Re: [RFC PATCH] Writer-biased low-latency rwlock v8
From: H. Peter Anvin
Date: Thu Aug 21 2008 - 17:32:01 EST
Linus Torvalds wrote:
Because that is already crap.
Go look at my code once more. Go look at how it has 128 bits of data,
exactly so that it DOES NOT HAVE TO LIMIT THE NUMBER OF READERS.
And then go look at it again.
Look at it five times, and until you can understand that it still uses
just a 32-bit word for the fast-path and no unnecessarily crap in it, but
it actually has 128 bits of data for all the slow paths, don't bother
emailing me any new versions.
Please. You -still- apparently haven't looked at it, at least not enough
to understand the _point_ of it. You still go on about trying to fit in
three or four different numbers in that one word. Even though the whole
point of my rwlock is that you need exactly _one_ count (active writers),
and _one_ bit (active reader) and _one_ extra bit ("contention, go to slow
path, look at the other bits ONLY IN THE SLOW PATH!")
That leaves 30 bits for readers. If you still think you need to "limit the
number of readers", then you aren't getting it.
First of all, let me say I don't pretend to understand formally how you
deal with overflow-after-the-fact, as unlikely as it is.
However, it seems to me to be an easy way to avoid it. Simply by
changing the read-test mask to $0x80000003, you will kick the code down
the slow path once the read counter reaches $0x80000004 (2^29+1
readers), where you can do any necessary fixup -- or BUG() -- at leisure.
This fastpath ends up being identical in size and performance to the one
you posted, although yours could be reduced by changing the test to a
testb instruction -- at the almost certainly unacceptable expense of
taking a partial-register stall on the CPUs that have those.
-hpa
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