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Em-mustapha (talk | contribs) m Reverted edits by 82.32.77.88 (talk) to last version by Serols |
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"In the case that ownership of the lock goes from thread A to thread B, A-post-gain-lock statements come before B-post-gain-lock statements."
▲Seems simple, right? The complication comes from the fact that the execution model does not have any means for the execution of "give up ownership of the lock" to have any influence over which execution of "gain ownership of the lock" in some other timeline (thread) follows. Very often, only certain handoffs give valid results. Thus, the programmer must think of all possible combinations of one thread giving up a lock and another thread getting it next, and make sure their code only allows valid combinations.
Note that the only effect is that A-post-gain-lock statements come before B-post-gain-lock statements. No other effect happens, and no other relative ordering can be relied upon. Specifically, A-post-give-up-lock and B-post-gain-lock have ''no relative ordering'' defined, which surprises many people. But thread A may have been swapped out after giving up ownership, so A-post-give-up-lock statements may happen long after many B-post-gain-lock statements have finished. That is one of the possibilities that must be thought about when designing locks, and illustrates why multi-threaded programming is difficult.
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