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

This chapter is from the book

7.6 Simultaneous Locking Pattern

The Simultaneous Locking Pattern is a pattern solely concerned with deadlock avoidance. It achieves this by breaking condition 2 (holding resources while waiting for others). The pattern works in an all-or-none fashion. Either all resources needed are locked at once or none are.

7.6.1 Abstract

Deadlock can be solved by breaking any of the four conditions required for its existence. This pattern prevents the condition of holding some resources by requesting others by allocating them all at once. This is similar to the Critical Section Pattern. However, it has the additional benefit of allowing higher-priority tasks to run if they don't need any of the locked resources.

7.6.2 Problem

The problem of deadlock is such a serious one in highly reliable computing that many systems design in specific mechanisms to detect it or avoid it. As previously discussed, deadlock occurs when a task is waiting on a condition that can never, in principle, be satisfied. There are four conditions that must be true for deadlock to occur, and it is sufficient to deny the existence of any one of these. The Simultaneous Locking Pattern breaks condition 2, not allowing any task to lock resources while waiting for other resources to be free.

7.6.3 Pattern Structure

Figure 7-15 shows the structure of the Simultaneous Locking Pattern. The special structural aspect of this pattern is the collaboration role MultiResource. Each MultiResource has a single mutex semaphore that locks only when the entire set of aggregated Shared Resources is available to be locked. Similarly, when the semaphore is released, all the aggregated Shared Resources are released.

Figure 7-15Figure 7-15: Simultaneous Locking Pattern

7.6.4 Collaboration Roles

  • MultiResource

    This object aggregates an entire set of resources needed (or possibly needed) by a Resource Client. MultiResource explicitly locks and unlocks the set of resources. This locking and unlocking action should be a noninterruptible critical section. If any of the aggregated Shared Resources is not available during the locking process, then the MultiResource must release all of the Shared Resources it successfully locked. MultiResource must define operations startCriticalSection() and endCriticalSection to prevent task switching from occurring during the locking or unlocking process. Also, areAnyLockedParts() returns TRUE if any of the Shared Resources aggregated by the MultiResource are still locked. For walking through the Shared Resources, the MultiResource also has the operations getFirstResource() and getNextResource(), both of which return a pointer to a Shared Resource (or NULL if at the end of the list) and isLocked(*Shared Resource), which returns TRUE only if the referenced Shared Resource is currently locked by the MultiResource. If either unlocked or not aggregated by the MultiResource, then it returns FALSE. Two more operations, lockNow() and unlockNow(), simply set the isLocked attribute of the MultiResource without checking the status of the aggregated parts.

  • Mutex

    The Mutex is a mutual exclusion semaphore object that associates with MultiResource. In this pattern the shared resources are locked for a longer duration than with the priority inheritance-based patterns. This is because Resource Client needs to own all the resources for the entire critical section so that the Resource Client never owns a resource while trying to lock another. The policy is that the Mutex is only locked if all of the required ShareResource PartLocks are successfully locked. Mutex is an OS-level mutex and signals the Scheduler to take care of blocking tasks that attempt to lock the SharedResource.

  • PartLock

    The PartLock is a special mutual exclusion semaphore that associates to Shared Resource. This Mutex is queryable as to its lock status, using the getIsLocked() operation. This semaphore does not signal the Scheduler unlike the Mutex, because there is no need; the OS-level locking is done by the Mutex and not by the PartLock. Nevertheless, the MultiResource needs to be able to ascertain the locking status of all the resources before attempting to lock any of them.

  • Resource Client

    The Resource Client is a user of Shared Resource objects. It locks potentially multiple Shared Resources via the MultiResource. The policy enforced in this pattern is that all resources used in a critical section must be locked at the same time, or the entire lock will fail. The Resource Client is specifically prohibited from locking one resource and later, while still owning that lock, attempting to lock another. Put another way, an attempt to lock a set of resources is only permitted if the Resource Client currently owns no locks at all, and if any of the requested resources are unavailable, the entire lock will fail and the Resource Client must wait and block on the set of resources (that is, it blocks on the mutex owned by its associated MultiResource).

