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In this solution of the readers/writers problem, the first reader must lock the resource (shared file) if such is available. Once the file is locked from writers, it may be used by many subsequent readers without having them to re-lock it again. Before entering the critical section, every new reader must go through the entry section. However ...
To release the lock upon exiting the critical section, process i sets level[i] to −1. That this algorithm achieves mutual exclusion can be proven as follows. Process i exits the inner loop when there is either no process with a higher level than level[i] , so the next waiting room is free; or, when i ≠ last_to_enter[ℓ] , so another ...
The scheduler will not migrate the process or thread to another processor, and it will not schedule another process or thread to run while the current process or thread is in a critical section. Similarly, if an interrupt occurs in a critical section, the interrupt information is recorded for future processing, and execution is returned to the ...
For example, when the operating system's lock library is used and a thread tries to acquire an already acquired lock, the operating system could suspend the thread using a context switch and swap it out with another thread that is ready to be run, or could put that processor into a low power state if there is no other thread that can be run.
Generally, locks are advisory locks, where each thread cooperates by acquiring the lock before accessing the corresponding data. Some systems also implement mandatory locks, where attempting unauthorized access to a locked resource will force an exception in the entity attempting to make the access. The simplest type of lock is a binary ...
not hold the lock across system or function calls where the entity is no longer running on the processor – this can lead to deadlock; ensure that if the entity is unexpectedly exited for any reason, the lock is freed. Non-holders of the lock (a.k.a. waiters) can be held in a list that is serviced in a round-robin fashion, or in a FIFO queue ...
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A lock file is often the best approach if the resource to be controlled is not a regular file at all, so using methods for locking files does not apply. For example, a lock file might govern access to a set of related resources, such as several different files, directories, a group of disk partitions, or selected access to higher level ...