Re: [PATCH 1/24] make atomic_read() behave consistently on alpha

From: Chris Snook
Date: Thu Aug 09 2007 - 15:36:28 EST


Segher Boessenkool wrote:
The compiler is within its rights to read a 32-bit quantity 16 bits at
at time, even on a 32-bit machine. I would be glad to help pummel any
compiler writer that pulls such a dirty trick, but the C standard really
does permit this.

Yes, but we don't write code for these compilers. There are countless pieces of kernel code which would break in this condition, and there doesn't seem to be any interest in fixing this.

"Other things are broken too". Great argument :-)

We make plenty of practical assumptions in the kernel, and declare incorrect things which violate them, even in cases where there's no commandment from the heavens forbidding them. Since the whole point of this exercise is to prevent badness with *optimizing* compilers, it's quite reasonable to declare broken any so-called optimizer which violates these trivial assumptions.

In short, please retain atomic_set()'s volatility, especially on those
architectures that declared the atomic_t's counter to be volatile.

Like i386 and x86_64? These used to have volatile in the atomic_t declaration. We removed it, and the sky did not fall.

And this proves what? Lots of stuff "works" by accident.

If something breaks because of this, it was already broken, but hidden a lot better. I don't see much of a downside to exposing and fixing those bugs.

-- Chris
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