Suppose that when the x.signal() operation is invoked by a process P, there is a suspended process Q associated with condition x (already in the monitor).
If no process is suspended, then the x.signal operation has no effect.
High-level synchronization construct that allows the safe sharing of an abstract data type among concurrent processes (e.g., synchronized in Java). A process may never be removed from the semaphore queue in which it is suspended. Let S and Q be two semaphores initialized to 1. Often employed on multiprocessor systems.ĭeadlock – two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes.When locks are expected to be held for short times, spinlocks are useful.Have an advantage in that no context switch is required.Spinlock – process spins while waiting for the lock. In general, busy waiting wastes CPU cycles that some other process might be able to use productively. Two Types of SemaphoresĬounting semaphore – integer value can range over an unrestricted domain.īinary semaphore – integer value can range only between 0 and 1 can be simpler to implement than counting semaphore also known as mutex lock. Semaphore S represents an integer variable that can only be accessed via two indivisible (atomic) operations. lock must be held before calling this function.Synchronization tool that does not require busy waiting. Wakes up all threads, if any, waiting on cond (protected by monitor lock lock). Void cond_broadcast (struct condition *cond, struct lock *lock) lock must be held before calling this function. If no threads are waiting, returns without performing any action. If any threads are waiting on cond (protected by monitor lock lock), then this function wakes up one of them. Void cond_signal (struct condition *cond, struct lock *lock) Thus, typically cond_wait’s caller must recheck the condition after the wait completes and, if necessary, wait again. Sending a signal and waking up from a wait are not an atomic operation. After cond is signaled, reacquires lock before returning. Void cond_wait (struct condition *cond, struct lock *lock)Ītomically releases lock (the monitor lock) and waits for cond to be signaled by some other piece of code. Initializes cond as a new condition variable. Their practical usage was later elaborated in a paper on the Mesa operating system.Ĭondition variable types and functions are declared in threads/synch.h. The theoretical framework for monitors was laid out by C. If, on the other hand, it has caused one of these conditions to become true, it “signals” the condition to wake up one waiter, or “broadcasts” the condition to wake all of them.
#LOCKS MONITOR AND SEMAPHOR CODE#
When code in the monitor needs to wait for a condition to become true, it “waits” on the associated condition variable, which releases the lock and waits for the condition to be signaled. “some data has arrived for processing” or “over 10 seconds has passed since the user’s last keystroke”. Each condition variable is associated with an abstract condition, e.g. When access to the protected data is complete, it releases the monitor lock.Ĭondition variables allow code in the monitor to wait for a condition to become true. While in the monitor, the thread has control over all the protected data, which it may freely examine or modify. Before it accesses the protected data, a thread first acquires the monitor lock.
#LOCKS MONITOR AND SEMAPHOR PLUS#
A monitor consists of data being synchronized, plus a lock, called the monitor lock, and one or more condition variables.
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