/*
* 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>
static int init_srcu_struct_fields(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;
}
#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 number of readers active on the specified rank
* of per-CPU counters. Also snapshots each counter's value in the
* corresponding element of sp->snap[] for later use validating
* the sum.
*/
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;
sp->snap[cpu] = t;
}
return sum & SRCU_REF_MASK;
}
/*
* To be called from the update side after an index flip. Returns true
* if the modulo sum of the counters is stably zero, false if there is
* some possibility of non-zero.
*/
static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
{
int cpu;
/*
* 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 modulo 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;
/*
* Since the caller recently flipped ->completed, we can see at
* most one increment of each CPU's counter from this point
* forward. The reason for this is that the reader CPU must have
* fetched the index before srcu_readers_active_idx checked
* that CPU's counter, but not yet incremented its counter.
* Its eventual counter increment will follow the read in
* srcu_readers_active_idx(), and that increment is immediately
* followed by smp_mb() B. Because smp_mb() D is between
* the ->completed flip and srcu_readers_active_idx()'s read,
* that CPU's subsequent load of ->completed must see the new
* value, and therefore increment the counter in the other rank.
*/
smp_mb(); /* A */
/*
* Now, we check the ->snap array that srcu_readers_active_idx()
* filled in from the per-CPU counter values. Since
* __srcu_read_lock() increments the upper bits of the per-CPU
* counter, an increment/decrement pair will change the value
* of the counter. Since there is only one possible increment,
* the only way to wrap the counter is to have a huge number of
* counter decrements, which requires a huge number of tasks and
* huge SRCU read-side critical-section nesting levels, even on
* 32-bit systems.
*
* All of the ways of confusing the readings require that the scan
* in srcu_readers_active_idx() see the read-side task's decrement,
* but not its increment. However, between that decrement and
* increment are smb_mb() B and C. Either or both of these pair
* with smp_mb() A above to ensure that the scan below will see
* the read-side tasks's increment, thus noting a difference in
* the counter values between the two passes.
*
* Therefore, if srcu_readers_active_idx() returned zero, and
* none of the counters changed, we know that the zero was the
* correct sum.
*
* Of course, it is possible that a task might be delayed
* for a very long time in __srcu_read_lock() after fetching
* the index but before incrementing its counter. This
* possibility will be dealt with in __synchronize_srcu().
*/
for_each_possible_cpu(cpu)
if (sp->snap[cpu] !=
ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]))
return false; /* False zero reading! */
return true;
}
/**
* 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;
}
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]) +=
SRCU_USAGE_COUNT + 1;
smp_mb(); /* B */ /* Avoid leaking the critical section. */
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 SYNCHRONIZE_SRCU_READER_DELAY 5
static void wait_idx(struct srcu_struct *sp, int idx, bool expedited)
{
int trycount = 0;
/*
* If a reader fetches the index before the ->completed increment,
* but increments its counter after srcu_readers_active_idx_check()
* sums it, then smp_mb() D will pair with __srcu_read_lock()'s
* smp_mb() B to ensure that the SRCU read-side critical section
* will see any updates that the current task performed before its
* call to synchronize_srcu(), or to synchronize_srcu_expedited(),
* as the case may be.
*/
smp_mb(); /* D */
/*
* SRCU read-side critical sections are normally short, so wait
* a small amount of time before possibly blocking.
*/
if (!srcu_readers_active_idx_check(sp, idx)) {
udelay(SYNCHRONIZE_SRCU_READER_DELAY);
while (!srcu_readers_active_idx_check(sp, idx)) {
if (expedited && ++ trycount < 10)
udelay(SYNCHRONIZE_SRCU_READER_DELAY);
else
schedule_timeout_interruptible(1);
}
}
/*
* The following smp_mb() E pairs with srcu_read_unlock()'s
* smp_mb C to ensure that if srcu_readers_active_idx_check()
* sees srcu_read_unlock()'s counter decrement, then any
* of the current task's subsequent code will happen after
* that SRCU read-side critical section.
*
* It also ensures the order between the above waiting and
* the next flipping.
*/
smp_mb(); /* E */
}
/*
* Flip the readers' index by incrementing ->completed, then wait
* until there are no more readers using the counters referenced by
* the old index value. (Recall that the index is the bottom bit
* of ->completed.)
*
* Of course, it is possible that a reader might be delayed for the
* full duration of flip_idx_and_wait() between fetching the
* index and incrementing its counter. This possibility is handled
* by the next __synchronize_srcu() invoking wait_idx() for such readers
* before starting a new grace period.
*/
static void flip_idx_and_wait(struct srcu_struct *sp, bool expedited)
{
int idx;
idx = sp->completed++ & 0x1;
wait_idx(sp, idx, expedited);
}
/*
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
*/
static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
{
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");
mutex_lock(&sp->mutex);
/*
* Suppose that during the previous grace period, a reader
* picked up the old value of the index, but did not increment
* its counter until after the previous instance of
* __synchronize_srcu() did the counter summation and recheck.
* That previous grace period was OK because the reader did
* not start until after the grace period started, so the grace
* period was not obligated to wait for that reader.
*
* However, the current SRCU grace period does have to wait for
* that reader. This is handled by invoking wait_idx() on the
* non-active set of counters (hence sp->completed - 1). Once
* wait_idx() returns, we know that all readers that picked up
* the old value of ->completed and that already incremented their
* counter will have completed.
*
* But what about readers that picked up the old value of
* ->completed, but -still- have not managed to increment their
* counter? We do not need to wait for those readers, because
* they will have started their SRCU read-side critical section
* after the current grace period starts.
*
* Because it is unlikely that readers will be preempted between
* fetching ->completed and incrementing their counter, wait_idx()
* will normally not need to wait.
*/
wait_idx(sp, (sp->completed - 1) & 0x1, expedited);
/*
* Now that wait_idx() has waited for the really old readers,
* invoke flip_idx_and_wait() to flip the counter and wait
* for current SRCU readers.
*/
flip_idx_and_wait(sp, expedited);
mutex_unlock(&sp->mutex);
}
/**
* 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, 0);
}
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, 1);
}
EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
/**
* 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);
