关于RCU-sched的研究

对于synchronize_rcu原理的研究，在现在的源码中发现：

 * 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.

也就是说一个CPU的synchronize_rcu需要等待其他所有CPU都脱离RCU read-side临界区！

 * In non-preemptible RCU implementations (pure TREE_RCU and TINY_RCU),* it is illegal to block while in an RCU read-side critical section.* In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPTION* kernel builds, RCU read-side critical sections may be preempted,* but explicit blocking is illegal.  Finally, in preemptible RCU* implementations in real-time (with -rt patchset) kernel builds, RCU* read-side critical sections may be preempted and they may also block, but* only when acquiring spinlocks that are subject to priority inheritance.

也就是说其实是有可抢占版的RCU的！

可抢占的RCU

如果config文件定义了CONFIG_TREE_PREEMPT_RCU=y，那么sychronize_rcu将默认使用rcu_preempt_state（sychronize_rcu）。这类rcu的特点就在于read_lock期间是允许其它进程抢占的，因此它判断宽限期度过的方法就不太一样。

从rcu_read_lock和rcu_read_unlock的定义就可以知道，TREE_PREEMPT_RCU并不是以简单的经过抢占为CPU渡过GP的标准，而是有个rcu_read_lock_nesting计数

在lwn文章中详细介绍了不同的内核配置导致RCU的不同：

CONFIG_PREEMPT=n and CONFIG_SMP=y implies CONFIG_TREE_RCU, selecting the non-preemptible tree-based RCU implementation that is appropriate for server-class SMP builds. It can accommodate a very large number of CPUs, but scales down sufficiently well for all but the most memory-constrained systems.
CONFIG_PREEMPT=y implies CONFIG_TREE_PREEMPT_RCU, selecting the preemptible tree-based RCU implementation that is appropriate for real-time and low-latency SMP builds. It can also accommodate a very large number of CPUs, and also scales down sufficiently well for all but the most memory-constrained systems. The boot parameters for CONFIG_TREE_RCU also apply to CONFIG_TREE_PREEMPT_RCU.
CONFIG_PREEMPT=n and CONFIG_SMP=n implies CONFIG_TINY_RCU, selecting the non-preemptible uniprocessor RCU implementation that is appropriate for non-real-time UP builds. It has the smallest memory footprint of any of the current in-kernel RCU implementations. In fact, its memory footprint is so small that it doesn’t even have any kernel boot parameters.

也就是说，CONFIG_PREEMPT=n内核用的是不可抢占RCU，而CONFIG_PREEMPT=y implies CONFIG_TREE_PREEMPT_RCU，所以CONFIG_PREEMPT=y用的是可抢占的RCU。查了下我自己的电脑，是CONFIG_PREEMPT_VOLUNTARY=y，因此用的是不可抢占内核！对于不可抢占的RCU来说，RCU和RCU-Sched是一样的，因为反正都是不可抢占。而对于可抢占的RCU来说，RCU是可以抢占的，而RCU-Sched是不可抢占！官方文档中介绍了Sched-Favor的RCU：
“Note well that in CONFIG_PREEMPT=y kernels, rcu_read_lock_sched() and rcu_read_unlock_sched() disable and re-enable preemption, respectively.” 也就是说，RCU-sched在CONFIG_PREEMPT=y的情况下，rcu_read_lock_sched()是关抢占的！

因此，在RCU中_sched API后缀的意义仅仅体现在当CONFIG_PREEMPT=y也就是将内核配置为可抢占的情况！在CONFIG_PREEMPT=y情况下，RCU read-side临界区是可抢占的，而RCU-sched read-side是不可抢占的！

在lwn文章中还讨论的一个RCU和RCU-Sched混用的问题：

Quick Quiz 3: What happens if you mix and match RCU and RCU-Sched?

假如我们把RCU和RCU-Sched混合使用了怎么办？

Answer: In a CONFIG_TREE_RCU or a CONFIG_TINY_RCU kernel, mixing these two works “by accident” because in those kernel builds, RCU and RCU-Sched map to the same implementation.

对于CONFIG_TREE_RCU or a CONFIG_TINY_RCU=y的内核来说，混用两者无所谓，因为在这种内核里RCU和RCU-Sched的实现是相同的

However, this mixture is fatal in CONFIG_TREE_PREEMPT_RCU builds, due to the fact that RCU’s read-side critical sections can then be preempted, which would permit synchronize_sched() to return before the RCU read-side critical section reached its rcu_read_unlock() call.

但是对于CONFIG_TREE_PREEMPT_RCU=y的内核来说，其RCU read-side是可以被抢占的，那么假如读端用的rcu_read_lock/unlock在写端使用synchronize_sched，读端的临界区发生抢占就会提前结束其宽限期（其实并没有）

This could, in turn, result in a data structure being freed before the read-side critical section was finished with it, which could, in turn, greatly increase the actuarial risk experienced by your kernel.

这可能会导致读者还没读完，写者就free掉了其读的内存，导致风险！

Even in CONFIG_TREE_RCU and CONFIG_TINY_RCU builds, such mixing and matching is of course very strongly discouraged. Mixing and matching other flavors of RCU is even worse: it can result in hard-to-debug bad-pointer bugs.

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