Your parch kept doing "spin_trylock()", didn't it?
That's a read-modify-write thing, and keeps bouncing the cacheline back
and forth, and together with the fact that even *after* you get the
spinlock the "wait_for_inactive()" would actually end up looping back,
releasing it, and re-getting it.
So the problem was that "wait_for_inactive()" kept the lock (because it
actually *got* it), and looped over getting it, and because it was an
exclusive cacheline ownership, that implies that somebody else is not
getting it, and is kept from ever getting it.
So trying to use "trylock" doesn't help. It still has all the same bad
sides - it still gets the lock (getting the lock wasn't the problem:
_holding_ the lock was the problem), and it still kept the cache line for
the lock on one core.
The only way to avoid lock contention is to avoid any exclusive use at
all.
Linus
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