/*
* 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, you can access it online at
* http://www.gnu.org/licenses/gpl-2.0.html.
*
* Copyright (C) IBM Corporation, 2006
* Copyright (C) Fujitsu, 2012
*
* Author: Paul McKenney <paulmck@us.ibm.com>
* Lai Jiangshan <laijs@cn.fujitsu.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_wait.h>
#include <linux/sched.h>
#include <linux/smp.h>
#include <linux/delay.h>
#include <linux/module.h>
#include <linux/srcu.h>
#include "rcu.h"
#include "rcu_segcblist.h"
/* Holdoff in nanoseconds for auto-expediting. */
#define DEFAULT_SRCU_EXP_HOLDOFF (25 * 1000)
static ulong exp_holdoff = DEFAULT_SRCU_EXP_HOLDOFF;
module_param(exp_holdoff, ulong, 0444);
/* Overflow-check frequency. N bits roughly says every 2**N grace periods. */
static ulong counter_wrap_check = (ULONG_MAX >> 2);
module_param(counter_wrap_check, ulong, 0444);
static void srcu_invoke_callbacks(struct work_struct *work);
static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
/*
* Initialize SRCU combining tree. Note that statically allocated
* srcu_struct structures might already have srcu_read_lock() and
* srcu_read_unlock() running against them. So if the is_static parameter
* is set, don't initialize ->srcu_lock_count[] and ->srcu_unlock_count[].
*/
static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
{
int cpu;
int i;
int level = 0;
int levelspread[RCU_NUM_LVLS];
struct srcu_data *sdp;
struct srcu_node *snp;
struct srcu_node *snp_first;
/* Work out the overall tree geometry. */
sp->level[0] = &sp->node[0];
for (i = 1; i < rcu_num_lvls; i++)
sp->level[i] = sp->level[i - 1] + num_rcu_lvl[i - 1];
rcu_init_levelspread(levelspread, num_rcu_lvl);
/* Each pass through this loop initializes one srcu_node structure. */
rcu_for_each_node_breadth_first(sp, snp) {
spin_lock_init(&snp->lock);
WARN_ON_ONCE(ARRAY_SIZE(snp->srcu_have_cbs) !=
ARRAY_SIZE(snp->srcu_data_have_cbs));
for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++) {
snp->srcu_have_cbs[i] = 0;
snp->srcu_data_have_cbs[i] = 0;
}
snp->srcu_gp_seq_needed_exp = 0;
snp->grplo = -1;
snp->grphi = -1;
if (snp == &sp->node[0]) {
/* Root node, special case. */
snp->srcu_parent = NULL;
continue;
}
/* Non-root node. */
if (snp == sp->level[level + 1])
level++;
snp->srcu_parent = sp->level[level - 1] +
(snp - sp->level[level]) /
levelspread[level - 1];
}
/*
* Initialize the per-CPU srcu_data array, which feeds into the
* leaves of the srcu_node tree.
*/
WARN_ON_ONCE(ARRAY_SIZE(sdp->srcu_lock_count) !=
ARRAY_SIZE(sdp->srcu_unlock_count));
level = rcu_num_lvls - 1;
snp_first = sp->level[level];
for_each_possible_cpu(cpu) {
sdp = per_cpu_ptr(sp->sda, cpu);
spin_lock_init(&sdp->lock);
rcu_segcblist_init(&sdp->srcu_cblist);
sdp->srcu_cblist_invoking = false;
sdp->srcu_gp_seq_needed = sp->srcu_gp_seq;
sdp->srcu_gp_seq_needed_exp = sp->srcu_gp_seq;
sdp->mynode = &snp_first[cpu / levelspread[level]];
for (snp = sdp->mynode; snp != NULL; snp = snp->srcu_parent) {
if (snp->grplo < 0)
snp->grplo = cpu;
snp->grphi = cpu;
}
sdp->cpu = cpu;
INIT_DELAYED_WORK(&sdp->work, srcu_invoke_callbacks);
sdp->sp = sp;
sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
if (is_static)
continue;
/* Dynamically allocated, better be no srcu_read_locks()! */
for (i = 0; i < ARRAY_SIZE(sdp->srcu_lock_count); i++) {
sdp->srcu_lock_count[i] = 0;
sdp->srcu_unlock_count[i] = 0;
}
}
}
/*
* Initialize non-compile-time initialized fields, including the
* associated srcu_node and srcu_data structures. The is_static
* parameter is passed through to init_srcu_struct_nodes(), and
* also tells us that ->sda has already been wired up to srcu_data.
