440253c17f
The purpose of the upper bit of SRCU's per-CPU counters is to guarantee that no reasonable series of srcu_read_lock() and srcu_read_unlock() operations can return the value of the counter to its original value. This guarantee is require only after the index has been switched to the other set of counters, so at most one srcu_read_lock() can affect a given CPU's counter. The number of srcu_read_unlock() operations on a given counter is limited to the number of tasks in the system, which given the Linux kernel's current structure is limited to far less than 2^30 on 32-bit systems and far less than 2^62 on 64-bit systems. (Something about a limited number of bytes in the kernel's address space.) Therefore, if srcu_read_lock() increments the upper bits, then srcu_read_unlock() need not do so. In this case, an srcu_read_lock() and an srcu_read_unlock() will flip the lower bit of the upper field of the counter. An unreasonably large additional number of srcu_read_unlock() operations would be required to return the counter to its initial value, thus preserving the guarantee. This commit takes this approach, which further allows it to shrink the size of the upper field to one bit, making the number of srcu_read_unlock() operations required to return the counter to its initial value even more unreasonable than before. Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
246 lines
8.3 KiB
C
246 lines
8.3 KiB
C
/*
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* Sleepable Read-Copy Update mechanism for mutual exclusion
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright (C) IBM Corporation, 2006
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*
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* Author: Paul McKenney <paulmck@us.ibm.com>
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU/ *.txt
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*
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*/
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#ifndef _LINUX_SRCU_H
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#define _LINUX_SRCU_H
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#include <linux/mutex.h>
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#include <linux/rcupdate.h>
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struct srcu_struct_array {
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unsigned long c[2];
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};
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/* Bit definitions for field ->c above and ->snap below. */
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#define SRCU_USAGE_BITS 1
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#define SRCU_REF_MASK (ULONG_MAX >> SRCU_USAGE_BITS)
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#define SRCU_USAGE_COUNT (SRCU_REF_MASK + 1)
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struct srcu_struct {
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unsigned completed;
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struct srcu_struct_array __percpu *per_cpu_ref;
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struct mutex mutex;
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unsigned long snap[NR_CPUS];
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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struct lockdep_map dep_map;
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#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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};
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#ifndef CONFIG_PREEMPT
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#define srcu_barrier() barrier()
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#else /* #ifndef CONFIG_PREEMPT */
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#define srcu_barrier()
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#endif /* #else #ifndef CONFIG_PREEMPT */
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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int __init_srcu_struct(struct srcu_struct *sp, const char *name,
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struct lock_class_key *key);
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#define init_srcu_struct(sp) \
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({ \
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static struct lock_class_key __srcu_key; \
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\
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__init_srcu_struct((sp), #sp, &__srcu_key); \
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})
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#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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int init_srcu_struct(struct srcu_struct *sp);
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#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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void cleanup_srcu_struct(struct srcu_struct *sp);
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int __srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
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void __srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
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void synchronize_srcu(struct srcu_struct *sp);
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void synchronize_srcu_expedited(struct srcu_struct *sp);
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long srcu_batches_completed(struct srcu_struct *sp);
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#ifdef CONFIG_DEBUG_LOCK_ALLOC
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/**
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* srcu_read_lock_held - might we be in SRCU read-side critical section?
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*
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* If CONFIG_DEBUG_LOCK_ALLOC is selected, returns nonzero iff in an SRCU
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* read-side critical section. In absence of CONFIG_DEBUG_LOCK_ALLOC,
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* this assumes we are in an SRCU read-side critical section unless it can
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* prove otherwise.
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*
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* Checks debug_lockdep_rcu_enabled() to prevent false positives during boot
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* and while lockdep is disabled.
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*
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* Note that if the CPU is in the idle loop from an RCU point of view
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* (ie: that we are in the section between rcu_idle_enter() and
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* rcu_idle_exit()) then srcu_read_lock_held() returns false even if
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* the CPU did an srcu_read_lock(). The reason for this is that RCU
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* ignores CPUs that are in such a section, considering these as in
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* extended quiescent state, so such a CPU is effectively never in an
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* RCU read-side critical section regardless of what RCU primitives it
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* invokes. This state of affairs is required --- we need to keep an
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* RCU-free window in idle where the CPU may possibly enter into low
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* power mode. This way we can notice an extended quiescent state to
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* other CPUs that started a grace period. Otherwise we would delay any
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* grace period as long as we run in the idle task.
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*
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* Similarly, we avoid claiming an SRCU read lock held if the current
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* CPU is offline.
