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/*
* Sleepable Read-Copy Update mechanism for mutual exclusion.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, write to the Free Software
* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
*
* Copyright (C) IBM Corporation, 2006
*
* Author: Paul McKenney <paulmck@us.ibm.com>
*
* For detailed explanation of Read-Copy Update mechanism see -
* Documentation/RCU/ *.txt
*
*/

#include <linux/export.h>
#include <linux/mutex.h>
#include <linux/percpu.h>
#include <linux/preempt.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/delay.h>
#include <linux/srcu.h>

/*
* Initialize an rcu_batch structure to empty.
*/
static inline void rcu_batch_init(struct rcu_batch *b)
{
b->head = NULL;
b->tail = &b->head;
}

/*
* Enqueue a callback onto the tail of the specified rcu_batch structure.
*/
static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
{
*b->tail = head;
b->tail = &head->next;
}

/*
* Is the specified rcu_batch structure empty?
*/
static inline bool rcu_batch_empty(struct rcu_batch *b)
{
return b->tail == &b->head;
}

/*
* Remove the callback at the head of the specified rcu_batch structure
* and return a pointer to it, or return NULL if the structure is empty.
*/
static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
{
struct rcu_head *head;

if (rcu_batch_empty(b))
return NULL;

head = b->head;
b->head = head->next;
if (b->tail == &head->next)
rcu_batch_init(b);

return head;
}

/*
* Move all callbacks from the rcu_batch structure specified by "from" to
* the structure specified by "to".
*/
static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
{
if (!rcu_batch_empty(from)) {
*to->tail = from->head;
to->tail = from->tail;
rcu_batch_init(from);
}
}

/* single-thread state-machine */
static void process_srcu(struct work_struct *work);

static int init_srcu_struct_fields(struct srcu_struct *sp)
{
sp->completed = 0;
spin_lock_init(&sp->queue_lock);
sp->running = false;
rcu_batch_init(&sp->batch_queue);
rcu_batch_init(&sp->batch_check0);
rcu_batch_init(&sp->batch_check1);
rcu_batch_init(&sp->batch_done);
INIT_DELAYED_WORK(&sp->work, process_srcu);
sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
return sp->per_cpu_ref ? 0 : -ENOMEM;
}

#ifdef CONFIG_DEBUG_LOCK_ALLOC

int __init_srcu_struct(struct srcu_struct *sp, const char *name,
struct lock_class_key *key)
{
/* Don't re-initialize a lock while it is held. */
debug_check_no_locks_freed((void *)sp, sizeof(*sp));
lockdep_init_map(&sp->dep_map, name, key, 0);
return init_srcu_struct_fields(sp);
}
EXPORT_SYMBOL_GPL(__init_srcu_struct);

#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/**
* init_srcu_struct - initialize a sleep-RCU structure
* @sp: structure to initialize.
*
* Must invoke this on a given srcu_struct before passing that srcu_struct
* to any other function. Each srcu_struct represents a separate domain
* of SRCU protection.
*/
int init_srcu_struct(struct srcu_struct *sp)
{
return init_srcu_struct_fields(sp);
}
EXPORT_SYMBOL_GPL(init_srcu_struct);

#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */

/*
* Returns approximate total of the readers' ->seq[] values for the
* rank of per-CPU counters specified by idx.
*/
static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long sum = 0;
unsigned long t;

for_each_possible_cpu(cpu) {
t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
sum += t;
}
return sum;
}

/*
* Returns approximate number of readers active on the specified rank
* of the per-CPU ->c[] counters.
*/
static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long sum = 0;
unsigned long t;

for_each_possible_cpu(cpu) {
t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
sum += t;
}
return sum;
}

/*
* Return true if the number of pre-existing readers is determined to
* be stably zero. An example unstable zero can occur if the call
* to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
* but due to task migration, sees the corresponding __srcu_read_unlock()
* decrement. This can happen because srcu_readers_active_idx() takes
* time to sum the array, and might in fact be interrupted or preempted
* partway through the summation.
*/
static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
{
unsigned long seq;

seq = srcu_readers_seq_idx(sp, idx);

/*
* The following smp_mb() A pairs with the smp_mb() B located in
* __srcu_read_lock(). This pairing ensures that if an
* __srcu_read_lock() increments its counter after the summation
* in srcu_readers_active_idx(), then the corresponding SRCU read-side
* critical section will see any changes made prior to the start
* of the current SRCU grace period.
*
* Also, if the above call to srcu_readers_seq_idx() saw the
* increment of ->seq[], then the call to srcu_readers_active_idx()
* must see the increment of ->c[].
*/
smp_mb(); /* A */

