diff options
| -rw-r--r-- | Documentation/RCU/checklist.txt | 38 | ||||
| -rw-r--r-- | Documentation/RCU/rcu.txt | 3 | ||||
| -rw-r--r-- | Documentation/RCU/whatisRCU.txt | 3 | ||||
| -rw-r--r-- | include/linux/srcu.h | 49 | ||||
| -rw-r--r-- | kernel/Makefile | 2 | ||||
| -rw-r--r-- | kernel/srcu.c | 257 |
6 files changed, 350 insertions, 2 deletions
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 1d50cf0..f4dffad 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -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. diff --git a/Documentation/RCU/rcu.txt b/Documentation/RCU/rcu.txt index 02e27bf..f84407c 100644 --- a/Documentation/RCU/rcu.txt +++ b/Documentation/RCU/rcu.txt @@ -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? diff --git a/Documentation/RCU/whatisRCU.txt b/Documentation/RCU/whatisRCU.txt index 820fee2..e0d6d99 100644 --- a/Documentation/RCU/whatisRCU.txt +++ b/Documentation/RCU/whatisRCU.txt @@ -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 diff --git a/include/linux/srcu.h b/include/linux/srcu.h new file mode 100644 index 0000000..947fdab --- /dev/null +++ b/include/linux/srcu.h @@ -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); diff --git a/kernel/Makefile b/kernel/Makefile index d948ca1..5e3f3b7 100644 --- a/kernel/Makefile +++ b/kernel/Makefile @@ -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/ diff --git a/kernel/srcu.c b/kernel/srcu.c new file mode 100644 index 0000000..7e1979f --- /dev/null +++ b/kernel/srcu.c @@ -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); |