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Symmetric-key encryption: the same key is used for both encryption and decryption. Symmetric-key algorithms [a] are algorithms for cryptography that use the same cryptographic keys for both the encryption of plaintext and the decryption of ciphertext. The keys may be identical, or there may be a simple transformation to go between the two keys. [1]
The above example does not guarantee that the task has the lock, since more than one task can be testing the lock at the same time. Since both tasks will detect that the lock is free, both tasks will attempt to set the lock, not knowing that the other task is also setting the lock.
On any multi-processor system that uses the MESI contention protocol, such a test-and-test-and-set lock (TTAS) performs much better than the simple test-and-set lock (TAS) approach. [ 4 ] With large numbers of processors, adding a random exponential backoff delay before re-checking the lock performs even better than TTAS.
volatile int lock = 0; void critical {// Spin lock: loop forever until we get the lock; we know the lock was // successfully obtained after exiting this while loop because the // test_and_set() function locks the lock and returns the previous lock // value. If the previous lock value was 1 then the lock was **already** // locked by another ...
[3] [4] [5] Hashing is an example of a space-time tradeoff. If memory is infinite, the entire key can be used directly as an index to locate its value with a single memory access. On the other hand, if infinite time is available, values can be stored without regard for their keys, and a binary search or linear search can be used to retrieve the ...
The key point to note from the example is that the initial attempts by all four processors to acquire the lock results in only the first to arrive actually getting the lock. All subsequent attempts, while the first still holds the lock, serves to form the queue of processors waiting their turn in the critical section.
In computer science, a readers–writer (single-writer lock, [1] a multi-reader lock, [2] a push lock, [3] or an MRSW lock) is a synchronization primitive that solves one of the readers–writers problems. An RW lock allows concurrent access for read-only operations, whereas
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 ...