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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jan 06, 2009 at 03:25:12AM +1100, Nick Piggin wrote:
> 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?

Assuming that "var" is an ordinary variable...

If you are asking whether the compiler guys believe that they are within
their rights to do a store to *var and then store the old value to *var
if !locked, the unfortunate answer is "yes".  Hence ACCESS_ONCE().

> > 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...

Another approach would be to change ACCESS_ONCE() to use the atomic
types.  Then we have a bigger hammer with which to beat the gcc guys
about optimizations.  We can hope, anyway...

							Thanx, Paul