Linux RCU Synchronization Mechanism

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In this post, we will get to know the insight about Linux Synchronization mechanism – RCU(Read Copy Update).
linusgeek.blogspot.com

RCU (Read Copy Update ) is one of the synchronization mechanism found under Linux for both User Space and Kernel Space. In this blog, I’ll be explaining about Kernel Space RCU implementation with a sample code.

 Read -Copy- Update is used as a synchronization mechanism when performance is pivotal for a system. Using RCU performance of "Read" operation can be made significantly more compared with other synchronization mechanisms like a semaphore.  As the name says Read-the list, Copy-the node, Update-update the list for writing into the linked list. Then what about read operation ?. Using RCU for reading a linked list is almost lock-free. In non-preempt Kernel, it is almost equal to nops. 

Below is the sample that explains RCU implementation using the miscellaneous driver and is compiled under GPL license.

#include <linux/module.h>
#include <linux/init.h> 
#include <linux/kernel.h>
#include <linux/miscdevice.h>
#include <linux/version.h>
#include <linux/kthread.h> 
#include <linux/list.h>
#include <linux/rculist.h>
#include <linux/spinlock.h>
#include <linux/preempt.h>
#include <linux/slab.h>

/** Structure to create a Linked List to store Employe DataBase */
typedef struct EmployDB {
    char empName[80];
    uint64_t empId;
    struct list_head node;
    struct rcu_head rcu;
}EmployDB_t;

static LIST_HEAD(EmployDBHeadNode);
static spinlock_t writeSpinLock;
static int empNum = 0;

/** Prototype declaration*/
static void addEmploy(char *pEmpName);
static void freeCallback(struct rcu_head *rcu);
static void deleteEmployNode(char *pEmpName, int syncFree);
static void displayEmployDB();
static void updateEmployInfo(char *pEmpName, char *pEmpNewName, int syncFree);

static struct miscdevice misc_struct={
    //Minor number is given, if we give it should be unique and not taken by any module
    .minor = MISC_DYNAMIC_MINOR,
    .name = "Trial_Driver",
};

static void addEmploy(char *pEmpName)
{
    EmployDB_t *newEmploy ;

    newEmploy = kzalloc(sizeof(EmployDB_t), GFP_KERNEL);
    if(!newEmploy)
    {
        return;
    }

    strcpy(newEmploy->empName, pEmpName);
    newEmploy->empId = empNum++;

    spin_lock(&writeSpinLock);
    list_add_rcu(&newEmploy->node, &EmployDBHeadNode);
    spin_unlock(&writeSpinLock);
}

/** Callback function for async reclaim */
static void freeCallback(struct rcu_head *rcu)
{
    EmployDB_t *empNode  = container_of(rcu, struct  EmployDB, rcu);
    kfree(empNode);
}

static void deleteEmployNode(char *pEmpName, int syncFree)
{
    EmployDB_t *empNode = NULL;
 
    spin_lock(&writeSpinLock);
    list_for_each_entry(empNode, &EmployDBHeadNode, node) {

        if(!strcmp(pEmpName, empNode->empName))
        {
            list_del_rcu(&empNode->node);
            spin_unlock(&writeSpinLock);

            if(syncFree) {
               printk("Calling synchronize_rcu.....\n");
                /** wait for all RCU read-side critical sections to complete*/
                synchronize_rcu();
                kfree(empNode);
            }
            else
            {
            call_rcu(&empNode->rcu, freeCallback);
            }
           return;
        }
    }
    spin_unlock(&writeSpinLock);
}

static void displayEmployDB()
{
    EmployDB_t *empNode = NULL;

    rcu_read_lock();

    list_for_each_entry_rcu(empNode, &EmployDBHeadNode, node) {
        printk(" Employ Name: {%s}\n", empNode->empName);
        pr_info(" Employ ID: {%d}\n", empNode->empId);
    }
    rcu_read_unlock();
}

