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Read-Write locking primitive
Synchronization object used in a solution of so-called second
readers-writers problem. In this problem, many readers can simultaneously
access a share, and a writer has an exclusive access to this share.
Additionally, the following constraints should be met:
1) no reader should be kept waiting if the share is currently opened for
reading unless a writer is also waiting for the share,
2) no writer should be kept waiting for the share longer than absolutely
necessary.
The implementation is based on [1, secs. 4.2.2, 4.2.6, 4.2.7]
with a modification -- adding an additional lock (C{self.__readers_queue})
-- in accordance with [2].
Sources:
[1] A.B. Downey: "The little book of semaphores", Version 2.1.5, 2008
[2] P.J. Courtois, F. Heymans, D.L. Parnas:
"Concurrent Control with 'Readers' and 'Writers'",
Communications of the ACM, 1971 (via [3])
[3] http://en.wikipedia.org/wiki/Readers-writers_problem
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