*/
static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
{
mutex_init(&sp->srcu_cb_mutex);
mutex_init(&sp->srcu_gp_mutex);
sp->srcu_idx = 0;
sp->srcu_gp_seq = 0;
sp->srcu_barrier_seq = 0;
mutex_init(&sp->srcu_barrier_mutex);
atomic_set(&sp->srcu_barrier_cpu_cnt, 0);
INIT_DELAYED_WORK(&sp->work, process_srcu);
if (!is_static)
sp->sda = alloc_percpu(struct srcu_data);
init_srcu_struct_nodes(sp, is_static);
sp->srcu_gp_seq_needed_exp = 0;
sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
smp_store_release(&sp->srcu_gp_seq_needed, 0); /* Init done. */
return sp->sda ? 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);
spin_lock_init(&sp->gp_lock);
return init_srcu_struct_fields(sp, false);
}
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)
{
spin_lock_init(&sp->gp_lock);
return init_srcu_struct_fields(sp, false);
}
EXPORT_SYMBOL_GPL(init_srcu_struct);
#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
/*
* First-use initialization of statically allocated srcu_struct
* structure. Wiring up the combining tree is more than can be
* done with compile-time initialization, so this check is added
* to each update-side SRCU primitive. Use ->gp_lock, which -is-
* compile-time initialized, to resolve races involving multiple
* CPUs trying to garner first-use privileges.
*/
static void check_init_srcu_struct(struct srcu_struct *sp)
{
unsigned long flags;
WARN_ON_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INIT);
/* The smp_load_acquire() pairs with the smp_store_release(). */
if (!rcu_seq_state(smp_load_acquire(&sp->srcu_gp_seq_needed))) /*^^^*/
return; /* Already initialized. */
spin_lock_irqsave(&sp->gp_lock, flags);
if (!rcu_seq_state(sp->srcu_gp_seq_needed)) {
spin_unlock_irqrestore(&sp->gp_lock, flags);
return;
}
init_srcu_struct_fields(sp, true);
spin_unlock_irqrestore(&sp->gp_lock, flags);
}
/*
* Returns approximate total of the readers' ->srcu_lock_count[] values
* for the rank of per-CPU counters specified by idx.
*/
static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long sum = 0;
for_each_possible_cpu(cpu) {
struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);
sum += READ_ONCE(cpuc->srcu_lock_count[idx]);
}
return sum;
}
/*
* Returns approximate total of the readers' ->srcu_unlock_count[] values
* for the rank of per-CPU counters specified by idx.
*/
static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
{
int cpu;
unsigned long sum = 0;
for_each_possible_cpu(cpu) {
struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);
sum += READ_ONCE(cpuc->srcu_unlock_count[idx]);
}
return sum;
}
/*
* Return true if the number of pre-existing readers is determined to
* be zero.
*/
static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
{
unsigned long unlocks;
unlocks = srcu_readers_unlock_idx(sp, idx);
/*
* Make sure that a lock is always counted if the corresponding
* unlock is counted. Needs to be a smp_mb() as the read side may
* contain a read from a variable that is written to before the
* synchronize_srcu() in the write side. In this case smp_mb()s
* A and B act like the store buffering pattern.
*
* This smp_mb() also pairs with smp_mb() C to prevent accesses
* after the synchronize_srcu() from being executed before the
* grace period ends.
*/
smp_mb(); /* A */
/*
* If the locks are the same as the unlocks, then there must have
* been no readers on this index at some time in between. This does
* not mean that there are no more readers, as one could have read
* the current index but not have incremented the lock counter yet.
*
* So suppose that the updater is preempted here for so long
* that more than ULONG_MAX non-nested readers come and go in
* the meantime. It turns out that this cannot result in overflow
* because if a reader modifies its unlock count after we read it
* above, then that reader's next load of ->srcu_idx is guaranteed
* to get the new value, which will cause it to operate on the
* other bank of counters, where it cannot contribute to the
* overflow of these counters. This means that there is a maximum
* of 2*NR_CPUS increments, which cannot overflow given current
* systems, especially not on 64-bit systems.
*
* OK, how about nesting? This does impose a limit on nesting
* of floor(ULONG_MAX/NR_CPUS/2), which should be sufficient,
* especially on 64-bit systems.
*/
return srcu_readers_lock_idx(sp, idx) == unlocks;
}
/**
* srcu_readers_active - returns true if there are readers. and false
* otherwise
* @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 bool srcu_readers_active(struct srcu_struct *sp)
{
int cpu;
unsigned long sum = 0;
for_each_possible_cpu(cpu) {
struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);
sum += READ_ONCE(cpuc->srcu_lock_count[0]);
sum += READ_ONCE(cpuc->srcu_lock_count[1]);
sum -= READ_ONCE(cpuc->srcu_unlock_count[0]);
sum -= READ_ONCE(cpuc->srcu_unlock_count[1]);
}
return sum;
}
#define SRCU_INTERVAL 1
/*
* Return grace-period delay, zero if there are expedited grace
* periods pending, SRCU_INTERVAL otherwise.