/*
* Note that srcu_readers_active_idx() can incorrectly return
* zero even though there is a pre-existing reader throughout.
* To see this, suppose that task A is in a very long SRCU
* read-side critical section that started on CPU 0, and that
* no other reader exists, so that the sum of the counters
* is equal to one. Then suppose that task B starts executing
* srcu_readers_active_idx(), summing up to CPU 1, and then that
* task C starts reading on CPU 0, so that its increment is not
* summed, but finishes reading on CPU 2, so that its decrement
* -is- summed. Then when task B completes its sum, it will
* incorrectly get zero, despite the fact that task A has been
* in its SRCU read-side critical section the whole time.
*
* We therefore do a validation step should srcu_readers_active_idx()
* return zero.
*/
if (srcu_readers_active_idx(sp, idx) != 0)
return false;

/*
* The remainder of this function is the validation step.
* The following smp_mb() D pairs with the smp_mb() C in
* __srcu_read_unlock(). If the __srcu_read_unlock() was seen
* by srcu_readers_active_idx() above, then any destructive
* operation performed after the grace period will happen after
* the corresponding SRCU read-side critical section.
*
* Note that there can be at most NR_CPUS worth of readers using
* the old index, which is not enough to overflow even a 32-bit
* integer. (Yes, this does mean that systems having more than
* a billion or so CPUs need to be 64-bit systems.) Therefore,
* the sum of the ->seq[] counters cannot possibly overflow.
* Therefore, the only way that the return values of the two
* calls to srcu_readers_seq_idx() can be equal is if there were
* no increments of the corresponding rank of ->seq[] counts
* in the interim. But the missed-increment scenario laid out
* above includes an increment of the ->seq[] counter by
* the corresponding __srcu_read_lock(). Therefore, if this
* scenario occurs, the return values from the two calls to
* srcu_readers_seq_idx() will differ, and thus the validation
* step below suffices.
*/
smp_mb(); /* D */

return srcu_readers_seq_idx(sp, idx) == seq;
}

/**
* srcu_readers_active - returns approximate number of readers.
* @sp: which srcu_struct to count active readers (holding srcu_read_lock).
*
* Note that this is not an atomic primitive, and can therefore suffer
* severe errors when invoked on an active srcu_struct. That said, it
* can be useful as an error check at cleanup time.
*/
static int srcu_readers_active(struct srcu_struct *sp)
{
int cpu;
unsigned long sum = 0;

for_each_possible_cpu(cpu) {
sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
}
return sum;
}

/**
* cleanup_srcu_struct - deconstruct a sleep-RCU structure
* @sp: structure to clean up.
*
* Must invoke this after you are finished using a given srcu_struct that
* was initialized via init_srcu_struct(), else you leak memory.
*/
void cleanup_srcu_struct(struct srcu_struct *sp)
{
int sum;

sum = srcu_readers_active(sp);
WARN_ON(sum); /* Leakage unless caller handles error. */
if (sum != 0)
return;
free_percpu(sp->per_cpu_ref);
sp->per_cpu_ref = NULL;
}
EXPORT_SYMBOL_GPL(cleanup_srcu_struct);

/*
* Counts the new reader in the appropriate per-CPU element of the
* srcu_struct. Must be called from process context.
* Returns an index that must be passed to the matching srcu_read_unlock().
*/
int __srcu_read_lock(struct srcu_struct *sp)
{
int idx;

preempt_disable();
idx = rcu_dereference_index_check(sp->completed,
rcu_read_lock_sched_held()) & 0x1;
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
smp_mb(); /* B */ /* Avoid leaking the critical section. */
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
preempt_enable();
return idx;
}
EXPORT_SYMBOL_GPL(__srcu_read_lock);

/*
* Removes the count for the old reader from the appropriate per-CPU
* element of the srcu_struct. Note that this may well be a different
* CPU than that which was incremented by the corresponding srcu_read_lock().
* Must be called from process context.
*/
void __srcu_read_unlock(struct srcu_struct *sp, int idx)
{
preempt_disable();
smp_mb(); /* C */ /* Avoid leaking the critical section. */
ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) -= 1;
preempt_enable();
}
EXPORT_SYMBOL_GPL(__srcu_read_unlock);

/*
* We use an adaptive strategy for synchronize_srcu() and especially for
* synchronize_srcu_expedited(). We spin for a fixed time period
* (defined below) to allow SRCU readers to exit their read-side critical
* sections. If there are still some readers after 10 microseconds,
* we repeatedly block for 1-millisecond time periods. This approach
* has done well in testing, so there is no need for a config parameter.
*/
#define SRCU_RETRY_CHECK_DELAY 5
#define SYNCHRONIZE_SRCU_TRYCOUNT 2
#define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12