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*/
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static inline int srcu_read_lock_held(struct srcu_struct *sp)
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{
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if (!debug_lockdep_rcu_enabled())
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return 1;
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if (rcu_is_cpu_idle())
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return 0;
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if (!rcu_lockdep_current_cpu_online())
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return 0;
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return lock_is_held(&sp->dep_map);
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}
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#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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static inline int srcu_read_lock_held(struct srcu_struct *sp)
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{
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return 1;
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}
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#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
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/**
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* srcu_dereference_check - fetch SRCU-protected pointer for later dereferencing
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* @p: the pointer to fetch and protect for later dereferencing
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* @sp: pointer to the srcu_struct, which is used to check that we
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* really are in an SRCU read-side critical section.
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* @c: condition to check for update-side use
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*
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* If PROVE_RCU is enabled, invoking this outside of an RCU read-side
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* critical section will result in an RCU-lockdep splat, unless @c evaluates
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* to 1. The @c argument will normally be a logical expression containing
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* lockdep_is_held() calls.
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*/
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#define srcu_dereference_check(p, sp, c) \
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__rcu_dereference_check((p), srcu_read_lock_held(sp) || (c), __rcu)
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/**
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* srcu_dereference - fetch SRCU-protected pointer for later dereferencing
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* @p: the pointer to fetch and protect for later dereferencing
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* @sp: pointer to the srcu_struct, which is used to check that we
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* really are in an SRCU read-side critical section.
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*
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* Makes rcu_dereference_check() do the dirty work. If PROVE_RCU
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* is enabled, invoking this outside of an RCU read-side critical
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* section will result in an RCU-lockdep splat.
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*/
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#define srcu_dereference(p, sp) srcu_dereference_check((p), (sp), 0)
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/**
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* srcu_read_lock - register a new reader for an SRCU-protected structure.
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* @sp: srcu_struct in which to register the new reader.
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*
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* Enter an SRCU read-side critical section. Note that SRCU read-side
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* critical sections may be nested. However, it is illegal to
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* call anything that waits on an SRCU grace period for the same
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* srcu_struct, whether directly or indirectly. Please note that
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* one way to indirectly wait on an SRCU grace period is to acquire
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* a mutex that is held elsewhere while calling synchronize_srcu() or
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* synchronize_srcu_expedited().
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*
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* Note that srcu_read_lock() and the matching srcu_read_unlock() must
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* occur in the same context, for example, it is illegal to invoke
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* srcu_read_unlock() in an irq handler if the matching srcu_read_lock()
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* was invoked in process context.
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*/
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static inline int srcu_read_lock(struct srcu_struct *sp) __acquires(sp)
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{
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int retval = __srcu_read_lock(sp);
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rcu_lock_acquire(&(sp)->dep_map);
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rcu_lockdep_assert(!rcu_is_cpu_idle(),
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"srcu_read_lock() used illegally while idle");
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return retval;
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}
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/**
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* srcu_read_unlock - unregister a old reader from an SRCU-protected structure.
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* @sp: srcu_struct in which to unregister the old reader.
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* @idx: return value from corresponding srcu_read_lock().
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*
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* Exit an SRCU read-side critical section.
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*/
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static inline void srcu_read_unlock(struct srcu_struct *sp, int idx)
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__releases(sp)
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{
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rcu_lockdep_assert(!rcu_is_cpu_idle(),
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"srcu_read_unlock() used illegally while idle");
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rcu_lock_release(&(sp)->dep_map);
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__srcu_read_unlock(sp, idx);
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}
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/**
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* srcu_read_lock_raw - register a new reader for an SRCU-protected structure.
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* @sp: srcu_struct in which to register the new reader.
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*
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* Enter an SRCU read-side critical section. Similar to srcu_read_lock(),
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* but avoids the RCU-lockdep checking. This means that it is legal to
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* use srcu_read_lock_raw() in one context, for example, in an exception
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* handler, and then have the matching srcu_read_unlock_raw() in another
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* context, for example in the task that took the exception.
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*
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* However, the entire SRCU read-side critical section must reside within a
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* single task. For example, beware of using srcu_read_lock_raw() in
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* a device interrupt handler and srcu_read_unlock() in the interrupted
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* task: This will not work if interrupts are threaded.
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*/
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static inline int srcu_read_lock_raw(struct srcu_struct *sp)
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{
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unsigned long flags;
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int ret;
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local_irq_save(flags);
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ret = __srcu_read_lock(sp);
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local_irq_restore(flags);
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return ret;
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}
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/**
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* srcu_read_unlock_raw - unregister reader from an SRCU-protected structure.
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* @sp: srcu_struct in which to unregister the old reader.
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* @idx: return value from corresponding srcu_read_lock_raw().
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*
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* Exit an SRCU read-side critical section without lockdep-RCU checking.
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* See srcu_read_lock_raw() for more details.
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*/
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static inline void srcu_read_unlock_raw(struct srcu_struct *sp, int idx)
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{
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unsigned long flags;
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local_irq_save(flags);
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__srcu_read_unlock(sp, idx);
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local_irq_restore(flags);
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}
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#endif
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