RCU (Read-Copy Update) 的实现机制

源代码摘自Linux内核2.6.24：

// Read-Copy Update mechanism for mutual exclusion 

所包含的头文件：

#include <linux/cache.h>#include <linux/spinlock.h>#include <linux/threads.h>#include <linux/percpu.h>#include <linux/cpumask.h>#include <linux/seqlock.h>#include <linux/lockdep.h>

rcu_head结构体：

// struct rcu_head - callback structure for use with RCU// @next: next update requests in a list//@func: actual update function to call after the grace period.struct rcu_head {struct rcu_head *next; //列表中的下一个请求项void (*func)(struct rcu_head *head);//};

哈哈

#define RCU_HEAD_INIT { .next = NULL, .func = NULL }#define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT#define INIT_RCU_HEAD(ptr) do { \       (ptr)->next = NULL; (ptr)->func = NULL; \} while (0)/* Global control variables for rcupdate callback mechanism. */struct rcu_ctrlblk {longcur;/* Current batch number.                      */longcompleted;/* Number of the last completed batch         */intnext_pending;/* Is the next batch already waiting?         */intsignaled;spinlock_tlock____cacheline_internodealigned_in_smp;cpumask_tcpumask; /* CPUs that need to switch in order    */                         /* for current batch to proceed.        */} ____cacheline_internodealigned_in_smp;/* Is batch a before batch b ? */static inline int rcu_batch_before(long a, long b){        return (a - b) < 0;}/* Is batch a after batch b ? */static inline int rcu_batch_after(long a, long b){        return (a - b) > 0;}/* * Per-CPU data for Read-Copy UPdate. * nxtlist - new callbacks are added here * curlist - current batch for which quiescent cycle started if any */struct rcu_data {/* 1) quiescent state handling : */longquiescbatch;     /* Batch # for grace period */intpassed_quiesc; /* User-mode/idle loop etc. */intqs_pending; /* core waits for quiesc state *//* 2) batch handling */long         batch;           /* Batch # for current RCU batch */struct rcu_head *nxtlist;struct rcu_head **nxttail;long            qlen;   /* # of queued callbacks */struct rcu_head *curlist;struct rcu_head **curtail;struct rcu_head *donelist;struct rcu_head **donetail;longblimit; /* Upper limit on a processed batch */int cpu;struct rcu_head barrier;};DECLARE_PER_CPU(struct rcu_data, rcu_data);DECLARE_PER_CPU(struct rcu_data, rcu_bh_data);/* * Increment the quiescent state counter. * The counter is a bit degenerated: We do not need to know * how many quiescent states passed, just if there was at least * one since the start of the grace period. Thus just a flag. */static inline void rcu_qsctr_inc(int cpu){struct rcu_data *rdp = &per_cpu(rcu_data, cpu);rdp->passed_quiesc = 1;}static inline void rcu_bh_qsctr_inc(int cpu){struct rcu_data *rdp = &per_cpu(rcu_bh_data, cpu);rdp->passed_quiesc = 1;}extern int rcu_pending(int cpu);extern int rcu_needs_cpu(int cpu);#ifdef CONFIG_DEBUG_LOCK_ALLOCextern struct lockdep_map rcu_lock_map;# define rcu_read_acquire()lock_acquire(&rcu_lock_map, 0, 0, 2, 1, _THIS_IP_)# define rcu_read_release()lock_release(&rcu_lock_map, 1, _THIS_IP_)#else# define rcu_read_acquire()do { } while (0)# define rcu_read_release()do { } while (0)#endif/** * rcu_read_lock - mark the beginning of an RCU read-side critical section. * * When synchronize_rcu() is invoked on one CPU while other CPUs * are within RCU read-side critical sections, then the * synchronize_rcu() is guaranteed to block until after all the other * CPUs exit their critical sections.  Similarly, if call_rcu() is invoked * on one CPU while other CPUs are within RCU read-side critical * sections, invocation of the corresponding RCU callback is deferred * until after the all the other CPUs exit their critical sections. * * Note, however, that RCU callbacks are permitted to run concurrently * with RCU read-side critical sections.  One way that this can happen * is via the following sequence of events: (1) CPU 0 enters an RCU * read-side critical section, (2) CPU 1 invokes call_rcu() to register * an RCU callback, (3) CPU 0 exits the RCU read-side critical section, * (4) CPU 2 enters a RCU read-side critical section, (5) the RCU * callback is invoked.  This is legal, because the RCU read-side critical * section that was running concurrently with the call_rcu() (and which * therefore might be referencing something that the corresponding RCU * callback would free up) has completed before the corresponding * RCU callback is invoked. * * RCU read-side critical sections may be nested.  