() sees prior change first! */
+ idx = sp->completed & 0x1;

- /*
- * The preceding synchronize_sched() ensures that any CPU that
- * sees the new value of sp->completed will also see any preceding
- * changes to data structures made by this CPU. This prevents
- * some other CPU from reordering the accesses in its SRCU
- * read-side critical section to precede the corresponding
- * srcu_read_lock() -- ensuring that such references will in
- * fact be protected.
- *
- * So it is now safe to do the flip.
- */
+#ifdef SMP__STORE_MB_LOAD_WORKS /* The fast path */
+ if (srcu_readers_active_idx(sp, idx) == 0)
+ goto done;
+#endif

- idx = sp->completed & 0x1;
sp->completed++;
-
- synchronize_sched(); /* Force memory barrier on all CPUs. */
+ synchronize_sched();

/*
* At this point, because of the preceding synchronize_sched(),
* all srcu_read_lock() calls using the old counters have completed.
* Their corresponding critical sections might well be still
* executing, but the srcu_read_lock() primitives themselves
- * will have finished executing.
+ * will have finished executing. The "old" rank of counters
+ * can therefore only decrease, never increase in value.
*/

while (srcu_readers_active_idx(sp, idx))
schedule_timeout_interruptible(1);

- synchronize_sched(); /* Force memory barrier on all CPUs. */
-
- /*
- * The preceding synchronize_sched() forces all srcu_read_unlock()
- * primitives that were executing concurrently with the preceding
- * for_each_possible_cpu() loop to have completed by this point.
- * More importantly, it also forces the corresponding SRCU read-side
- * critical sections to have also completed, and the corresponding
- * references to SRCU-protected data items to be dropped.
- *
- * Note:
- *
- * Despite what you might think at first glance, the
- * preceding synchronize_sched() -must- be within the
- * critical section ended by the following mutex_unlock().
- * Otherwise, a task taking the early exit can race
- * with a srcu_read_unlock(), which might have executed
- * just before the preceding srcu_readers_active() check,
- * and whose CPU might have reordered the srcu_read_unlock()
- * with the preceding critical section. In this case, there
- * is nothing preventing the synchronize_sched() task that is
- * taking the early exit from freeing a data structure that
- * is still being referenced (out of order) by the task
- * doing the srcu_read_unlock().
- *
- * Alternatively, the comparison with "2" on the early exit
- * could be changed to "3", but this increases synchronize_srcu()
- * latency for bulk loads. So the current code is preferred.
- */
+ smp_mb(); /* must see critical section prior to srcu_read_unlock() */

+done:
mutex_unlock(&sp->mutex);
}

-
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