[SOLVED] what does __rcu stands for in linux?

Issue

I am new to linux kernel. My question is about the task_struct. I know that Each task_struct has a reference to its parent process via a pointer to the task_struct of the parent.

After looking at the sched.h in the task_struct definition I noticed the following :

struct task_struct __rcu *real_parent; /* real parent process */

I found that it is referenced to compiler.h. I guess that the "__rcu" stands for "read copy update"

Can someone clarify the syntax ?

Solution

Read-copy-update is an algorithm that enables concurrent access to readers of a data structure without having to lock the structure. It can be read about here.

If the kernel is built with the CONFIG_SPARSE_RCU_POINTER config option, __rcu is defined in include/linux/compiler.h as

# define __rcu          __attribute__((noderef, address_space(4)))

This is an annotation for a the Sparse code analysis tool that can warn about certain things the programmer may have overlooked. How this is relevant to RCU is explained in Documentation/RCU/checklist.txt:

__rcu sparse checks: tag the pointer to the RCU-protected data structure with __rcu, and sparse will warn you if you access that pointer without the services of one of the variants of rcu_dereference().

rcu_dereference() returns a pointer that can be safely dereferenced by the code and documents the programmer's intention to protect the pointer with the RCU mechanism, enabling tools like Sparse to check for programming errors and omissions.

Answered By - idoby

Answer Checked By - Katrina (WPSolving Volunteer)