*/
static unsigned long srcu_get_delay(struct srcu_struct *sp)
{
if (ULONG_CMP_LT(READ_ONCE(sp->srcu_gp_seq),
READ_ONCE(sp->srcu_gp_seq_needed_exp)))
return 0;
return SRCU_INTERVAL;
}
/**
* 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 cpu;
if (WARN_ON(!srcu_get_delay(sp)))
return; /* Leakage unless caller handles error. */
if (WARN_ON(srcu_readers_active(sp)))
return; /* Leakage unless caller handles error. */
flush_delayed_work(&sp->work);
for_each_possible_cpu(cpu)
flush_delayed_work(&per_cpu_ptr(sp->sda, cpu)->work);
if (WARN_ON(rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
WARN_ON(srcu_readers_active(sp))) {
pr_info("cleanup_srcu_struct: Active srcu_struct %p state: %d\n", sp, rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)));
return; /* Caller forgot to stop doing call_srcu()? */
}
free_percpu(sp->sda);
sp->sda = NULL;
}
EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
/*
* Counts the new reader in the appropriate per-CPU element of the
* srcu_struct.
* Returns an index that must be passed to the matching srcu_read_unlock().
*/
int __srcu_read_lock(struct srcu_struct *sp)
{
int idx;
idx = READ_ONCE(sp->srcu_idx) & 0x1;
this_cpu_inc(sp->sda->srcu_lock_count[idx]);
smp_mb(); /* B */ /* Avoid leaking the critical section. */
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().
*/
void __srcu_read_unlock(struct srcu_struct *sp, int idx)
{
smp_mb(); /* C */ /* Avoid leaking the critical section. */
this_cpu_inc(sp->sda->srcu_unlock_count[idx]);
}
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 a few microseconds,
* we repeatedly block for 1-millisecond time periods.
*/
#define SRCU_RETRY_CHECK_DELAY 5
/*
* Start an SRCU grace period.
*/
static void srcu_gp_start(struct srcu_struct *sp)
{
struct srcu_data *sdp = this_cpu_ptr(sp->sda);
int state;
RCU_LOCKDEP_WARN(!lockdep_is_held(&sp->gp_lock),
"Invoked srcu_gp_start() without ->gp_lock!");
WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
rcu_segcblist_advance(&sdp->srcu_cblist,
rcu_seq_current(&sp->srcu_gp_seq));
(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
rcu_seq_snap(&sp->srcu_gp_seq));
smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
rcu_seq_start(&sp->srcu_gp_seq);
state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
}
/*
* Track online CPUs to guide callback workqueue placement.
*/
DEFINE_PER_CPU(bool, srcu_online);
void srcu_online_cpu(unsigned int cpu)
{
WRITE_ONCE(per_cpu(srcu_online, cpu), true);
}
void srcu_offline_cpu(unsigned int cpu)
{
WRITE_ONCE(per_cpu(srcu_online, cpu), false);
}
/*
* Place the workqueue handler on the specified CPU if online, otherwise
* just run it whereever. This is useful for placing workqueue handlers
* that are to invoke the specified CPU's callbacks.
*/
static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
struct delayed_work *dwork,
unsigned long delay)
{
bool ret;
preempt_disable();
if (READ_ONCE(per_cpu(srcu_online, cpu)))
ret = queue_delayed_work_on(cpu, wq, dwork, delay);
else
ret = queue_delayed_work(wq, dwork, delay);
preempt_enable();
return ret;
}
/*
* Schedule callback invocation for the specified srcu_data structure,
* if possible, on the corresponding CPU.
*/
static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
{
srcu_queue_delayed_work_on(sdp->cpu, system_power_efficient_wq,
&sdp->work, delay);
}
/*
* Schedule callback invocation for all srcu_data structures associated
* with the specified srcu_node structure that have callbacks for the
* just-completed grace period, the one corresponding to idx. If possible,
* schedule this invocation on the corresponding CPUs.
*/
static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp,
unsigned long mask, unsigned long delay)
{
int cpu;
for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
if (!(mask & (1 << (cpu - snp->grplo))))
continue;
srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu), delay);
}
}
/*
* Note the end of an SRCU grace period. Initiates callback invocation
* and starts a new grace period if needed.
*
* The ->srcu_cb_mutex acquisition does not protect any data, but
* instead prevents more than one grace period from starting while we
* are initiating callback invocation. This allows the ->srcu_have_cbs[]
* array to have a finite number of elements.
