Kernels and locking

Proper locking can be tough—real tough. Improper locking can result in random crashes and other oddities. Poorly designed locking can result in code that is hard to read, performs poorly and makes your fellow kernel developers cringe.

In this article, I explain why kernel code requires locking, provide general rules for proper kernel locking semantics and then outline the various locking primitives in the Linux kernel.

The fundamental issue surrounding locking is the need to provide synchronization in certain code paths in the kernel. These code paths, called critical sections, require some combination of concurrency or re-entrancy protection and proper ordering with respect to other events. The typical result without proper locking is called a race condition.

Consider the case where one processor reads i, then another, then they both increment it, then they both write i back to memory. If i were originally 2, it should now be 4, but in fact it would be 3! This is not to say that the only locking issues arise from SMP symmetric multiprocessing. Interrupt handlers create locking issues, as does the new preemptible kernel, and any code can block go to sleep. Of these, only SMP is considered true concurrency, i. The other situations—interrupt handlers, preempt-kernel and blocking methods—provide pseudo concurrency as code is not actually executed concurrently, but separate code can mangle one another's data.

These critical regions require locking. The Linux kernel provides a family of locking primitives that developers can use to write safe and efficient code. Whether or not you have an SMP machine, people who use your code may. Further, code that does not handle locking issues properly is typically not accepted into the Linux kernel. Finally, with a preemptible kernel even UP uniprocessor systems require proper locking. Thus, do not forget: you must implement locking.

This allows certain locks not to exist at all in a UP kernel. It does not matter, however, to the developer: lock everything appropriately and all situations will be covered.

We cover atomic operators initially for two reasons. First, they are the simplest of the approaches to kernel synchronization and thus the easiest to understand and use. Second, the complex locking primitives are built off them. In this sense, they are the building blocks of the kernel's locks. Atomic operators are operations, like add and subtract, which perform in one uninterruptible operation. If we could read i, increment it and write it back to memory in one uninterruptible operation, the race condition discussed above would not be an issue.

Atomic operators provide these uninterruptible operations. Two types exist: methods that operate on integers and methods that operate on bits.

The integer operations work like this:.

kernels and locking

They are simple. There are, however, little caveats to keep in mind when using atomics. Function Reference. The next group of atomic methods is those that operate on individual bits. They are simpler than the integer methods because they work on the standard C data types.Containers have taken the place of virtual machines as the go-to technology if multiple programs are required to run in isolation of one another on a single server.

Virtual machines made processes such as cloud computing and web hosting possible. With virtual machines, the operating system and program code are stored together and any single operating system is running on the assumption that it is doing so on its own server. This is made possible by the host operating system. If this sounds confusing, think of it this way: Several virtual machines are running independently of one another on the same server.

While this approach solved many problems in computing, it also has some major issues—namely, the processing overhead required to run numerous emulated servers. Most laptops today are not capable of running multiple virtual machines efficiently. This is where container technology has stepped in. To mitigate the heavy computer costs often associated with virtual machines, containers have only the application and the necessary namespaces.

This program interacts directly with the host operating system. To make it clear, the only operating system on the server is the host server interacting with the containers. To use containers, something called a kernel is required.

A kernel acts as a middle man between the operating system and a container. In fact, many containers may share a single kernel. The kernel limits what programs can access what data.

For example, it may not be desirable that program A has full access to the data and information stored within program B, even though they will need to share information at some point. The operating system, on the other hand, has full access to all data in kernel mode when all memory is accessible. Although container technology has made computing less costly than the previous virtual machine approach, it has created a new type of security concerns.

Next, we will look at some things to keep in mind when locking down a kernel to secure the container. Failure to lock down a kernel can result in a wide variety of issues, the most damaging being a malicious actor gaining access to parts of a container they were not meant to.

Here are some best practices for preventing this. Keep the kernel updated: The kernel should be updated to the newest version as soon as the host is created. The issue is not usually with the kernel itself, but rather with containers.

It is not uncommon for containers to have vulnerabilities. Although these vulnerabilities are usually resolved quickly, failure to update to the latest version will still leave the container exposed to these issues. Checking if one has the latest kernel installed is simple; it requires running the code shown below:. Use only SSH authentication and remove root user: The purpose of doing this is simple: If a container escapes isolation we do not want to special privileges of the root user exposed to the host.

Furthermore, SSH authentication passwords are, by design, insecure and best disabled. To disable SSH authentication passwords execute the following code:. Use container security tools: Many quality container tools perform scans and alert the user if there is a security issue with a container.

Docker is one of the most popular programs that offer this service. The three tips listed above are some of the most simple yet effective ways to lock down kernels and ensure container security.

kernels and locking

That being said, no container is ever fully secure or safe from attack. It is important to always monitor. It is also recommended that those concerned with container security look into host security as well.

kernels and locking

These two topics go hand in hand concerning information security. Michael Dehoyos is a content marketer and editor at Phd Kingdom and Academic brits. He assists companies in their marketing strategy concepts, and contributes to numerous sites and publications.Due to high volumes, response times in the community may be delayed over the next few days.

