Nick Piggin wrote:

Satyam Sharma wrote:

quoted

And we have driver / subsystem maintainers such as Stefan
coming up and admitting that often a lot of code that's written to use
atomic_read() does assume the read will not be elided by the compiler.

So these are broken on i386 and x86-64?

The ieee1394 and firewire subsystems have open, undiagnosed bugs, also
on i386 and x86-64.  But whether there is any bug because of wrong
assumptions about atomic_read among them, I don't know.  I don't know
which assumptions the authors made, I only know that I wasn't aware of
all the properties of atomic_read until now.

Are they definitely safe on SMP and weakly ordered machines with
just a simple compiler barrier there? Because I would not be
surprised if there are a lot of developers who don't really know
what to assume when it comes to memory ordering issues.

This is not a dig at driver writers: we still have memory ordering
problems in the VM too (and probably most of the subtle bugs in
lockless VM code are memory ordering ones). Let's not make up a
false sense of security and hope that sprinkling volatile around
will allow people to write bug-free lockless code. If a writer
can't be bothered reading API documentation

...or, if there is none, the implementation specification (as in case of
the atomic ops), or, if there is none, the implementation (as in case of
a some infrastructure code here and there)...

and learning the Linux memory model, they can still be productive
writing safely locked code.

Provided they are aware that they might not have the full picture of the
lockless primitives.  :-)
-- 
Stefan Richter
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http://arcgraph.de/sr/