*/
static void srcu_gp_end(struct srcu_struct *sp)
{
unsigned long cbdelay;
bool cbs;
int cpu;
unsigned long flags;
unsigned long gpseq;
int idx;
int idxnext;
unsigned long mask;
struct srcu_data *sdp;
struct srcu_node *snp;
/* Prevent more than one additional grace period. */
mutex_lock(&sp->srcu_cb_mutex);
/* End the current grace period. */
spin_lock_irq(&sp->gp_lock);
idx = rcu_seq_state(sp->srcu_gp_seq);
WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
cbdelay = srcu_get_delay(sp);
sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
rcu_seq_end(&sp->srcu_gp_seq);
gpseq = rcu_seq_current(&sp->srcu_gp_seq);
if (ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, gpseq))
sp->srcu_gp_seq_needed_exp = gpseq;
spin_unlock_irq(&sp->gp_lock);
mutex_unlock(&sp->srcu_gp_mutex);
/* A new grace period can start at this point. But only one. */
/* Initiate callback invocation as needed. */
idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
idxnext = (idx + 1) % ARRAY_SIZE(snp->srcu_have_cbs);
rcu_for_each_node_breadth_first(sp, snp) {
spin_lock_irq(&snp->lock);
cbs = false;
if (snp >= sp->level[rcu_num_lvls - 1])
cbs = snp->srcu_have_cbs[idx] == gpseq;
snp->srcu_have_cbs[idx] = gpseq;
rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
snp->srcu_gp_seq_needed_exp = gpseq;
mask = snp->srcu_data_have_cbs[idx];
snp->srcu_data_have_cbs[idx] = 0;
spin_unlock_irq(&snp->lock);
if (cbs) {
smp_mb(); /* GP end before CB invocation. */
srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);
}
/* Occasionally prevent srcu_data counter wrap. */
if (!(gpseq & counter_wrap_check))
for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
sdp = per_cpu_ptr(sp->sda, cpu);
spin_lock_irqsave(&sdp->lock, flags);
if (ULONG_CMP_GE(gpseq,
sdp->srcu_gp_seq_needed + 100))
sdp->srcu_gp_seq_needed = gpseq;
spin_unlock_irqrestore(&sdp->lock, flags);
}
}
/* Callback initiation done, allow grace periods after next. */
mutex_unlock(&sp->srcu_cb_mutex);
/* Start a new grace period if needed. */
spin_lock_irq(&sp->gp_lock);
gpseq = rcu_seq_current(&sp->srcu_gp_seq);
if (!rcu_seq_state(gpseq) &&
ULONG_CMP_LT(gpseq, sp->srcu_gp_seq_needed)) {
srcu_gp_start(sp);
spin_unlock_irq(&sp->gp_lock);
/* Throttle expedited grace periods: Should be rare! */
srcu_reschedule(sp, rcu_seq_ctr(gpseq) & 0x3ff
? 0 : SRCU_INTERVAL);
} else {
spin_unlock_irq(&sp->gp_lock);
}
}
/*
* Funnel-locking scheme to scalably mediate many concurrent expedited
* grace-period requests. This function is invoked for the first known
* expedited request for a grace period that has already been requested,
* but without expediting. To start a completely new grace period,
* whether expedited or not, use srcu_funnel_gp_start() instead.
*/
static void srcu_funnel_exp_start(struct srcu_struct *sp, struct srcu_node *snp,
unsigned long s)
{
unsigned long flags;
for (; snp != NULL; snp = snp->srcu_parent) {
if (rcu_seq_done(&sp->srcu_gp_seq, s) ||
ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
return;
spin_lock_irqsave(&snp->lock, flags);
if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
spin_unlock_irqrestore(&snp->lock, flags);
return;
}
WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
spin_unlock_irqrestore(&snp->lock, flags);
}
spin_lock_irqsave(&sp->gp_lock, flags);
if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
sp->srcu_gp_seq_needed_exp = s;
spin_unlock_irqrestore(&sp->gp_lock, flags);
}
/*
* Funnel-locking scheme to scalably mediate many concurrent grace-period
* requests. The winner has to do the work of actually starting grace
* period s. Losers must either ensure that their desired grace-period
* number is recorded on at least their leaf srcu_node structure, or they
* must take steps to invoke their own callbacks.
*/
static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
unsigned long s, bool do_norm)
{
unsigned long flags;
int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
struct srcu_node *snp = sdp->mynode;
unsigned long snp_seq;
/* Each pass through the loop does one level of the srcu_node tree. */
for (; snp != NULL; snp = snp->srcu_parent) {
if (rcu_seq_done(&sp->srcu_gp_seq, s) && snp != sdp->mynode)
return; /* GP already done and CBs recorded. */
spin_lock_irqsave(&snp->lock, flags);
if (ULONG_CMP_GE(snp->srcu_have_cbs[idx], s)) {
snp_seq = snp->srcu_have_cbs[idx];
if (snp == sdp->mynode && snp_seq == s)
snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
spin_unlock_irqrestore(&snp->lock, flags);
if (snp == sdp->mynode && snp_seq != s) {
smp_mb(); /* CBs after GP! */
srcu_schedule_cbs_sdp(sdp, do_norm
? SRCU_INTERVAL
: 0);
return;
}
if (!do_norm)
srcu_funnel_exp_start(sp, snp, s);
return;
}
snp->srcu_have_cbs[idx] = s;
if (snp == sdp->mynode)
snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
snp->srcu_gp_seq_needed_exp = s;
spin_unlock_irqrestore(&snp->lock, flags);
}
/* Top of tree, must ensure the grace period will be started. */
spin_lock_irqsave(&sp->gp_lock, flags);
if (ULONG_CMP_LT(sp->srcu_gp_seq_needed, s)) {
/*
* Record need for grace period s. Pair with load
* acquire setting up for initialization.
