Re: [PATCH 1/24] make atomic_read() behave consistently on alpha
From: Chris Snook
Date: Thu Aug 09 2007 - 15:56:05 EST
Geert Uytterhoeven wrote:
On Thu, 9 Aug 2007, Chris Snook wrote:
Segher Boessenkool wrote:
The only safe way to get atomic accesses is to write
assembler code. Are there any downsides to that? I don't
see any.
The assumption that aligned word reads and writes are atomic, and that
words are aligned unless explicitly packed otherwise, is endemic in the
kernel. No sane compiler violates this assumption. It's true that we're
not portable to insane compilers after this patch, but we never were in
the first place.
You didn't answer my question: are there any downsides to using
explicit coded-in-assembler accesses for atomic accesses? You
can handwave all you want that it should "just work" with
volatile accesses, but volatility != atomicity, volatile in C
is really badly defined, GCC never officially gave stronger
guarantees, and we have a bugzilla full of PRs to show what a
minefield it is.
So, why not use the well-defined alternative?
Because we don't need to, and it hurts performance.
It hurts performance by implementing 32-bit atomic reads in assembler?
No, I misunderstood the question. Implementing 32-bit atomic reads in assembler
is redundant, because any sane compiler, *particularly* and optimizing compiler
(and we're only in this mess because of optimizing compilers) will give us that
automatically without the assembler. Yes, it is legal for a compiler to violate
this assumption. It is also legal for us to refuse to maintain compatibility
with compilers that suck this badly. That decision was made a very long time
ago, and I consider it the correct decision.
-- Chris
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