Re: [RFC][PATCH 0/5] arch: atomic rework

[Torvald Riegel]

On Mon, 2014-02-24 at 09:28 -0800, Paul E. McKenney wrote:
> On Mon, Feb 24, 2014 at 05:55:50PM +0100, Michael Matz wrote:
> > Hi,
> > 
> > On Mon, 24 Feb 2014, Linus Torvalds wrote:
> > 
> > > > To me that reads like
> > > >
> > > >   int i;
> > > >   int *q = &i;
> > > >   int **p = &q;
> > > >
> > > >   atomic_XXX (p, CONSUME);
> > > >
> > > > orders against accesses '*p', '**p', '*q' and 'i'.  Thus it seems they
> > > > want to say that it orders against aliased storage - but then go further
> > > > and include "indirectly through a chain of pointers"?!  Thus an
> > > > atomic read of a int * orders against any 'int' memory operation but
> > > > not against 'float' memory operations?
> > > 
> > > No, it's not about type at all, and the "chain of pointers" can be
> > > much more complex than that, since the "int *" can point to within an
> > > object that contains other things than just that "int" (the "int" can
> > > be part of a structure that then has pointers to other structures
> > > etc).
> > 
> > So, let me try to poke holes into your definition or increase my 
> > understanding :) .  You said "chain of pointers"(dereferences I assume), 
> > e.g. if p is result of consume load, then access to 
> > p->here->there->next->prev->stuff is supposed to be ordered with that load 
> > (or only when that last load/store itself is also an atomic load or 
> > store?).
> > 
> > So, what happens if the pointer deref chain is partly hidden in some 
> > functions:
> > 
> > A * adjustptr (B *ptr) { return &ptr->here->there->next; }
> > B * p = atomic_XXX (&somewhere, consume);
> > adjustptr(p)->prev->stuff = bla;
> > 
> > As far as I understood you, this whole ptrderef chain business would be 
> > only an optimization opportunity, right?  So if the compiler can't be sure 
> > how p is actually used (as in my function-using case, assume adjustptr is 
> > defined in another unit), then the consume load would simply be 
> > transformed into an acquire (or whatever, with some barrier I mean)?  Only 
> > _if_ the compiler sees all obvious uses of p (indirectly through pointer 
> > derefs) can it, yeah, do what with the consume load?
> 
> Good point, I left that out of my list.  Adding it:
> 
> 13.	By default, pointer chains do not propagate into or out of functions.
> 	In implementations having attributes, a [[carries_dependency]]
> 	may be used to mark a function argument or return as passing
> 	a pointer chain into or out of that function.
> 
> 	If a function does not contain memory_order_consume loads and
> 	also does not contain [[carries_dependency]] attributes, then
> 	that function may be compiled using any desired dependency-breaking
> 	optimizations.
> 
> 	The ordering effects are implementation defined when a given
> 	pointer chain passes into or out of a function through a parameter
> 	or return not marked with a [[carries_dependency]] attributed.
> 
> Note that this last paragraph differs from the current standard, which
> would require ordering regardless.

I would prefer if we could get rid off [[carries_dependency]] as well;
currently, it's a hint whose effectiveness really depends on how the
particular implementation handles this attribute.  If we still need
something like it in the future, it would be good if it had a clearer
use and performance effects.

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