/*
* 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.
*/
static 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);
int init_qrcu_struct(struct qrcu_struct *qp)
{
qp->completed = 0;
atomic_set(qp->ctr + 0, 1);
atomic_set(qp->ctr + 1, 0);
init_waitqueue_head(&qp->wq);
mutex_init(&qp->mutex);
return 0;
}
int qrcu_read_lock(struct qrcu_struct *qp)
{
for (;;) {
int idx = qp->completed & 0x1;
if (likely(atomic_inc_not_zero(qp->ctr + idx)))
return idx;
}
}
void qrcu_read_unlock(struct qrcu_struct *qp, int idx)
{
if (atomic_dec_and_test(qp->ctr + idx))
wake_up(&qp->wq);
}
void synchronize_qrcu(struct qrcu_struct *qp)
{
int idx;
smp_mb(); /* Force preceding change to happen before fastpath check. */
/*
* Fastpath: If the two counters sum to "1" at a given point in
* time, there are no readers. However, it takes two separate
* loads to sample both counters, which won't occur simultaneously.
* So we might race with a counter switch, so that we might see
* ctr[0]==0, then the counter might switch, then we might see
* ctr[1]==1 (unbeknownst to us because there is a reader still
* there). So we do a read memory barrier and recheck. If the
* same race happens again, there must have been a second counter
* switch. This second counter switch could not have happened
* until all preceding readers finished, so if the condition
* is true both times, we may safely proceed.
*
* This relies critically on the atomic increment and atomic
* decrement being seen as executing in order.
*/
if (atomic_read(&qp->ctr[0]) + atomic_read(&qp->ctr[1]) <= 1) {
smp_rmb(); /* Keep two checks independent. */
if (atomic_read(&qp->ctr[0]) + atomic_read(&qp->ctr[1]) <= 1)
goto out;
}
mutex_lock(&qp->mutex);
idx = qp->completed & 0x1;
if (atomic_read(qp->ctr + idx) == 1)
goto out_unlock;
atomic_inc(qp->ctr + (idx ^ 0x1));
/*
* Prevent subsequent decrement from being seen before previous
* increment -- such an inversion could cause the fastpath
* above to falsely conclude that there were no readers. Also,
* reduce the likelihood that qrcu_read_lock() will loop.
*/
smp_mb__after_atomic_inc();
qp->completed++;
atomic_dec(qp->ctr + idx);
__wait_event(qp->wq, !atomic_read(qp->ctr + idx));
out_unlock:
mutex_unlock(&qp->mutex);
out:
smp_mb(); /* force subsequent free after qrcu_read_unlock(). */
}
EXPORT_SYMBOL_GPL(init_qrcu_struct);
EXPORT_SYMBOL_GPL(qrcu_read_lock);
EXPORT_SYMBOL_GPL(qrcu_read_unlock);
EXPORT_SYMBOL_GPL(synchronize_qrcu);
