Segher Boessenkool wrote:

quoted

quoted

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.