rcu_dereference()对rcu_dereference_protected()？

Question

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

rcu_dereference()包含障碍代码，而rcu_dereference_protected()不包含。

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

Answer 1

简言之：

• rcu_dereference()应在受rcu_read_lock()或类似保护的读取端中使用.
• rcu_dereference_protected()应由单个写入器在写端中使用，或者由锁保护，以防止多个写入器并发修改取消引用的指针。在这种情况下，指针不能在当前线程之外进行修改，因此不需要编译器--也不需要cpu--屏障。

如果有疑问的话，使用rcu_dereference总是安全的，它的性能惩罚(与rcu_dereference_protected相比)很低。

描述用于内核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.
 */