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

  • Print
  • + Share This
This chapter is from the book

7.4 Highest Locker Pattern

The Highest Locker Pattern defines a priority ceiling with each resource. The basic idea is that the task owning the resource runs at the highest-priority ceiling of all the resources that it currently owns, provided that it is blocking one or more higher-priority tasks. This limits priority inversion to at most one level.

7.4.1 Abstract

The Highest Locker Pattern is another solution to the unbounded blocking/unbounded priority inversion problem. It is perhaps a minor elaboration from the Priority Inheritance Pattern, but it is different enough to have some different properties with respects to schedulability. The Highest Locker Pattern limits priority inversion to a single level as long as a task does not suspend itself while owning a resource. In this case, you may get chained blocking similar to the Priority Inheritance Pattern. Unlike the Priority Inheritance Pattern, however, you cannot get chained blocking if a task is preempted while owning a resource.

7.4.2 Problem

The unbounded priority inversion problem is discussed in the chapter introduction in some detail. The problem addressed by this pattern is to limit the maximum amount of priority inversion to a single level—that is, there is at most a single lower-priority task blocking a higher-priority task from executing.

7.4.3 Pattern Structure

The Highest Locker Pattern is shown in Figure 7-10. The structural elements of the pattern are the same as for the Priority Inheritance Pattern, with the addition of an attribute priorityCeiling for the SharedResource.

Figure 7-10Figure 7-10: Highest Locker Pattern


The pattern works by defining each lockable resource with a priority ceiling. The priority ceiling is just greater than the priority of the highest-priority client of the resource—this is known at design time in a static priority scheme. When the resource is locked, the priority of the locking task is augmented to the priority ceiling of the resource.

7.4.4 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.

  • Mutex

    The Mutex is a mutual exclusion semaphore object that permits only a single caller through at a time. The operations of the Shared Resource invoke it whenever a relevant service is called, locking it prior to starting the service and unlocking it once the service is complete. Threads that attempt to invoke a service when the services are already locked become blocked until the Mutex is in its unlocked state. This is done by the Mutex semaphore signaling the Scheduler that a call attempt was made by the currently active thread, the Mutex ID (necessary to unlock it later when the mutex is released), and the entry point—the place at which to continue execution of the Thread.

  • Scheduler

    This object orchestrates the execution of multiple threads based on their priority according to a simple rule: Always run the ready thread with the highest priority. 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 some special duties when the Mutex signals an attempt to access a locked resource. Specifically, it must block the requesting task (done by stopping that task and placing a reference to it in the Blocked Queue (not shown—for details of the Blocked Queue, see the Static Priority Pattern in Chapter 5), and it must elevate the priority of the task owning the resource to the Shared Resource's priorityCeiling.

  • Shared Resource

    A resource is an object shared by one or more Threads. For the system to operate properly in all cases, all Shared Resources must either be reentrant (meaning that corruption from simultaneous access cannot occur), or they must be protected. In the case of a protected resource, when a Thread attempts to use the resource, the associated Mutex semaphore is checked, and if locked, the calling task is placed into the Blocked Queue. The task is terminated with its reentry point noted in the TCB.

    The SharedResource has a constant attribute (note the «frozen» constraint in Figure 7-10), called priorityCeiling. This is set during design to just greater than the priority of the highest-priority task that can ever access it. In some RTOSs, this means that the priority will be one more (when a larger number indicates a higher priority), and in some it will be one less (when a lower number indicates a higher priority). This ensures that when the resource is locked, no other task using that resource can preempt it.

  • 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 7-10. The TCB tracks both the current priority of the thread (which may have been elevated due to resource access and blocking) and its nominal priority.

