Re: [PATCH 0/24] make atomic_read() behave consistently across all architectures

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

Nick Piggin wrote:
> Satyam Sharma wrote:
>> 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/