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In software engineering, double-checked locking (also known as "double-checked locking optimization" [1]) is a software design pattern used to reduce the overhead of acquiring a lock by testing the locking criterion (the "lock hint") before acquiring the lock. Locking occurs only if the locking criterion check indicates that locking is required.
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.
A snippet of Python code with keywords highlighted in bold yellow font. The syntax of the Python programming language is the set of rules that defines how a Python program will be written and interpreted (by both the runtime system and by human readers). The Python language has many similarities to Perl, C, and Java. However, there are some ...
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 ...
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.
The downside is that write-preferring locks allows for less concurrency in the presence of writer threads, compared to read-preferring RW locks. Also the lock is less performant because each operation, taking or releasing the lock for either read or write, is more complex, internally requiring taking and releasing two mutexes instead of one.
All wait-free algorithms are lock-free. In particular, if one thread is suspended, then a lock-free algorithm guarantees that the remaining threads can still make progress. Hence, if two threads can contend for the same mutex lock or spinlock, then the algorithm is not lock-free. (If we suspend one thread that holds the lock, then the second ...
Timsort is a stable sorting algorithm (order of elements with same key is kept) and strives to perform balanced merges (a merge thus merges runs of similar sizes). In order to achieve sorting stability, only consecutive runs are merged. Between two non-consecutive runs, there can be an element with the same key inside the runs.