7.4.5 Consequences

The Highest Locker Pattern has even better priority inversion-bounding properties than the Priority Inheritance Pattern. It allows higher-priority tasks to run, but only if they have a priority higher than the priority ceiling of the resource. The priority ceiling can be determined at design time for each resource by examining the clients of a given resource and identifying to which active object they belong and selecting the highest from among those. The priority ceiling is this value augmented by one. Computation of worst-case blocking is the length of the longest critical section (that is, resource locking time) of any task of lesser priority as long as a task never suspends itself while owning a resource.

The pattern has the disadvantage that while it bounds priority inversion to a single level, that level happens more frequently than with some other approaches. For example, if the lowest-priority task locks a resource with the highest-priority ceiling, and during that time an intermediate priority task becomes ready to run, then it is blocked even though in this case one would prefer that the normal priority rules apply. One way to handle that is to elevate the priority of the task owning the resource only when another task attempts to lock it; until then, the locking tasks runs at its nominal priority.

In this pattern, care must be taken to ensure that a task never suspends itself while owning a resource. It is fine if it is preempted, but voluntary preemption while owning a resource can lead to chain blocking, a problem previously identified with the Priority Inheritance Pattern in the previous section. If the system allows tasks to suspend themselves while owning a resource, then the computation of worst-case blocking is computed in the same way as with the Priority Inheritance Pattern—the longest case of chain blocked must be traversed.

This pattern avoids deadlock as long as no task suspends itself while owning a resource because no other task is permitted to wait on the resource (condition 4). This is because the locking task runs at a priority higher than any of the other clients of the resource. As previously noted, there is also a consequence of computational overhead associated with the Highest Locker Pattern.

7.4.6 Implementation Strategies

Fewer RTOSs support the Highest Locker Pattern more than the basic Priority Inheritance Pattern. Implementation of this pattern in your own RTOS is fairly straightforward, with the addition of priority ceiling attributes in the Shared Resource. When the mutex is locked, it must notify the Scheduler to elevate the priority of the locking task to that resource's priority ceiling.

7.4.7 Related Patterns

The Highest Locker Pattern exists to help solve a particular problem peculiar to priority-based preemption multitasking, so all of the concurrency patterns having to do with that style of multitasking can be mixed with this pattern.

7.4.8 Sample Model

In the example shown in Figure 7-11, there are four tasks with their priorities shown using constraints, two of which, Waveform Draw and Message Display, share a common resource, Display. The tasks, represented as active objects in order of their priority, are Message Display (priority Low), Switch Monitor (priority Medium Low), Waveform Draw (priority Medium High), and Safety Monitor (priority Very High), leaving priority High unused at the outset. Message Display and Waveform Draw share Display, so the priority ceiling of Display is just above Waveform Draw (that is, High).

The scenario runs as follows: First, the lowest-priority task, Message Display, runs, calling the operation Display.displayMsg(). Because the Display has a mutex semaphore, this locks the resource, and the Scheduler (not shown in Figure 7-11) escalates the priority of the locking task, Message Display, to the priority ceiling of the resource—that is, the value High.

Figure 7-11Figure 7-11: Highest Locker Pattern


While this operation executes, first the Switch Monitor and then the Waveform Draw tasks both become ready to run but cannot because the Message Display task is running at a higher priority than either of them. The Safety Monitor task becomes ready to run. Because it runs at a priority Very High, it can, and does, preempt the Message Display task.

After the Safety Monitor task returns control to the Scheduler, the Scheduler continues the execution of the Message Display task. Once it releases the resource, the mutex signals the Scheduler, and the latter deescalates the priority of the Message Display task to its nominal priority level of Low. At this point, there are two tasks of a higher priority waiting to run, so the higher-priority waiting task (Waveform Draw) runs, and when it completes, the remaining higher-priority task (Switch Monitor) runs. When this last task completes, the Message Display task can finally resume its work and complete.

  • + Share This
  • 🔖 Save To Your Account

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.

Overview


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.

Surveys

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.

Newsletters

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.

Security


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

Children


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

Marketing


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.

Choice/Opt-out


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.

Links


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