de7546e12b
If there were exactly rowner_retries/asx_retries (by default: 10) transitions between read and write state and the waiters still did not get the lock, the next owner -> reader transition would result in the code correctly falling back to turnstile/sleepq where it would incorrectly think it was waiting for a writer and decide to leave turnstile/sleepq to loop back. From this point it would take ts/sq trips until the lock gets released. The bug sometimes manifested itself in stalls during -j 128 package builds. Refactor the code to fix the bug, while here remove some of the gratituous differences between rw and sx locks.
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