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Locking Down the Kernel and Securing the Container

Hi John. If your Power Options settings are set correctly then the next suspect is the screensaver which is mysteriously setting a 1 minute timeout to blank Screensaver when some Themes are chosen.

This could also pertain to an issue of Unattended Sleep Timeout setting in Registry. I hope this helps. Feel free to ask back any questions and let us know how it goes. I will keep working with you until it's resolved. The pages appear to be providing accurate, safe information. Watch out for ads on the sites that may advertise products frequently classified as a PUP Potentially Unwanted Products. Thoroughly research any product advertised on the sites before you decide to download and install it.

I will walk you through any steps and will not let you fail. Reply 23 people were helped by this reply. Did this solve your problem? Yes No. Sorry this didn't help. Thanks for your feedback. Reply 8 people were helped by this reply.

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Also see fixes here for random Shutdowns which appears similar:. Reply 1 person was helped by this reply. Reply 4 people were helped by this reply.Knee locking is when the leg gets stuck in one position, making it impossible to bend or straighten the knee. A locked knee may only last a few seconds, it may last longer.

It all depends on what is causing it. Most cases fall into one of two categories:. True Knee Locking: True locking at the knee is where the knee gets physically stuck and you physically can't move the knee for a period of time. True knee locking is caused by a mechanical block where something gets caught inside the joint, preventing movement.

True locking is fairly rare and typically occurs as you move the knee into full extension, i. Here, knee movement is limited by temporary by muscle spasming as the body tries to protect itself in response to pain.

Here, we will look at the common causes of both types of knee locking and the different treatment options. The knee joint is designed to bend up and down, flexion and extension, and rotate slightly. If something gets caught inside the knee joint, it blocks the movement and the leg gets stuck. When this happens, the knee is totally blocked, unable to move at all. This is known as true locking, i. True knee locking is usually caused by:. The meniscus is a thick piece of cartilage which lines the knee joint to provide cushioning and allow smooth movement.

If the cartilage gets torn, the loose fragment may get stuck in the joint stopping it from being able to move. The most common type of meniscus tear that causes knee locking is known as a bucket-handle tear.

This is where part of the cartilage gets torn, but remains partially attached producing a movable flap, usually "C" shaped. As the knee moves around, if the cartilage flap is large enough, it can get wedged in the wrong position, blocking the joint and causing knee locking. Your knee just won't be able to move until you are able to manoeuvre the flap out of the way, freeing up the joint.

A meniscus tear can be caused by an injury, usually from a twisting movement or from gradual wear and tear on the joint. You can find out more in the meniscus tear section. Another thing that can block the joint and cause true knee locking is when a small fragment of bone breaks off from the knee joint, known as a loose body, and floats around.

As with a meniscus tear, if it moves into the wrong place, it can get wedged in place and cause the joint to lock in a specific position. True knee locking may or may not be accompanied by pain, depending on the cause. It is usually extension that is limited in this type of knee locking, preventing you from being able to fully extend your leg.

Pseudo knee locking is always accompanied by pain. It usually does this by causing the muscles to spasm, holding the leg in position. The difference with true locking is that there is nothing actually stuck inside the joint, and whilst the knee may at first appear to be stuck, it usually unlocks quickly.

Pseudo locking can limit both flexion and extension, bending and straightening the knee, whereas true locking is usually a block to extension only.This document describes the locking systems in the Linux Kernel in 2. With the wide availability of HyperThreading, and preemption in the Linux Kernel, everyone hacking on the kernel needs to know the fundamentals of concurrency and locking for SMP. This overlap, where the result depends on the relative timing of multiple tasks, is called a race condition.

The piece of code containing the concurrency issue is called a critical region. And especially since Linux starting running on SMP machines, they became one of the major issues in kernel design and implementation. Preemption can have the same effect, even if there is only one CPU: by preempting one task during the critical region, we have exactly the same race condition.

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In this case the thread which preempts might run the critical region itself. The solution is to recognize when these simultaneous accesses occur, and use locks to make sure that only one instance can enter the critical region at any time. There are many friendly primitives in the Linux kernel to help you do this. If I could give you one piece of advice: never sleep with anyone crazier than yourself.

But if I had to give you advice on locking: keep it simple.

Linux Kernel 5.4 Released With Kernel Lockdown, ExFAT Support & Improvements to AMD Radeon Graphics

Strangely enough, this last one is the exact reverse of my advice when you have slept with someone crazier than yourself. And you should think about getting a big dog. There are two main types of kernel locks. Spinlocks are very small and fast, and can be used anywhere. This means the CPU can do something else while you are waiting. Neither type of lock is recursive: see Deadlock: Simple and Advanced.

This is an excellent design decision: when no-one else can run at the same time, there is no reason to have a lock. For most purposes, we can think of preemption as equivalent to SMP, and not worry about it separately. Mutexes still exist, because they are required for synchronization between user contexts, as we will see below. This is the most trivial case: you initialize the mutex.

Registration and de-registration are only done on module load and unload and boot time, where there is no concurrencyand the list of registrations is only consulted for an unknown setsockopt or getsockopt system call. If a softirq shares data with user context, you have two problems. Firstly, the current user context can be interrupted by a softirq, and secondly, the critical region could be entered from another CPU.