*/
smp_store_release(&sp->srcu_gp_seq_needed, s); /*^^^*/
}
if (!do_norm && ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
sp->srcu_gp_seq_needed_exp = s;
/* If grace period not already done and none in progress, start it. */
if (!rcu_seq_done(&sp->srcu_gp_seq, s) &&
rcu_seq_state(sp->srcu_gp_seq) == SRCU_STATE_IDLE) {
WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
srcu_gp_start(sp);
queue_delayed_work(system_power_efficient_wq, &sp->work,
srcu_get_delay(sp));
}
spin_unlock_irqrestore(&sp->gp_lock, flags);
}
/*
* Wait until all readers counted by array index idx complete, but
* loop an additional time if there is an expedited grace period pending.
* The caller must ensure that ->srcu_idx is not changed while checking.
*/
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 + !srcu_get_delay(sp) <= 0)
return false;
udelay(SRCU_RETRY_CHECK_DELAY);
}
}
/*
* Increment the ->srcu_idx counter so that future SRCU readers will
* use the other rank of the ->srcu_(un)lock_count[] arrays. This allows
* us to wait for pre-existing readers in a starvation-free manner.
*/
static void srcu_flip(struct srcu_struct *sp)
{
/*
* Ensure that if this updater saw a given reader's increment
* from __srcu_read_lock(), that reader was using an old value
* of ->srcu_idx. Also ensure that if a given reader sees the
* new value of ->srcu_idx, this updater's earlier scans cannot
* have seen that reader's increments (which is OK, because this
* grace period need not wait on that reader).
*/
smp_mb(); /* E */ /* Pairs with B and C. */
WRITE_ONCE(sp->srcu_idx, sp->srcu_idx + 1);
/*
* Ensure that if the updater misses an __srcu_read_unlock()
* increment, that task's next __srcu_read_lock() will see the
* above counter update. Note that both this memory barrier
* and the one in srcu_readers_active_idx_check() provide the
* guarantee for __srcu_read_lock().
*/
smp_mb(); /* D */ /* Pairs with C. */
}
/*
* If SRCU is likely idle, return true, otherwise return false.
*
* Note that it is OK for several current from-idle requests for a new
* grace period from idle to specify expediting because they will all end
* up requesting the same grace period anyhow. So no loss.
*
* Note also that if any CPU (including the current one) is still invoking
* callbacks, this function will nevertheless say "idle". This is not
* ideal, but the overhead of checking all CPUs' callback lists is even
* less ideal, especially on large systems. Furthermore, the wakeup
* can happen before the callback is fully removed, so we have no choice
* but to accept this type of error.
*
* This function is also subject to counter-wrap errors, but let's face
* it, if this function was preempted for enough time for the counters
* to wrap, it really doesn't matter whether or not we expedite the grace
* period. The extra overhead of a needlessly expedited grace period is
* negligible when amoritized over that time period, and the extra latency
* of a needlessly non-expedited grace period is similarly negligible.
*/
static bool srcu_might_be_idle(struct srcu_struct *sp)
{
unsigned long curseq;
unsigned long flags;
struct srcu_data *sdp;
unsigned long t;
/* If the local srcu_data structure has callbacks, not idle. */
local_irq_save(flags);
sdp = this_cpu_ptr(sp->sda);
if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
local_irq_restore(flags);
return false; /* Callbacks already present, so not idle. */
}
local_irq_restore(flags);
/*
* No local callbacks, so probabalistically probe global state.
* Exact information would require acquiring locks, which would
* kill scalability, hence the probabalistic nature of the probe.
*/
/* First, see if enough time has passed since the last GP. */
t = ktime_get_mono_fast_ns();
if (exp_holdoff == 0 ||
time_in_range_open(t, sp->srcu_last_gp_end,
sp->srcu_last_gp_end + exp_holdoff))
return false; /* Too soon after last GP. */
/* Next, check for probable idleness. */
curseq = rcu_seq_current(&sp->srcu_gp_seq);
smp_mb(); /* Order ->srcu_gp_seq with ->srcu_gp_seq_needed. */
if (ULONG_CMP_LT(curseq, READ_ONCE(sp->srcu_gp_seq_needed)))
return false; /* Grace period in progress, so not idle. */
smp_mb(); /* Order ->srcu_gp_seq with prior access. */
if (curseq != rcu_seq_current(&sp->srcu_gp_seq))
return false; /* GP # changed, so not idle. */
return true; /* With reasonable probability, idle! */
}
/*
* SRCU callback function to leak a callback.
