1. Sleep and Wake-up of Linux Processes
In Linux, processes that are waiting for CPU time are referred to as ready processes. These are kept in a run queue, and their status flag is set to TASK_RUNNING. When a process exhausts its time slice, the Linux kernel's scheduler takes control away from it and selects another process from the run queue to execute.
Processes can also voluntarily release the CPU by calling the schedule() function. This function allows the process to yield control to another process. Once the process is rescheduled, it resumes execution from the point where it last called schedule().
Sometimes, a process needs to wait for a specific event—such as device initialization, I/O completion, or a timer trigger. In such cases, the process is removed from the run queue and added to a wait queue, entering a sleep state.
Linux defines two types of sleep states: interruptible sleep (TASK_INTERRUPTIBLE) and uninterruptible sleep (TASK_UNINTERRUPTIBLE). A process in an interruptible sleep state will wake up when a condition is met, such as a hardware interrupt, resource availability, or a signal. However, a process in an uninterruptible sleep state will not respond to signals, making it less common but useful in critical situations where the process must wait until a specific event occurs.
Modern Linux systems typically use the schedule() function to put a process to sleep. Here's an example:
sleeping_task = current;
set_current_state(TASK_INTERRUPTIBLE);
schedule();
func1();
The first line stores the current process structure, while set_current_state() changes the process’s state to TASK_INTERRUPTIBLE. If the process was in TASK_RUNNING before calling schedule(), it will be replaced by another process. If it was already in a sleep state, it will be removed from the run queue, effectively putting it to sleep.
To wake up a sleeping process, the wake_up_process() function is used. It sets the process’s state back to TASK_RUNNING and adds it to the run queue. However, the process will only resume execution when it is scheduled again.
2. The Problem of Invalid Wake-ups
In most cases, a process checks a condition and goes to sleep if it is not met. However, sometimes a process may go to sleep even when the condition is already true. This results in an invalid wake-up problem, where the process sleeps indefinitely because no one wakes it up.
This issue often arises due to race conditions, where multiple processes access shared data. For example, consider two processes, A and B. Process A checks if a linked list is empty and sleeps if it is. Process B adds nodes to the list and tries to wake up A. But if B runs before A actually goes to sleep, the wake-up call is ineffective, and A may sleep unnecessarily.
3. Avoiding Invalid Wake-ups
To avoid this, the process should ensure that checking the condition and setting the sleep state happen as a single atomic operation. One approach is to set the process’s state to TASK_INTERRUPTIBLE before checking the condition. This way, if a wake-up call happens after the check but before the sleep, the process will be in the correct state to respond.
For example, modifying the code to:
set_current_state(TASK_INTERRUPTIBLE);
spin_lock(&list_lock);
if (list_empty(&list_head)) {
spin_unlock(&list_lock);
schedule();
spin_lock(&list_lock);
}
set_current_state(TASK_RUNNING);
ensures that the process is in the correct state before checking the condition, preventing unnecessary sleep.
4. Examples from the Linux Kernel
The Linux kernel uses a similar approach to handle sleep and wake-up operations safely. It creates a wait queue, adds the process to it, sets the state to TASK_INTERRUPTIBLE, and then loops until the condition is met. Only then does it call schedule() and proceed.
An example from the Linux kernel (from sched.c):
set_current_state(TASK_INTERRUPTIBLE);
while (!kthread_should_stop()) {
schedule();
set_current_state(TASK_INTERRUPTIBLE);
}
__set_current_state(TASK_RUNNING);
This ensures that the process only sleeps when necessary, avoiding invalid wake-up scenarios.
Summary
To prevent invalid wake-ups in Linux, processes should set their state to TASK_INTERRUPTIBLE or TASK_UNINTERRUPTIBLE before checking conditions. If the condition is already satisfied, the state should be reset to TASK_RUNNING. This ensures that the process does not sleep unnecessarily, maintaining system stability and correctness.
VCM Spacer is a kind of metal etching spring sheets and is an important part of the VCM. VCM is include of Shield Case, Frame, F.Spacer, F.Spring, Yoke, Magnet, Coil, Carrier, B.Spacer, B.Spring, Base.
VCM is a camera important component, its main function is to allow the position of the lens to be adjusted to give a clear image. It is mainly used in camera micro zoom motor (VCM) for mobile phone camera, security camera, UAV camera, tablet computer camera, computer camera, as well as smart wear, high-end electronics, electronic scanning payment equipment and photo electricity and other fields. The material we usually use for metal etching VCM spacer are SUS304.C7701.BF158-TM10.C1990-GSH. We use fine metal etching process, and we can guarantee that our etched products have high precision, uniform etching lines, no burr surface, no notch and no deformation. We are the best supplier and manufacturer of VCM spacer in China, our VCM spacer have high quality but low price. We custom precision VCM spacer with drawings provided by customers.
VCM Spacer for Camera, Metal Etching VCM Spacer, High Precision VCM Spacer, Custom Precision VCM Spacer
SHAOXING HUALI ELECTRONICS CO., LTD. , https://www.cnsxhuali.com