Книга: Embedded Linux Primer: A Practical, Real-World Approach

17.3.4. Critical Section Management

17.3.4. Critical Section Management

When writing kernel code, such as a custom device driver, you will encounter data structures that you must protect from concurrent access. The easiest way to protect critical data is to disable preemption around the critical section. Keep the critical path as short as possible to maintain a low maximum latency for your system. Listing 17-5 shows an example.

Listing 17-5. Protecting Critical Section in Kernel Code

...
/*
* Declare and initialize a global lock for your
* critical data
*/
DEFINE_SPINLOCK(my_lock);
...
int operate_on_critical_data() {
 ...
 spin_lock(&my_lock);
 ...
 /* Update critical/shared data */
 ...
 spin_unlock(&my_lock);
 ...
}

When a task successfully acquires a spinlock, preemption is disabled and the task that acquired the spinlock is allowed into the critical section. No task switches can occur until a spin_unlock operation takes place. The spin_lock() function is actually a macro that has several forms, depending on the kernel configuration. They are defined at the top level (architecture-independent definitions) in .../include/linux/spinlock.h. When the kernel is patched with the real-time patch, these spinlocks are promoted to mutexes to allow preemption of higher-priority processes when a spinlock is held.

Because the real-time patch is largely transparent to the device driver and kernel developer, the familiar constructs can be used to protect critical sections, as described in Listing 17-5. This is a major advantage of the real-time patch for real-time applications; it preserves the well-known semantics for locking and interrupt service routines.

Using the macro DEFINE_SPINLOCK as in Listing 17-5 preserves future compatibility. These macros are defined in .../include/linux/spinlock_types.h.

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