try_upgrade for rwlock #7284
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try_upgrade for rwlock #7284
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the code is a rough draft and is mostly here to help the discussion in #7282
Motivation
in #7282 there was an idea to add an upgrade method to write locks.
I hope to achieve this without changing any existing code as it is a small feature
Solution
the key idea is that we can use compare_exchange to try and fish for specifically the situation where we have 1 reader and exchange the semiphore to 1 writer. this should be safe because of the invariant of compare_exchange