*/
static void srcu_leak_callback(struct rcu_head *rhp)
{
}
/*
* Enqueue an SRCU callback on the srcu_data structure associated with
* the current CPU and the specified srcu_struct structure, initiating
* grace-period processing if it is not already running.
*
* Note that all CPUs must agree that the grace period extended beyond
* all pre-existing SRCU read-side critical section. On systems with
* more than one CPU, this means that when "func()" is invoked, each CPU
* is guaranteed to have executed a full memory barrier since the end of
* its last corresponding SRCU read-side critical section whose beginning
* preceded the call to call_rcu(). It also means that each CPU executing
* an SRCU read-side critical section that continues beyond the start of
* "func()" must have executed a memory barrier after the call_rcu()
* but before the beginning of that SRCU read-side critical section.
* Note that these guarantees include CPUs that are offline, idle, or
* executing in user mode, as well as CPUs that are executing in the kernel.
*
* Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
* resulting SRCU callback function "func()", then both CPU A and CPU
* B are guaranteed to execute a full memory barrier during the time
* interval between the call to call_rcu() and the invocation of "func()".
* This guarantee applies even if CPU A and CPU B are the same CPU (but
* again only if the system has more than one CPU).
*
* Of course, these guarantees apply only for invocations of call_srcu(),
* srcu_read_lock(), and srcu_read_unlock() that are all passed the same
* srcu_struct structure.
*/
void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
rcu_callback_t func, bool do_norm)
{
unsigned long flags;
bool needexp = false;
bool needgp = false;
unsigned long s;
struct srcu_data *sdp;
check_init_srcu_struct(sp);
if (debug_rcu_head_queue(rhp)) {
/* Probable double call_srcu(), so leak the callback. */
WRITE_ONCE(rhp->func, srcu_leak_callback);
WARN_ONCE(1, "call_srcu(): Leaked duplicate callback\n");
return;
}
rhp->func = func;
local_irq_save(flags);
sdp = this_cpu_ptr(sp->sda);
spin_lock(&sdp->lock);
rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp, false);
rcu_segcblist_advance(&sdp->srcu_cblist,
rcu_seq_current(&sp->srcu_gp_seq));
s = rcu_seq_snap(&sp->srcu_gp_seq);
(void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
sdp->srcu_gp_seq_needed = s;
needgp = true;
}
if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
sdp->srcu_gp_seq_needed_exp = s;
needexp = true;
}
spin_unlock_irqrestore(&sdp->lock, flags);
if (needgp)
srcu_funnel_gp_start(sp, sdp, s, do_norm);
else if (needexp)
srcu_funnel_exp_start(sp, sdp->mynode, s);
}
void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
rcu_callback_t func)
{
__call_srcu(sp, rhp, func, true);
}
EXPORT_SYMBOL_GPL(call_srcu);
/*
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
*/
static void __synchronize_srcu(struct srcu_struct *sp, bool do_norm)
{
struct rcu_synchronize rcu;
RCU_LOCKDEP_WARN(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 in RCU) read-side critical section");
if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
return;
might_sleep();
check_init_srcu_struct(sp);
init_completion(&rcu.completion);
init_rcu_head_on_stack(&rcu.head);
__call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm);
wait_for_completion(&rcu.completion);
destroy_rcu_head_on_stack(&rcu.head);
}
/**
* 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 synchronize_srcu_expedited() has the same deadlock and
* memory-ordering properties as does synchronize_srcu().
*/
void synchronize_srcu_expedited(struct srcu_struct *sp)
{
__synchronize_srcu(sp, rcu_gp_is_normal());
}
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
/**
* synchronize_srcu - wait for prior SRCU read-side critical-section completion
* @sp: srcu_struct with which to synchronize.
*
* Wait for the count to drain to zero of both indexes. To avoid the
* possible starvation of synchronize_srcu(), it waits for the count of
* the index=((->srcu_idx & 1) ^ 1) to drain to zero at first,
* and then flip the srcu_idx and wait for the count of the other index.
*
* 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,
* as long as the resulting graph of srcu_structs is acyclic.
*
* There are memory-ordering constraints implied by synchronize_srcu().
* On systems with more than one CPU, when synchronize_srcu() returns,
* each CPU is guaranteed to have executed a full memory barrier since
* the end of its last corresponding SRCU-sched read-side critical section
* whose beginning preceded the call to synchronize_srcu(). In addition,
* each CPU having an SRCU read-side critical section that extends beyond
* the return from synchronize_srcu() is guaranteed to have executed a
* full memory barrier after the beginning of synchronize_srcu() and before
* the beginning of that SRCU read-side critical section. Note that these
* guarantees include CPUs that are offline, idle, or executing in user mode,
* as well as CPUs that are executing in the kernel.
