Re: Memory allocator semantics

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

On Mon, Feb 10, 2014 at 01:07:58PM -0600, Christoph Lameter wrote:
> On Sat, 8 Feb 2014, Pekka Enberg wrote:
> 
> > So to be completely honest, I don't understand what is the race in (A) that
> > concerns the *memory allocator*.  I also don't what the memory allocator can
> > do in (B) and (C) which look like double-free and use-after-free,
> > respectively, to me. :-)
> 
> Well it seems to be some academic mind game to me.
> 
> Does an invocation of the allocator have barrier semantics or not?

In case (A), I don't see why the allocator should have barrier semantics
from kmalloc() to a matching kfree().  I would argue that any needed
barrier semantics must be provided by the caller.

In contrast, from kfree() to a kmalloc() returning some of the kfree()ed
memory, I believe the kfree()/kmalloc() implementation must do any needed
synchronization and ordering.  But that is a different set of examples,
for example, this one:

	CPU 0			CPU 1
	p->a = 42;		q = kmalloc(...); /* returning p */
	kfree(p);		q->a = 5;
				BUG_ON(q->a != 5);

Unlike the situation with (A), (B), and (C), in this case I believe
that it is kfree()'s and kmalloc()'s responsibility to ensure that
the BUG_ON() never triggers.

Make sense?

							Thanx, Paul

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