linux - rcu_dereference() 与 rcu_dereference_protected()？

Question

rcu_dereference()谁能解释和有什么区别rcu_dereference_protected()？

rcu_dereference()包含屏障代码并且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，受保护rcu_read_lock()或类似。
• rcu_dereference_protected()应该由单个写入者在写入端（更新端）使用，或者受锁保护，以防止多个写入者同时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被修改，因此既不需要编译器也不需要 cpu 屏障。

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

内核 4.6 中的确切描述：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.
 */