/*
* @@@ Wait until all pre-existing readers complete. Such readers
* will have used the index specified by "idx".
* the caller should ensures the ->completed is not changed while checking
* and idx = (->completed & 1) ^ 1
*/
static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
{
for (;;) {
if (srcu_readers_active_idx_check(sp, idx))
return true;
if (--trycount <= 0)
return false;
udelay(SRCU_RETRY_CHECK_DELAY);
}
}

/*
* Increment the ->completed counter so that future SRCU readers will
* use the other rank of the ->c[] and ->seq[] arrays. This allows
* us to wait for pre-existing readers in a starvation-free manner.
*/
static void srcu_flip(struct srcu_struct *sp)
{
sp->completed++;
}

/*
* Enqueue an SRCU callback on the specified srcu_struct structure,
* initiating grace-period processing if it is not already running.
*/
void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
void (*func)(struct rcu_head *head))
{
unsigned long flags;

head->next = NULL;
head->func = func;
spin_lock_irqsave(&sp->queue_lock, flags);
rcu_batch_queue(&sp->batch_queue, head);
if (!sp->running) {
sp->running = true;
queue_delayed_work(system_nrt_wq, &sp->work, 0);
}
spin_unlock_irqrestore(&sp->queue_lock, flags);
}
EXPORT_SYMBOL_GPL(call_srcu);

struct rcu_synchronize {
struct rcu_head head;
struct completion completion;
};

/*
* Awaken the corresponding synchronize_srcu() instance now that a
* grace period has elapsed.
*/
static void wakeme_after_rcu(struct rcu_head *head)
{
struct rcu_synchronize *rcu;

rcu = container_of(head, struct rcu_synchronize, head);
complete(&rcu->completion);
}

static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
static void srcu_reschedule(struct srcu_struct *sp);

/*
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
*/
static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
{
struct rcu_synchronize rcu;
struct rcu_head *head = &rcu.head;
bool done = false;

rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
!lock_is_held(&rcu_bh_lock_map) &&
!lock_is_held(&rcu_lock_map) &&
!lock_is_held(&rcu_sched_lock_map),
"Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");

init_completion(&rcu.completion);

head->next = NULL;
head->func = wakeme_after_rcu;
spin_lock_irq(&sp->queue_lock);
if (!sp->running) {
/* steal the processing owner */
sp->running = true;
rcu_batch_queue(&sp->batch_check0, head);
spin_unlock_irq(&sp->queue_lock);

srcu_advance_batches(sp, trycount);
if (!rcu_batch_empty(&sp->batch_done)) {
BUG_ON(sp->batch_done.head != head);
rcu_batch_dequeue(&sp->batch_done);
done = true;
}
/* give the processing owner to work_struct */
srcu_reschedule(sp);
} else {
rcu_batch_queue(&sp->batch_queue, head);
spin_unlock_irq(&sp->queue_lock);
}

if (!done)
wait_for_completion(&rcu.completion);
}

/**
* synchronize_srcu - wait for prior SRCU read-side critical-section completion
* @sp: srcu_struct with which to synchronize.
*
* Flip the completed counter, and wait for the old count to drain to zero.
* As with classic RCU, the updater must use some separate means of
* synchronizing concurrent updates. Can block; must be called from
* process context.
*
* Note that it is illegal to call synchronize_srcu() from the corresponding
* SRCU read-side critical section; doing so will result in deadlock.
* However, it is perfectly legal to call synchronize_srcu() on one
* srcu_struct from some other srcu_struct's read-side critical section.
*/
void synchronize_srcu(struct srcu_struct *sp)
{
__synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);

/**
* synchronize_srcu_expedited - Brute-force SRCU grace period
* @sp: srcu_struct with which to synchronize.
*
* Wait for an SRCU grace period to elapse, but be more aggressive about
* spinning rather than blocking when waiting.
*
* Note that it is illegal to call this function while holding any lock
* that is acquired by a CPU-hotplug notifier. It is also illegal to call
* synchronize_srcu_expedited() from the corresponding SRCU read-side
* critical section; doing so will result in deadlock. However, it is
* perfectly legal to call synchronize_srcu_expedited() on one srcu_struct
* from some other srcu_struct's read-side critical section, as long as
* the resulting graph of srcu_structs is acyclic.
*/
void synchronize_srcu_expedited(struct srcu_struct *sp)
{
__synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
}
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);

/**
* srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
*/
void srcu_barrier(struct srcu_struct *sp)
{
synchronize_srcu(sp);
}
EXPORT_SYMBOL_GPL(srcu_barrier);

