rcu_dereference() vs rcu_dereference_protected()?

Question

有人能解释rcu_dereference()和rcu_dereference_protected()之间的区别吗？

rcu_dereference()包含屏障代码，而rcu_dereference_protected()不包含。

何时使用rcu_dereference()，何时使用rcu_dereference_protected()？

Answer 1

简而言之：

• rcu_dereference()应在读取端使用，受rcu_read_lock()或类似保护
• rcu_dereference_protected()应由单个写入程序在写入端(更新端部修改，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，并且其性能惩罚(与rcu_dereference_protected相比(较低。

内核4.6:中rcu_dereference_protected的精确描述

/**
 * 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.
 */