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Many systems provide hardware support for implementing the critical section code. • All solutions below based on idea of locking. – Protecting critical regions via locks. • Modern machines provide special atomic hardware instructions. • Atomic = non-interruptible. – test memory word and set value. – swap contents of two
critical section remainder section. until false. Test-and-Set(target). result := target target := TRUE return result. Information common to both processes: target = FALSE. Read S&G, Section 6.4. Swap. another type of hardware assistance for process synchronization; a special hardware instruction that does a complete swap
Correct critical sections have the following requirements. – mutual exclusion. • at most one thread is in the critical section. – progress. • if thread T is outside the critical section, then T cannot prevent thread S from entering the critical section. – bounded waiting (no starvation). • if thread T is waiting on the critical section, then
If two concurrent threads are accessing a shared variable, and that variable is read/modified/written by those threads, then access to the variable must be controlled to avoid erroneous behavior. 0. Over the next couple of lectures, we will look at. ?. Mechanisms to control access to shared resources. » Locks, mutexes
on interrupts after the critical section. Guaranteed atomicity. It is also fairly efficient. Early versions of Unix did this. Why not just use turning off interrupts? Two main disadvantages: can't use in a multiprocessor, and can't use directly from user program for synchronization. Test-And-Set. The test and set instruction atomically
Synchronization Hardware. • Many systems provide hardware support for critical section code. • Uniprocessors. – could disable interrupts. – Currently running code would execute without preemption. – Generally too inefficient on multiprocessor systems. • Operating systems using this not broadly scalable. • Modern
In computer science, the test-and-set instruction is an instruction used to write 1 (set) to a memory location and return its old value as a single atomic (i.e., non-interruptible) operation. The calling process obtains the lock if the old value was 0 otherwise while-loop spins waiting to acquire the lock.
6 Apr 2010 Atomic execution of a set of instructions means that the instructions are treated as a single step that cannot be interrupted. // example process using mutual exclusion void process() { int mutex; init_lock (&mutex); do { lock (&mutex); // critical section unlock (&mutex); //remainder section } while(TRUE); }
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