[PATCH] srcu-3: RCU variant permitting read-side blocking

Updated patch adding a variant of RCU that permits sleeping in read-side
critical sections.  SRCU is as follows:

o	Each use of SRCU creates its own srcu_struct, and each
	srcu_struct has its own set of grace periods.  This is
	critical, as it prevents one subsystem with a blocking
	reader from holding up SRCU grace periods for other
	subsystems.

o	The SRCU primitives (srcu_read_lock(), srcu_read_unlock(),
	and synchronize_srcu()) all take a pointer to a srcu_struct.

o	The SRCU primitives must be called from process context.

o	srcu_read_lock() returns an int that must be passed to
	the matching srcu_read_unlock().  Realtime RCU avoids the
	need for this by storing the state in the task struct,
	but SRCU needs to allow a given code path to pass through
	multiple SRCU domains -- storing state in the task struct
	would therefore require either arbitrary space in the
	task struct or arbitrary limits on SRCU nesting.  So I
	kicked the state-storage problem up to the caller.

	Of course, it is not permitted to call synchronize_srcu()
	while in an SRCU read-side critical section.

o	There is no call_srcu().  It would not be hard to implement
	one, but it seems like too easy a way to OOM the system.
	(Hey, we have enough trouble with call_rcu(), which does
	-not- permit readers to sleep!!!)  So, if you want it,
	please tell me why...

[josht@us.ibm.com: sparse notation]
Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com>
Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Paul E. McKenney 2006-10-04 02:17:02 -07:00 committed by Linus Torvalds
parent 95d77884c7
commit 621934ee7e
6 changed files with 350 additions and 2 deletions

View file

@ -221,3 +221,41 @@ over a rather long period of time, but improvements are always welcome!
disable irq on a given acquisition of that lock will result in
deadlock as soon as the RCU callback happens to interrupt that
acquisition's critical section.
13. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())
may only be invoked from process context. Unlike other forms of
RCU, it -is- permissible to block in an SRCU read-side critical
section (demarked by srcu_read_lock() and srcu_read_unlock()),
hence the "SRCU": "sleepable RCU". Please note that if you
don't need to sleep in read-side critical sections, you should
be using RCU rather than SRCU, because RCU is almost always
faster and easier to use than is SRCU.
Also unlike other forms of RCU, explicit initialization
and cleanup is required via init_srcu_struct() and
cleanup_srcu_struct(). These are passed a "struct srcu_struct"
that defines the scope of a given SRCU domain. Once initialized,
the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
and synchronize_srcu(). A given synchronize_srcu() waits only
for SRCU read-side critical sections governed by srcu_read_lock()
and srcu_read_unlock() calls that have been passd the same
srcu_struct. This property is what makes sleeping read-side
critical sections tolerable -- a given subsystem delays only
its own updates, not those of other subsystems using SRCU.
Therefore, SRCU is less prone to OOM the system than RCU would
be if RCU's read-side critical sections were permitted to
sleep.
The ability to sleep in read-side critical sections does not
come for free. First, corresponding srcu_read_lock() and
srcu_read_unlock() calls must be passed the same srcu_struct.
Second, grace-period-detection overhead is amortized only
over those updates sharing a given srcu_struct, rather than
being globally amortized as they are for other forms of RCU.
Therefore, SRCU should be used in preference to rw_semaphore
only in extremely read-intensive situations, or in situations
requiring SRCU's read-side deadlock immunity or low read-side
realtime latency.
Note that, rcu_assign_pointer() and rcu_dereference() relate to
SRCU just as they do to other forms of RCU.

View file

@ -45,7 +45,8 @@ o How can I see where RCU is currently used in the Linux kernel?
Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
"synchronize_rcu", and "synchronize_net".
"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
"synchronize_net", and "synchronize_srcu".
o What guidelines should I follow when writing code that uses RCU?

View file

@ -778,6 +778,8 @@ Markers for RCU read-side critical sections:
rcu_read_unlock
rcu_read_lock_bh
rcu_read_unlock_bh
srcu_read_lock
srcu_read_unlock
RCU pointer/list traversal:
@ -804,6 +806,7 @@ RCU grace period:
synchronize_net
synchronize_sched
synchronize_rcu
synchronize_srcu
call_rcu
call_rcu_bh

49
include/linux/srcu.h Normal file
View file

@ -0,0 +1,49 @@
/*
* 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
*
*/
struct srcu_struct_array {
int c[2];
};
struct srcu_struct {
int completed;
struct srcu_struct_array *per_cpu_ref;
struct mutex mutex;
};
#ifndef CONFIG_PREEMPT
#define srcu_barrier() barrier()
#else /* #ifndef CONFIG_PREEMPT */
#define srcu_barrier()
#endif /* #else #ifndef CONFIG_PREEMPT */
void init_srcu_struct(struct srcu_struct *sp);
void cleanup_srcu_struct(struct srcu_struct *sp);
int srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
void synchronize_srcu(struct srcu_struct *sp);
long srcu_batches_completed(struct srcu_struct *sp);
void cleanup_srcu_struct(struct srcu_struct *sp);

View file

@ -8,7 +8,7 @@ obj-y = sched.o fork.o exec_domain.o panic.o printk.o profile.o \
signal.o sys.o kmod.o workqueue.o pid.o \
rcupdate.o extable.o params.o posix-timers.o \
kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
hrtimer.o rwsem.o latency.o nsproxy.o
hrtimer.o rwsem.o latency.o nsproxy.o srcu.o
obj-$(CONFIG_STACKTRACE) += stacktrace.o
obj-y += time/

257
kernel/srcu.c Normal file
View file

@ -0,0 +1,257 @@
/*
* 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.
*/
void init_srcu_struct(struct srcu_struct *sp)
{
sp->completed = 0;
sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
mutex_init(&sp->mutex);
}
/*
* 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);