/**
* srcu_batches_completed - return batches completed.
* @sp: srcu_struct on which to report batch completion.
*
* Report the number of batches, correlated with, but not necessarily
* precisely the same as, the number of grace periods that have elapsed.
*/
long srcu_batches_completed(struct srcu_struct *sp)
{
return sp->completed;
}
EXPORT_SYMBOL_GPL(srcu_batches_completed);

#define SRCU_CALLBACK_BATCH 10
#define SRCU_INTERVAL 1

/*
* Move any new SRCU callbacks to the first stage of the SRCU grace
* period pipeline.
*/
static void srcu_collect_new(struct srcu_struct *sp)
{
if (!rcu_batch_empty(&sp->batch_queue)) {
spin_lock_irq(&sp->queue_lock);
rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
spin_unlock_irq(&sp->queue_lock);
}
}

/*
* Core SRCU state machine. Advance callbacks from ->batch_check0 to
* ->batch_check1 and then to ->batch_done as readers drain.
*/
static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
{
int idx = 1 ^ (sp->completed & 1);

/*
* Because readers might be delayed for an extended period after
* fetching ->completed for their index, at any point in time there
* might well be readers using both idx=0 and idx=1. We therefore
* need to wait for readers to clear from both index values before
* invoking a callback.
*/

if (rcu_batch_empty(&sp->batch_check0) &&
rcu_batch_empty(&sp->batch_check1))
return; /* no callbacks need to be advanced */

if (!try_check_zero(sp, idx, trycount))
return; /* failed to advance, will try after SRCU_INTERVAL */

/*
* The callbacks in ->batch_check1 have already done with their
* first zero check and flip back when they were enqueued on
* ->batch_check0 in a previous invocation of srcu_advance_batches().
* (Presumably try_check_zero() returned false during that
* invocation, leaving the callbacks stranded on ->batch_check1.)
* They are therefore ready to invoke, so move them to ->batch_done.
*/
rcu_batch_move(&sp->batch_done, &sp->batch_check1);

if (rcu_batch_empty(&sp->batch_check0))
return; /* no callbacks need to be advanced */
srcu_flip(sp);

/*
* The callbacks in ->batch_check0 just finished their
* first check zero and flip, so move them to ->batch_check1
* for future checking on the other idx.
*/
rcu_batch_move(&sp->batch_check1, &sp->batch_check0);

/*
* SRCU read-side critical sections are normally short, so check
* at least twice in quick succession after a flip.
*/
trycount = trycount < 2 ? 2 : trycount;
if (!try_check_zero(sp, idx^1, trycount))
return; /* failed to advance, will try after SRCU_INTERVAL */

/*
* The callbacks in ->batch_check1 have now waited for all
* pre-existing readers using both idx values. They are therefore
* ready to invoke, so move them to ->batch_done.
*/
rcu_batch_move(&sp->batch_done, &sp->batch_check1);
}

/*
* Invoke a limited number of SRCU callbacks that have passed through
* their grace period. If there are more to do, SRCU will reschedule
* the workqueue.
*/
static void srcu_invoke_callbacks(struct srcu_struct *sp)
{
int i;
struct rcu_head *head;

for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
head = rcu_batch_dequeue(&sp->batch_done);
if (!head)
break;
local_bh_disable();
head->func(head);
local_bh_enable();
}
}

/*
* Finished one round of SRCU grace period. Start another if there are
* more SRCU callbacks queued, otherwise put SRCU into not-running state.
*/
static void srcu_reschedule(struct srcu_struct *sp)
{
bool pending = true;

if (rcu_batch_empty(&sp->batch_done) &&
rcu_batch_empty(&sp->batch_check1) &&
rcu_batch_empty(&sp->batch_check0) &&
rcu_batch_empty(&sp->batch_queue)) {
spin_lock_irq(&sp->queue_lock);
if (rcu_batch_empty(&sp->batch_done) &&
rcu_batch_empty(&sp->batch_check1) &&
rcu_batch_empty(&sp->batch_check0) &&
rcu_batch_empty(&sp->batch_queue)) {
sp->running = false;
pending = false;
}
spin_unlock_irq(&sp->queue_lock);
}

if (pending)
queue_delayed_work(system_nrt_wq, &sp->work, SRCU_INTERVAL);
}

/*
* This is the work-queue function that handles SRCU grace periods.
*/
static void process_srcu(struct work_struct *work)
{
struct srcu_struct *sp;

sp = container_of(work, struct srcu_struct, work.work);

srcu_collect_new(sp);
srcu_advance_batches(sp, 1);
srcu_invoke_callbacks(sp);
srcu_reschedule(sp);
}
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