linux - rcu_dereference() vs rcu_dereference_protected()？

Question

谁能解释一下 rcu_dereference() 和 rcu_dereference_protected() 之间的区别？

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

何时使用 rcu_dereference() 以及何时使用 rcu_dereference_protected()？

Answer 1

简而言之:

• rcu_dereference() 应该用在读端，受rcu_read_lock() 或类似的保护。
• rcu_dereference_protected() 应该由单个 作者在写入端(更新端)使用，或受锁保护，该锁可防止多个编写者并发修改取消引用的指针。在这种情况下，不能在当前线程的外部修改指针，因此既不需要编译器屏障也不需要 cpu 屏障。

如果有疑问，使用 rcu_dereference 总是安全的，并且它的性能损失(与 rcu_dereference_protected 相比)很低。

精确description对于内核 4.6 中的 rcu_dereference_protected:

/**
 * 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.
 */