linux-kernel - Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an

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Message-ID: <20061211023054.2622.qmail@science.horizon.com>
Date:	10 Dec 2006 21:30:54 -0500
From:	linux@...izon.com
To:	nickpiggin@...oo.com.au, torvalds@...l.org
Cc:	linux@...izon.com, linux-arch@...r.kernel.org,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an

> Even if ARM is able to handle any arbitrary C code between the
> "load locked" and store conditional API, other architectures can not
> by definition.

Maybe so, but I think you and Linus are missing the middle ground.

While I agree that LL/SC can't be part of the kernel API for people to
get arbitrarily clever with in the device driver du jour, they are *very*
nice abstractions for shrinking the arch-specific code size.

The semantics are widely enough shared that it's quite possible in
practice to write a good set of atomic primitives in terms of LL/SC
and then let most architectures define LL/SC and simply #include the
generic atomic op implementations.

If there's a restriction that would pessimize the generic implementation,
that function can be implemented specially for that arch.

Then implementing things like backoff on contention can involve writing
a whole lot less duplicated code.

Just like you can write a set of helpers for, say, CPUs with physically
addressed caches, even though the "real" API has to be able to handle the
virtually addressed ones, you can write a nice set of helpers for machines
with sane LL/SC.
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