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

[Nick Piggin]

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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