  • ResourceMaster

    The ResourceMaster orchestrates the locking and unlocking of Mutexes associated with MultiResources. Whenever a MultiResource locks a Mutex, the ResourceMaster searches its list of all MultiResources and locks any that share one of the SharedResources. That way, if a Thread tries to lock its MultiResource and another one owns a needed SharedResource, the Thread can block on the Mutex of its associated MultiResource. Conversely, when a MultiResource releases all of its Shared Resources, that MultiResource notifies the ResourceMaster and it tracks down all of the other MultiResources and sees if it can unlock them as well (it may not be able to if another MultiResource has locked a SharedResource unused by the first).

  • Shared Resource

    A resource is a part object owned by the MultiResource object. In this pattern, a Shared Resource does not connect to a Mutex because it is not locked individually. As implied by its name, the same Shared Resource object may be an aggregated part of different MultiResource objects. The pattern policy is that no resource that is aggregated by one MultiResource is allowed to be directly locked by a Thread, although it may be accessed by a Thread to perform services. The Shared Resource contains operations to explicitly lock, unlock, and to query its locked status, and these simply invoke services in the associated PartLock.

7.6.5 Consequences

The Simultaneous Locking Pattern prevents deadlock by breaking condition 2, required for deadlock to occur—namely locking some resources while waiting for others to become available. It does this by locking all resources needed at once and releasing them all at once. This resource management pattern can easily be used in most scheduling patterns, such as the Static Priority Pattern.

There are two primary negatives to the use of this pattern. First, priority inversion is not bounded. A higher-priority task is free to preempt and run as long as it doesn't use any currently locked resource. This pattern could be mixed in with the priority inheritance pattern to address that problem.

The second issue is that this pattern invokes some computational overhead, which may become severe in situations in which there are many shared resources. Each time a request to lock a resource is made, each of the Shared Resources must be locked and all of the other MultiResources must be checked to see if they aggregate any of these locked Shared Resources. Any MultiResource that shares one of the just-locked Shared Resources must itself be locked. On release of a lock on a particular MultiResource, all of its Shared Resources must be unlocked, and then each of the other MultiResources must be examined using the areAnyLockedParts() operation. If it returns TRUE, then that MultiResource must remain locked; otherwise is must be unlocked.

Another issue is that programmer/designer discipline is required not to access the Shared Resources without first obtaining a lock by going through the MultiResource mechanism. Because Shared Resources don't use standard OS mutexes for locking (since we don't want Threads blocking on them rather then the MultiResources), it is possible to directly access the Shared Resource, bypassing the locking mechanisms. This is a Bad Idea. One possible solution to enforce the locking is to propagate all of the operations from the resources to the MultiResource, make the operations public in the MultiResource and private in the Shared Resource, and making the MultiResource a friend of the Shared Resource. This adds some additional computational overhead, but in some languages the propagated operations could be made inline to minimize this. Alternatively, each Shared Resource could be told, during the locking process, who its owner is. Then on each service call, the owner would have to pass an owner ID to prove it had rights to request the service.

7.6.6 Implementation Strategies

Care must be taken that the locking of all the resources in MultiResource.lock() and MultiResource.unlock() must be done in a critical section to prevent deadlock condition 2 from occurring. Other than that, the implementation of this pattern is straightforward.

7.6.7 Related Patterns

This pattern removes deadlock by breaking condition 2 required for deadlock. There are other approaches to avoiding deadlock. One of this is presented in the Ceiling Priority Pattern and another in the Ordered Locking Pattern, both presented in this chapter. This pattern is normally mixed with a concurrency management policy, such as the Static Priority Pattern, but other patterns can be used as well. If it is desirable to limit priority inversion, then this pattern can be mixed with the Priority Inheritance Pattern.

7.6.8 Sample Model

Figure 7-16a shows a simple example of the application of this pattern. Two Concrete Threads, Machine 1 and Machine 2, share three resources: MsgQueue 1 (Machine 1 only), Command Queue (both), and MsgQueue 2 (Machine 2 only). To avoid the possibility of a deadlock occurring, the Simultaneous Locking Pattern is used. Two MultiResources (Multi 1 and Multi 2) are created as composite parts of an instance of ResourceMaster. Figure 7-16b shows the behavior when Machine 1 locks its resources, does some work (moving messages from the Command Queue to MsgQueue 1), and then unlocks the resources.

Figure 7-16Figure 7-16: Simultaneous Locking Pattern (continued)

What is not shown is what happens if Machine 2 runs during the execution of the get() and put() operations, but it is clear that as soon as Machine 2 attempts to lock its MultiResource, it will be blocked.

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