对linux rcu机制的理解.doc

对linux rcu机制的理解RCU（ Read-copy update）锁机制是kernel2.6的重大进步，使用rcu可以获得比使用rwlock更高的性能，而且代码简单，不易死锁。Linux 文档如下描述：So the typical RCU update sequence goes something like the following:a.Remove pointers to a data structure, so that subsequentreaders cannot gain a reference to it.b.Wait for all previous readers to complete their RCU read-sidecritical sections.c.At this point, there cannot be any readers who hold referencesto the data structure, so it now may safely be reclaimed(e.g., kfree()d).如上所述，RCU原理其实很简单，现在假设你自己要实现一个基于rcu原理的模型。假定cpu处理报文的过程是原子的，这样可以认为处理完一个报文就经过了一个quiescent state，RCU引用的过时数据就可以释放了。接收报文开始进入一个新的周期，当发送报文后，标记一个周期的结束。假设有8个cpu，轮流处理报文，而报文处理依赖的信息存储在共享内存中。那末，现在的问题是：请用rcu机制实现共享内存的释放。假设所有的cpu处理一个报文结束，就代表一个grace period (因为只有处理报文时才会使用共享内存)，即每个cpu都经过了一个quiscent state . 那么现在就有三个问题：1 如何确定所有的cpu都经过了一个quiscent state?2 在rcu处理时，如何保证别的cpu不对要释放的数据操作？3 在rcu处理后，如何开启和判断下一个rcu周期？带着这三个问题，我们可以看一下linux是如何实现rcu机制的。以下的分析是基于linux-2.6.27代码：Rcu有两个重要的数据结构：/* 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;上述结构是rcu全局控制结构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;上述结构是每个cpu用的rcu数据结构。DECLARE_PER_CPU(struct rcu_data, rcu_data);static struct rcu_ctrlblk rcu_ctrlblk = .cur = -300,.completed = -300,.lock = _SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),.cpumask = CPU_MASK_NONE,;Linux在rcu代码中声明了两个全局变量，一个是rcu_data，每个cpu一个；一个是rcu_ctrlblk，全局一个。Linux rcu 处理流程图：Linux rcu 状态转换图一：Init-start new period，注意rdp，rcp的数据变化Linux rcu 状态转换图二最后引用rcu代码中的一段注释作为总结/* * Grace period handling: * The grace period handling consists out of two steps: * - A new grace period is started. * This is done by rcu_start_batch. The start is not broadcasted to * all cpus, they must pick this up by comparing rcp-cur with * rdp-quiescbatch. All cpus are recorded in the * rcu_ctrlblk.cpumask bitmap. * - All cpus must go through a quiescent state. * Since the start of the grace period is not broadcasted, at least two * calls to rcu_check_quiescent_state are required: * The first call just notices that a new grace period is running. The * following calls check if there was a quiescent state since the beginning * of the grace period. If so, it updates rcu_ctrlblk.cpumask. If * the