Any deferred actions * will be deferred until the outermost RCU read-side critical section * completes. * * It is illegal to block while in an RCU read-side critical section. */#define rcu_read_lock() \do { \preempt_disable(); \__acquire(RCU); \rcu_read_acquire(); \} while(0)/** * rcu_read_unlock - marks the end of an RCU read-side critical section. * * See rcu_read_lock() for more information. */#define rcu_read_unlock() \do { \rcu_read_release(); \__release(RCU); \preempt_enable(); \} while(0)/* * So where is rcu_write_lock()?  It does not exist, as there is no * way for writers to lock out RCU readers.  This is a feature, not * a bug -- this property is what provides RCU's performance benefits. * Of course, writers must coordinate with each other.  The normal * spinlock primitives work well for this, but any other technique may be * used as well.  RCU does not care how the writers keep out of each * others' way, as long as they do so. *//** * rcu_read_lock_bh - mark the beginning of a softirq-only RCU critical section * * This is equivalent of rcu_read_lock(), but to be used when updates * are being done using call_rcu_bh(). Since call_rcu_bh() callbacks * consider completion of a softirq handler to be a quiescent state, * a process in RCU read-side critical section must be protected by * disabling softirqs. Read-side critical sections in interrupt context * can use just rcu_read_lock(). * */#define rcu_read_lock_bh() \do { \local_bh_disable(); \__acquire(RCU_BH); \rcu_read_acquire(); \} while(0)/* * rcu_read_unlock_bh - marks the end of a softirq-only RCU critical section * * See rcu_read_lock_bh() for more information. */#define rcu_read_unlock_bh() \do { \rcu_read_release(); \__release(RCU_BH); \local_bh_enable(); \} while(0)/* * Prevent the compiler from merging or refetching accesses.  The compiler * is also forbidden from reordering successive instances of ACCESS_ONCE(), * but only when the compiler is aware of some particular ordering.  One way * to make the compiler aware of ordering is to put the two invocations of * ACCESS_ONCE() in different C statements. * * This macro does absolutely -nothing- to prevent the CPU from reordering, * merging, or refetching absolutely anything at any time. */#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))/** * rcu_dereference - fetch an RCU-protected pointer in an * RCU read-side critical section.  This pointer may later * be safely dereferenced. * * Inserts memory barriers on architectures that require them * (currently only the Alpha), and, more importantly, documents * exactly which pointers are protected by RCU. */#define rcu_dereference(p)     ({ \typeof(p) _________p1 = ACCESS_ONCE(p); \smp_read_barrier_depends(); \(_________p1); \})/** * rcu_assign_pointer - assign (publicize) a pointer to a newly * initialized structure that will be dereferenced by RCU read-side * critical sections.  Returns the value assigned. * * Inserts memory barriers on architectures that require them * (pretty much all of them other than x86), and also prevents * the compiler from reordering the code that initializes the * structure after the pointer assignment.  More importantly, this * call documents which pointers will be dereferenced by RCU read-side * code. */#define rcu_assign_pointer(p, v)({ \smp_wmb(); \(p) = (v); \})/** * synchronize_sched - block until all CPUs have exited any non-preemptive * kernel code sequences. * * This means that all preempt_disable code sequences, including NMI and * hardware-interrupt handlers, in progress on entry will have completed * before this primitive returns.  However, this does not guarantee that * softirq handlers will have completed, since in some kernels, these * handlers can run in process context, and can block. * * This primitive provides the guarantees made by the (now removed) * synchronize_kernel() API.  In contrast, synchronize_rcu() only * guarantees that rcu_read_lock() sections will have completed. * In "classic RCU", these two guarantees happen to be one and * the same, but can differ in realtime RCU implementations. */#define synchronize_sched() synchronize_rcu()extern void rcu_init(void);extern void rcu_check_callbacks(int cpu, int user);extern void rcu_restart_cpu(int cpu);extern long rcu_batches_completed(void);extern long rcu_batches_completed_bh(void);/* Exported interfaces */extern void FASTCALL(call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *head)));extern void FASTCALL(call_rcu_bh(struct rcu_head *head,void (*func)(struct rcu_head *head)));extern void synchronize_rcu(void);extern void rcu_barrier(void);#endif /* __KERNEL__ */#endif /* __LINUX_RCUPDATE_H */

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