18108ebfeb
The safety of SRCU is provided byy wait_idx() rather than flipping. The flipping actually prevents starvation. This commit therefore updates the comments to more accurately and precisely describe what is going on. Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
416 lines
14 KiB
C
416 lines
14 KiB
C
/*
|
|
* 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
|
|
|
|
/*
|
|
* Wait until all pre-existing readers complete. Such readers
|
|
* will have used the index specified by "idx".
|
|
*/
|
|
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 */
|
|
}
|
|
|
|
static void srcu_flip(struct srcu_struct *sp)
|
|
{
|
|
sp->completed++;
|
|
}
|
|
|
|
/*
|
|
* Helper function for synchronize_srcu() and synchronize_srcu_expedited().
|
|
*/
|
|
static void __synchronize_srcu(struct srcu_struct *sp, bool expedited)
|
|
{
|
|
int busy_idx;
|
|
|
|
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);
|
|
busy_idx = sp->completed & 0X1UL;
|
|
|
|
/*
|
|
* If we recently flipped the index, there will be some readers
|
|
* using idx=0 and others using idx=1. Therefore, two calls to
|
|
* wait_idx()s suffice to ensure that all pre-existing readers
|
|
* have completed:
|
|
*
|
|
* __synchronize_srcu() {
|
|
* wait_idx(sp, 0, expedited);
|
|
* wait_idx(sp, 1, expedited);
|
|
* }
|
|
*
|
|
* Starvation is prevented by the fact that we flip the index.
|
|
* While we wait on one index to clear out, almost all new readers
|
|
* will be using the other index. The number of new readers using the
|
|
* index we are waiting on is sharply bounded by roughly the number
|
|
* of CPUs.
|
|
*
|
|
* How can new readers possibly using the old pre-flip value of
|
|
* the index? Consider the following sequence of events:
|
|
*
|
|
* 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, this sequence of events is quite improbable, so
|
|
* this call to wait_idx(), which waits on really old readers
|
|
* describe in this comment above, will almost never need to wait.
|
|
*/
|
|
wait_idx(sp, 1 - busy_idx, expedited);
|
|
|
|
/* Flip the index to avoid reader-induced starvation. */
|
|
srcu_flip(sp);
|
|
|
|
/* Wait for recent pre-existing readers. */
|
|
wait_idx(sp, busy_idx, 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);
|