有人能解释一下rcu_dereference()和rcu_dereference_protected()之间的区别吗？rcu_dereference()包含屏障代码，rcu_dereference_protected()不包含。
何时使用rcu_dereference()，何时使用rcu_dereference_protected()？
最佳答案
简而言之：rcu_dereference()应在读取端使用，由rcu_read_lock()或类似的保护。rcu_dereference_protected()应该由单个写入程序在写端（更新端）使用，或者由防止多个写入程序同时修改取消引用指针的锁保护。在这种情况下，指针不能在当前线程之外修改，因此不需要编译器或CPU屏障。
如果有疑问，使用rcu_dereference总是安全的，其性能惩罚（与rcu_dereference_protected相比）很低。
内核4.6中rcu_dereference_protected的精确description：
/**
* rcu_dereference_protected() - fetch RCU pointer when updates prevented
* @p: The pointer to read, prior to dereferencing
* @c: The conditions under which the dereference will take place
*
* Return the value of the specified RCU-protected pointer, but omit
* both the smp_read_barrier_depends() and the READ_ONCE(). This
* is useful in cases where update-side locks prevent the value of the
* pointer from changing. Please note that this primitive does -not-
* prevent the compiler from repeating this reference or combining it
* with other references, so it should not be used without protection
* of appropriate locks.
*
* This function is only for update-side use. Using this function
* when protected only by rcu_read_lock() will result in infrequent
* but very ugly failures.
*/
关于linux - rcu_dereference()和rcu_dereference_protected()？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/39251287/