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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/module.h>

#include <linux/mutex.h>

#include <linux/percpu.h>

#include <linux/preempt.h>

#include <linux/rcupdate.h>

#include <linux/sched.h>

#include <linux/slab.h>

#include <linux/smp.h>

#include <linux/srcu.h>

/**

 * 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)

{

        sp->completed = 0;

        mutex_init(&sp->mutex);

        sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);

        return (sp->per_cpu_ref ? 0 : -ENOMEM);

}

/*

 * srcu_readers_active_idx -- returns approximate number of readers

 *      active on the specified rank of per-CPU counters.

 */

static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)

{

        int cpu;

        int sum;

        sum = 0;

        for_each_possible_cpu(cpu)

                sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];

        return sum;

}

/**

 * 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.

 */

int srcu_readers_active(struct srcu_struct *sp)

{

        return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);

}

/**

 * 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;

}

/**

 * 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.

 * 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 = 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. */

        preempt_enable();

        return idx;

}

/**

 * srcu_read_unlock - unregister a old reader from an SRCU-protected structure.

 * @sp: srcu_struct in which to unregister the old reader.

 * @idx: return value from corresponding 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();

        srcu_barrier();  /* ensure compiler won't misorder critical section. */

        per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;

        preempt_enable();

}

/**

 * 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 synchornize_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)

{

        int idx;

        idx = sp->completed;

        mutex_lock(&sp->mutex);

        /*

         * Check to see if someone else did the work for us while we were

         * waiting to acquire the lock.  We need -two- advances of

         * the counter, not just one.  If there was but one, we might have

         * shown up -after- our helper's first synchronize_sched(), thus

         * having failed to prevent CPU-reordering races with concurrent

         * srcu_read_unlock()s on other CPUs (see comment below).  So we

         * either (1) wait for two or (2) supply the second ourselves.

         */

        if ((sp->completed - idx) >= 2) {

                mutex_unlock(&sp->mutex);

                return;

        }

        synchronize_sched();  /* Force memory barrier on all CPUs. */

        /*

         * 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.

         */

        idx = sp->completed & 0x1;

        sp->completed++;

        synchronize_sched();  /* Force memory barrier on all CPUs. */

        /*

         * 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.

         */

        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.

         */

        mutex_unlock(&sp->mutex);

}

/**

 * 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(init_srcu_struct);

EXPORT_SYMBOL_GPL(cleanup_srcu_struct);

EXPORT_SYMBOL_GPL(srcu_read_lock);

EXPORT_SYMBOL_GPL(srcu_read_unlock);

EXPORT_SYMBOL_GPL(synchronize_srcu);

EXPORT_SYMBOL_GPL(srcu_batches_completed);

EXPORT_SYMBOL_GPL(srcu_readers_active);