Re: atomics: document that linux expects certain atomic behaviour from unsigned long

On Tuesday 06 January 2009 03:05:01 Paul E. McKenney wrote:
On Mon, Jan 05, 2009 at 11:00:24PM +1100, Nick Piggin wrote:
On Monday 05 January 2009 22:23:50 Alan Cox wrote:
Pretty much everywhere that uses RCU for example does so using atomic
pointer loads and stores. The nastiest issue IMO actually is
reloading the value through the pointer even if it isn't explicitly
dereferenced. RCU gets this right with ACCESS_ONCE. Probably a lot of
code using basic types does not. x86 atomic_read maybe should be
using ACCESS_ONCE too...

I'm pretty sure it should. gcc makes no guarantees about not being
clever with accesses.

Arguably it should. I don't know what the concurrent C standard looks
like, but prohibiting reloads of potentially concurrently modified memory
when there is no explicit pointer dereference is the natural complement
to prohibiting stores to potentially concurrently read memory when there
is no explicit store (which I think is begrudgingly agreed to be a
problem).

http://lkml.org/lkml/2007/10/24/673

I think I would like to see multiple reloads to local variables
prohibited, to avoid potential really subtle problems... But if
ACCESS_ONCE is here to stay, then I do think that atomic_read etc should
use it.

The concurrency stuff in c++0x still permits the compiler to have its
way with loads and stores to normal variables, but provides an "atomic"
type that must be loaded and stored as specified in the program.

So:
if (trylock())
locked = 1;
...
if (locked)
*var = blah;
...
if (locked)
unlock();

So the second part can still be transformed into a predicated calculation
of blah, then an unconditional store to *var?


The issue with ACCESS_ONCE() is that gcc doesn't do any optimizations on
volatile accesses, even the obvious ones. Speaking of which, the gcc
guys kicked out my bug 33102, which was complaining about this
situation. :-/

Hmm. It's still quite annoying even to have to switch everything to the
atomic type. I guarantee there will be bugs in Linux caused by the
compiler reloading pointers/longs/ints to access local variables...


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