Notes of Linux Device Driver 3rd (2)

* Concurrency and race condition
Two aspects of resource sharing:
- Resource sync
- Reference count

Utils for resource sync provided in Linux Kernel
- Semaphore and mutexes
Process could be blocked and sleep. Not used in ISR.
Semaphore here is not preferred to use as event sync util. And it is ususally optimized for the "available" case. Completions could be used for event sycn.
- RW semaphores
best used when write access is required only rarely and writer access is held for short period of time.
- Spinlocks
Process could NOT be blocked. Higher performance than semaphores but has a lot of constraints:
-- better used on SMP systems
-- when holding a lock, the code must be atomic, non-blocking and non-sleep. so preemption would be disabled for this core when holding the lock; the interrupts are disabled either for this core.
-- when holding a lock, the time should be as little as possible.
- RW spinlocks

Alternatives to locking
- Lock-free algorithms: circular buffer
- Atomic variables
- Bit operations
- seqlocks: small, simple and frequently accessed and where write access is rare but must be fast. Non pointers in the protected data.
- Read-Copy-Update: reads are common and writes are rare. The resources must be accessed via pointers, and all references to those resources must be held only by atomic code.