The first 9 Chapter Introduction to kernel synchronization

Critical zones and competitive conditions

Critical areas are snippets of code that access and manipulate shared data . It is usually unsafe for multiple threads to access the same resource concurrently , It must be ensured that these operations are atomic . If two execution threads are in a critical area, they may execute at the same time , So this is a bug, Call it a competitive condition （race conditions）. Mistakes caused by competition are very difficult to reproduce , So debugging is very difficult . Avoiding concurrency and preventing race conditions is called synchronization （synchronization）.

Causes of concurrent execution

User space , The user program will be preempted and rescheduled by the scheduler .
Kernel space , For a similar reason ：

1. interrupt
2. Soft interrupt and tasklet
3. Kernel preemption
4. Sleep and synchronization with user space
5. Symmetric multiprocessing

What data needs to be locked

1. overall situation ？ Whether other threads can access ？
2. Will share in context ？ Will it be shared between two different interrupt handlers ？
3. Can it be preempted ？ Will the scheduled new program access the same data ？
4. Will it block some resources , Processing of shared data ？
   …

Deadlock

There should be one or more execution threads and one or more resources , Each thread is waiting for one of the resources , But all resources are occupied . But the thread will not release the resources already occupied , A deadlock occurs .
The prevention of ：

1. Lock in sequence
2. Prevent hunger
3. Do not repeatedly request the same lock
4. The design is simple

Contention and scalability

Lock contention （lock contention）, The lock is held frequently or for a long time , Will degrade the performance of the system .
Extensibility （scalability） Is a measure of system scalability .
Lock granularity is used to describe the data scale of lock protection . It is between too large and too small when used .

summary

Protect data from concurrent access ！