Home > Articles > Software Development & Management > Architecture and Design

This chapter is from the book

7.2 Critical Section Pattern

The Critical Section Pattern is the simplest pattern to share resources that cannot be shared simultaneously. It is lightweight and easy to implement, but it may prevent high priority tasks, even ones that don't use any resources, from meeting their deadlines if the critical section lasts too long.

7.2.1 Abstract

This pattern has been long used in the design of real-time and embedded systems whenever a resource must have at most a single owner at any given time. The basic idea is to lock the Scheduler whenever a resource is accessed to prevent another task from simultaneously accessing it. The primary advantage of this pattern is its simplicity, both in terms of understandability and in terms of implementation. It becomes less applicable when the resource access may take a long time because it means that higher-priority tasks may be blocked from execution for a long period of time.

7.2.2 Problem

The main problem addressed by the Critical Section Pattern is how to robustly share resources that may have, at most, a single owner at any given time.

7.2.3 Pattern Structure

Figure 7-4 shows the basic structural elements in the Critical Section Pattern.

Figure 7-4Figure 7-4: Critical Section Pattern

7.2.4 Collaboration Roles

  • Abstract Thread

    The Abstract Thread class is an abstract (noninstantiable) superclass for Concrete Thread. Abstract Thread associates with the Scheduler. Since Concrete Thread is a subclass, it has the same interface to the Scheduler as the Abstract Thread. This enforces interface compliance. The Abstract Thread is an «active» object, meaning that when it is created, it creates an OS thread in which to run. It contains (that is, it has composition relations with) more primitive application objects that execute in the thread of the composite «active» object.

  • Concrete Thread

    The Concrete Thread is an «active» object most typically constructed to contain passive "semantic" objects (via the composition relation) that do the real work of the system. The Concrete Thread object provides a straightforward means of attaching these semantic objects into the concurrency architecture. Concrete Thread is an instantiable subclass of Abstract Thread.

  • Scheduler

    This object orchestrates the execution of multiple threads based on some scheme requiring preemption. When the «active» Thread object is created, it (or its creator) calls the createThread operation to create a thread for the «active» object. Whenever this thread is executed by the Scheduler, it calls the StartAddr address (except when the thread has been blocked or preempted—in which case it calls the EntryPoint address).

    In this pattern, the Scheduler has a Boolean attribute called taskSwitchingEnabled and two operations, startCriticalSection() and endCriticalSection(), which manipulate this attribute. When FALSE, it means that the Scheduler will not perform any task switching; when TRUE, tasks will be switched according to the task scheduling policies in force.

  • Shared Resource

    A resource is an object shared by one or more Threads but cannot be reliably accessed by more than one client at any given time. All operations defined on this resource that access any part of the resource that is not simultaneously sharable (its nonreentrant parts) should call Scheduler.startCriticalSection() before they manipulate the internal values of the resource and should call Scheduler.endCriticalSection() when they are done.

  • Task Control Block

    The TCB contains the scheduling information for its corresponding Thread object. This includes the priority of the thread, the default start address, and the current entry address if it was preempted or blocked prior to completion. The Scheduler maintains a TCB object for each existing Thread. Note that TCB typically also has a reference off to a call and parameter stack for its Thread, but that level of detail is not shown in Figure 7-4.

7.2.5 Consequences

The designers and programmers must show good discipline in ensuring that every resource access locks the resource before performing any manipulation of the source. This pattern works by effectively making the current task the highest-priority task in the system. While quite successful at preventing resource corruption due to simultaneous access, it locks out all higher-priority tasks from executing during the critical section, even if they don't require the use of the resource. Many systems find this blocking delay unacceptable and must use more elaborate means for resource sharing. Further, if the initial task that locks the resource neglects to deescalate its priority, then all other tasks are permanently prevented from running. Calculation of the worst-case blocking for each task is trivial with this pattern: It is simply the longest critical section of any single task of lesser priority.

It is perhaps obvious, but should nevertheless be stated, that when using this pattern a task should never suspend itself while owning a resource because task switching is disabled so that in a situation like that no tasks are permitted to run at all. This pattern has the advantage in that it avoids deadlock by breaking the second condition (holding resources while waiting for others) as long as the task releases the resource (and reenables task switching) before it suspends itself.

7.2.6 Implementation Strategies

All commercial RTOSs have a means for beginning and ending a critical section. Invoking this Scheduler operation prevents all task switching from occurring during the critical section. If you write your own RTOS, the most common way to do this is to set the Disable Interrupts bit on your processor's flags register. The precise details of this vary, naturally, depending on the specific processor.

7.2.7 Related Patterns

As mentioned, this is the simplest pattern that addresses the issue of sharing nonreentrant resources. Other resource sharing approaches, such as Priority Inheritance, Highest Locker, and Priority Ceiling Patterns, solve this problem as well with less impact on the schedulability of the overall system but at the cost of increased complexity. This pattern can be mixed with all of the concurrency patterns from Chapter 5, except the Cyclic Executive Pattern, for which resource sharing is a nonissue.

7.2.8 Sample Model

An example of the use of this pattern is shown in Figure 7-5. This example contains three tasks: Device Test (highest priority), Motor Control (medium priority), and Data Processing (lowest priority). Device Test and Data Processing share a resource called Sensor, whereas Motor Control has its own resource called Motor.

Figure 7-5Figure 7-5: Critical Section Pattern Example (continued)

The scenario starts off with the lowest-priority task, Data Processing, accessing the resource that starts up a critical section. During this critical section both the Motor Control task and the Device Test task become ready to run but cannot because task switching is disabled. When the call to the resource is almost done, the Sensor.gimme() operation makes a call to the scheduler to end the critical section. The scenario shows three critical sections, one for each of the running tasks. Finally, at the end, the lowest-priority task is allowed to complete its work and then returns to its Idle state.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020