/** Update a Employ information*/
static void updateEmployInfo(char *pEmpName, char *pEmpNewName, int syncFree)
{
    EmployDB_t *empNode = NULL;
    EmployDB_t *empOldNode = NULL;
    EmployDB_t *empNewNode = NULL;
   
    rcu_read_lock();
    list_for_each_entry(empNode, &EmployDBHeadNode, node) {

     if(!strcmp(empNode->empName, pEmpName))
     {
            empOldNode = empNode;
            break;
        }
    }

    if(!empOldNode)
        return;

    empNewNode = kzalloc(sizeof(EmployDB_t), GFP_ATOMIC);
    if(!empNewNode)
    {
        return;
    }

    memcpy(empNewNode, empOldNode, sizeof(EmployDB_t));
    strcpy(empNewNode->empName, pEmpNewName);
    spin_lock(&writeSpinLock);
    list_replace_rcu(&empOldNode->node, &empNewNode->node );
    spin_unlock(&writeSpinLock);
    rcu_read_unlock();

    if(syncFree) {
        synchronize_rcu();
        kfree(empOldNode);
    }
    else
    {
        call_rcu(&empOldNode->rcu, freeCallback);     
    }
}

/*Initialization function */
static int __init trialInit(void)
{
    int result = 0;

    printk(KERN_INFO "In function intit module inserted\n");

    //0 - Success, -ve  on failure
    result = misc_register(&misc_struct);
    if(result < 0)
        printk(KERN_ALERT "misc_register Failed..?");
    else
        printk(KERN_INFO " device Registered with minor number = %i \n", misc_struct.minor);

   //initialize spin lock
    spin_lock_init(&writeSpinLock);

    printk("\nAdding Employee's to DataBase:\n");
    //Initialize for Trident array
    addEmploy("Employ1");
    addEmploy("Employ2");
    addEmploy("Employ3");
    addEmploy("Employ4");

    printk("Displaying Employ list:\n");
    //Display List
    displayEmployDB();

    printk("Deleting Employ from DB:\n");
    //Delete a Employ node
    deleteEmployNode("Employ3", 1);

    printk("Displaying Employ list:\n");
     //Display List
    displayEmployDB();

    printk("Updating Employ Name:\n");
    //Update a employ name
    updateEmployInfo("Employ1", "Employ3", 1);

    printk("Displaying Employ list:\n");
     //Display List
    displayEmployDB();
 
    return result;
}

static void __exit trialExit(void)
{
    misc_deregister(&misc_struct);
    printk("\nDevice Unregistered\n");
}

module_init(trialInit);
module_exit(trialExit);

MODULE_AUTHOR("linusgeek.blogspot.com");
MODULE_DESCRIPTION("Trial driver for RCU");
MODULE_LICENSE("GPL");

Makefile:
obj-m += rcu_example.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

In this sample module, RCU synchronization mechanism is used to protect the Employee Database.
The list has to be protected for a concurrent write operation and read operation can be done concurrently. 

"trialInit" is the entry point into the miscellaneous driver. misc_register API is used to register the kernel module. Then the spinlock is initialized and is used to protect the Linked List against concurrent write to the list.  Before Reading a List rcu lock(rcu_read_lock) is taken to avoid data corruption and unlocked after completing read operation. rcu_read_lock and rcu_read_unlock
are the lightweight operation, which improves the performance compare to the conventional synchronization technique.

During update/deletion operation there will be 2 versions of Linked List exist. 1st version (old list) is used by pre-existing reader's and 2nd version(updated version) is used by the new Reader's.
When updating to the linked list is complete then the node can be freed once the wait-for read-side critical section is complete. Wait-for read-side critical section is the code that is protected by rcu_read_lock and rcu_read_unlock.

This freeing up the removed/updated node can be performed in two ways either synchronously using sync_rcu() or asynchronously using call_rcu(), which calls registered callback function. 

This module can be inserted and run like:
         insmod rcu_example.ko
         dmesg
         rmmod rcu_example

Throughout this blog, we went through the RCU synchronization technique. In the next blog, we'll be going through  Kernel side implementation of RCU methods. Till then goodbye

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