It disables softirqs on that CPU, then grabs the lock. This, too, is exactly the same as above, because timers are actually run from a softirq.Robert explains the various locking primitives in the Linux kernel, why you need them and how kernel developers can use them to write safe code.

Proper locking can be tough--real tough. Improper locking can result in random crashes and other oddities. Poorly designed locking can result in code that is hard to read, performs poorly and makes your fellow kernel developers cringe.

In this article, I explain why kernel code requires locking, provide general rules for proper kernel locking semantics and then outline the various locking primitives in the Linux kernel. The fundamental issue surrounding locking is the need to provide synchronization in certain code paths in the kernel.

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These code paths, called critical sections, require some combination of concurrency or re-entrancy protection and proper ordering with respect to other events. The typical result without proper locking is called a race condition. Consider the case where one processor reads i, then another, then they both increment it, then they both write i back to memory.

If i were originally 2, it should now be 4, but in fact it would be 3! This is not to say that the only locking issues arise from SMP symmetric multiprocessing.

Interrupt handlers create locking issues, as does the new preemptible kernel, and any code can block go to sleep. Of these, only SMP is considered true concurrency, i. The other situations--interrupt handlers, preempt-kernel and blocking methods--provide pseudo concurrency as code is not actually executed concurrently, but separate code can mangle one another's data.

These critical regions require locking. The Linux kernel provides a family of locking primitives that developers can use to write safe and efficient code. Whether or not you have an SMP machine, people who use your code may.

Further, code that does not handle locking issues properly is typically not accepted into the Linux kernel. Finally, with a preemptible kernel even UP uniprocessor systems require proper locking. Thus, do not forget: you must implement locking.

This allows certain locks not to exist at all in a UP kernel. It does not matter, however, to the developer: lock everything appropriately and all situations will be covered.

THE SCREEN SAYS " LOCKING " and it goes into sleep mode

We cover atomic operators initially for two reasons. First, they are the simplest of the approaches to kernel synchronization and thus the easiest to understand and use.

Second, the complex locking primitives are built off them.

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In this sense, they are the building blocks of the kernel's locks. Atomic operators are operations, like add and subtract, which perform in one uninterruptible operation.

If we could read i, increment it and write it back to memory in one uninterruptible operation, the race condition discussed above would not be an issue. Atomic operators provide these uninterruptible operations.Skip to Content. How did popping and locking come to be? Find out more about the dance style and the moves to create more emotive dances. Popping and locking have inspired hip hop, dubstep, and other modern dance moves for a long time. These two dance styles differ in their movements, though.

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While both dance styles tend to intertwine, popping and locking is a combined style of dance that has a very distinct rhythm. This unique move is the combination of two movements: the pop and the lock. What Is Popping and Locking? These two moves combine to form a basis for hip hop, dubstep, and contemporary dancers to play into their choreography. Here's a guide to help you understand the moves and how you can up your popping and locking game so you can get better and create more emotive dances.

Dive into perfecting your popping by learning some of the basics. First of all, popping involves contracting and relaxing your muscles, creating a hit effect when you put it to the beat of the music you're dancing to.

You can use popping in quick successions or in slow and controlled movements to add emphasis to certain beats in the music. The Forearm Hit. To get the hang of popping, start with a basic forearm hit or forearm pop. Just contract and relax the muscles of your forearm to the beat of the music, either in fast hits or slow hits to reflect the different points in the rhythm that you want to make stand out in your dance. The Chest Pop. When you do a chest popyou're pretty much doing the same thing as you do with a forearm hit, except the movement is in your chest muscles.

Contract your chest inward quickly, almost where it feels like you're jerking your body in, but keep it controlled.

Relax the tension in your chest muscles as quickly as you contract, and repeat the motion in the pattern you need for your choreography. Play with this move by contracting and relaxing the muscles in your chest as you shift your body parts in different directions during your dance routine. Combine your forearm hits with your chest pops to create a flow to the music.

You can use these two popping moves to form the basis of other moves you put into your dances. Next Comes Locking. Locking is just stopping right in the middle of a quick movement and holding that position for a moment before flowing back into your dance. It's pretty easy to do.

Just go through your routine and put these stops at different points in the music that you want to accent with your body. Freeze your dance in a position that drives your choreography to the rhythm, hold it for a couple of beats, and continue the routine. Here are some signature lock dance moves you can practice to hone your skills as a dubstep or hip hop dancer.

Ask for Five. This locking dance step looks like the hand signal for a high five. To pull off this lock, listen for the points in the music where you want to put your stop, and start playing it out in your choreography.

Dance your routine and lock your hand in a high-five position for several beats. This looks awesome when you follow through with a forearm hit to emphasize the beat of the music. You can use the ask for five move in a few different ways.

In one, you can do it one-handed where you freeze your hand right in front of your body, or you can use both hands as you flow with forearm hits. It's really up to you how much or how little you put the move into your dance steps. Locking Points.

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To pull off this lock, incorporate pointing in your choreography.


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