*
* Furthermore, if CPU A invoked synchronize_srcu(), which returned
* to its caller on CPU B, then both CPU A and CPU B are guaranteed
* to have executed a full memory barrier during the execution of
* synchronize_srcu(). This guarantee applies even if CPU A and CPU B
* are the same CPU, but again only if the system has more than one CPU.
*
* Of course, these memory-ordering guarantees apply only when
* synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
* passed the same srcu_struct structure.
*
* If SRCU is likely idle, expedite the first request. This semantic
* was provided by Classic SRCU, and is relied upon by its users, so TREE
* SRCU must also provide it. Note that detecting idleness is heuristic
* and subject to both false positives and negatives.
*/
void synchronize_srcu(struct srcu_struct *sp)
{
if (srcu_might_be_idle(sp) || rcu_gp_is_expedited())
synchronize_srcu_expedited(sp);
else
__synchronize_srcu(sp, true);
}
EXPORT_SYMBOL_GPL(synchronize_srcu);
/*
* Callback function for srcu_barrier() use.
*/
static void srcu_barrier_cb(struct rcu_head *rhp)
{
struct srcu_data *sdp;
struct srcu_struct *sp;
sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
sp = sdp->sp;
if (atomic_dec_and_test(&sp->srcu_barrier_cpu_cnt))
complete(&sp->srcu_barrier_completion);
}
/**
* srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
* @sp: srcu_struct on which to wait for in-flight callbacks.
*/
void srcu_barrier(struct srcu_struct *sp)
{
int cpu;
struct srcu_data *sdp;
unsigned long s = rcu_seq_snap(&sp->srcu_barrier_seq);
check_init_srcu_struct(sp);
mutex_lock(&sp->srcu_barrier_mutex);
if (rcu_seq_done(&sp->srcu_barrier_seq, s)) {
smp_mb(); /* Force ordering following return. */
mutex_unlock(&sp->srcu_barrier_mutex);
return; /* Someone else did our work for us. */
}
rcu_seq_start(&sp->srcu_barrier_seq);
init_completion(&sp->srcu_barrier_completion);
/* Initial count prevents reaching zero until all CBs are posted. */
atomic_set(&sp->srcu_barrier_cpu_cnt, 1);
/*
* Each pass through this loop enqueues a callback, but only
* on CPUs already having callbacks enqueued. Note that if
* a CPU already has callbacks enqueue, it must have already
* registered the need for a future grace period, so all we
* need do is enqueue a callback that will use the same
* grace period as the last callback already in the queue.
*/
for_each_possible_cpu(cpu) {
sdp = per_cpu_ptr(sp->sda, cpu);
spin_lock_irq(&sdp->lock);
atomic_inc(&sp->srcu_barrier_cpu_cnt);
sdp->srcu_barrier_head.func = srcu_barrier_cb;
debug_rcu_head_queue(&sdp->srcu_barrier_head);
if (!rcu_segcblist_entrain(&sdp->srcu_cblist,
&sdp->srcu_barrier_head, 0)) {
debug_rcu_head_unqueue(&sdp->srcu_barrier_head);
atomic_dec(&sp->srcu_barrier_cpu_cnt);
}
spin_unlock_irq(&sdp->lock);
}
/* Remove the initial count, at which point reaching zero can happen. */
if (atomic_dec_and_test(&sp->srcu_barrier_cpu_cnt))
complete(&sp->srcu_barrier_completion);
wait_for_completion(&sp->srcu_barrier_completion);
rcu_seq_end(&sp->srcu_barrier_seq);
mutex_unlock(&sp->srcu_barrier_mutex);
}
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.
*/
unsigned long srcu_batches_completed(struct srcu_struct *sp)
{
return sp->srcu_idx;
}
EXPORT_SYMBOL_GPL(srcu_batches_completed);
/*
* Core SRCU state machine. Push state bits of ->srcu_gp_seq
* to SRCU_STATE_SCAN2, and invoke srcu_gp_end() when scan has
* completed in that state.
*/
static void srcu_advance_state(struct srcu_struct *sp)
{
int idx;
mutex_lock(&sp->srcu_gp_mutex);
/*
* Because readers might be delayed for an extended period after
* fetching ->srcu_idx 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.
*
* The load-acquire ensures that we see the accesses performed
* by the prior grace period.
