Atomicity and semaphores

update Oct 22, 2017 15:19

Kinds of atomicity

synthetic (voluntary) atomicity: programs and/or threads obey locking to avoid operating at the same time. kernel atomicity: when a system call is executing, the scheduler won't interrupt it. instruction atomicity: some machine instructions preclude concurrent operations. special-purpose system calls: some specific system calls execute program instructions in an atomic (uninterruptible) fashion.

Kinds of locks

Between threads: mutexes user mode (except when they block!) exploit shared memory binary: locked or unlocked

Between threads: unnamed semaphores user mode (except when they block!) exploit shared memory they're integers Unlimited numbers available.

Between processes: named semaphores kernel mode: live in kernel memory. no shared memory. They're integers. An array offset in a kernel table. Limited number available.

Between machines: network semaphores network objects use network communications no shared architecture.

What is a mutex

Basically, one word of user memory. Utilize a simple race condition to acquire lock, based upon atomicity of operations in assembly language. If the word of memory is > 0, then a lock has been acquired. If the word of memory == 0, then there is no lock. Testset: if word==0 then word=1 as an atomic operation.

what is a semaphore

A semaphore is an integer to which access is atomic. Initialized to some integer value. locks decrement it. unlocks increment it. value determines number of concurrent locks available.

Mutex versus semphores

• Mutexes can only be locked or unlocked, they are sometimes called "binary semaphores".

• Semaphores have an integer value that is the number of locks available. Can read a semaphore's value via sem_getvalue(&sem,&val) where val is an int.

Using a semphore