*/
idx = rcu_seq_state(smp_load_acquire(&sp->srcu_gp_seq)); /* ^^^ */
if (idx == SRCU_STATE_IDLE) {
spin_lock_irq(&sp->gp_lock);
if (ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed)) {
WARN_ON_ONCE(rcu_seq_state(sp->srcu_gp_seq));
spin_unlock_irq(&sp->gp_lock);
mutex_unlock(&sp->srcu_gp_mutex);
return;
}
idx = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
if (idx == SRCU_STATE_IDLE)
srcu_gp_start(sp);
spin_unlock_irq(&sp->gp_lock);
if (idx != SRCU_STATE_IDLE) {
mutex_unlock(&sp->srcu_gp_mutex);
return; /* Someone else started the grace period. */
}
}
if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
idx = 1 ^ (sp->srcu_idx & 1);
if (!try_check_zero(sp, idx, 1)) {
mutex_unlock(&sp->srcu_gp_mutex);
return; /* readers present, retry later. */
}
srcu_flip(sp);
rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
}
if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
/*
* SRCU read-side critical sections are normally short,
* so check at least twice in quick succession after a flip.
*/
idx = 1 ^ (sp->srcu_idx & 1);
if (!try_check_zero(sp, idx, 2)) {
mutex_unlock(&sp->srcu_gp_mutex);
return; /* readers present, retry later. */
}
srcu_gp_end(sp); /* Releases ->srcu_gp_mutex. */
}
}
/*
* 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. Note that needed memory barriers have been executed
* in this task's context by srcu_readers_active_idx_check().
*/
static void srcu_invoke_callbacks(struct work_struct *work)
{
bool more;
struct rcu_cblist ready_cbs;
struct rcu_head *rhp;
struct srcu_data *sdp;
struct srcu_struct *sp;
sdp = container_of(work, struct srcu_data, work.work);
sp = sdp->sp;
rcu_cblist_init(&ready_cbs);
spin_lock_irq(&sdp->lock);
smp_mb(); /* Old grace periods before callback invocation! */
rcu_segcblist_advance(&sdp->srcu_cblist,
rcu_seq_current(&sp->srcu_gp_seq));
if (sdp->srcu_cblist_invoking ||
!rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
spin_unlock_irq(&sdp->lock);
return; /* Someone else on the job or nothing to do. */
}
/* We are on the job! Extract and invoke ready callbacks. */
sdp->srcu_cblist_invoking = true;
rcu_segcblist_extract_done_cbs(&sdp->srcu_cblist, &ready_cbs);
spin_unlock_irq(&sdp->lock);
rhp = rcu_cblist_dequeue(&ready_cbs);
for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
debug_rcu_head_unqueue(rhp);
local_bh_disable();
rhp->func(rhp);
local_bh_enable();
}
/*
* Update counts, accelerate new callbacks, and if needed,
* schedule another round of callback invocation.
*/
spin_lock_irq(&sdp->lock);
rcu_segcblist_insert_count(&sdp->srcu_cblist, &ready_cbs);
(void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
rcu_seq_snap(&sp->srcu_gp_seq));
sdp->srcu_cblist_invoking = false;
more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
spin_unlock_irq(&sdp->lock);
if (more)
srcu_schedule_cbs_sdp(sdp, 0);
}
/*
* 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, unsigned long delay)
{
bool pushgp = true;
spin_lock_irq(&sp->gp_lock);
if (ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed)) {
if (!WARN_ON_ONCE(rcu_seq_state(sp->srcu_gp_seq))) {
/* All requests fulfilled, time to go idle. */
pushgp = false;
}
} else if (!rcu_seq_state(sp->srcu_gp_seq)) {
/* Outstanding request and no GP. Start one. */
srcu_gp_start(sp);
}
spin_unlock_irq(&sp->gp_lock);
if (pushgp)
queue_delayed_work(system_power_efficient_wq, &sp->work, delay);
}
/*
* This is the work-queue function that handles SRCU grace periods.
*/
void process_srcu(struct work_struct *work)
{
struct srcu_struct *sp;
sp = container_of(work, struct srcu_struct, work.work);
srcu_advance_state(sp);
srcu_reschedule(sp, srcu_get_delay(sp));
}
EXPORT_SYMBOL_GPL(process_srcu);
void srcutorture_get_gp_data(enum rcutorture_type test_type,
struct srcu_struct *sp, int *flags,
unsigned long *gpnum, unsigned long *completed)
{
if (test_type != SRCU_FLAVOR)
return;
*flags = 0;
*completed = rcu_seq_ctr(sp->srcu_gp_seq);
*gpnum = rcu_seq_ctr(sp->srcu_gp_seq_needed);
}
EXPORT_SYMBOL_GPL(srcutorture_get_gp_data);
static int __init srcu_bootup_announce(void)
{
pr_info("Hierarchical SRCU implementation.\n");
if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF)
pr_info("\tNon-default auto-expedite holdoff of %lu ns.\n", exp_holdoff);
return 0;
}
early_initcall(srcu_bootup_announce);
