https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66842
--- Comment #7 from Richard Henderson <rth at gcc dot gnu.org> ---
(In reply to Bin Fan from comment #6)
> Could you clarify what does aliased pages mean? Do you mean the same object
> is mapped into two or more different processes with different virtual
> addresses? And the locks in libatomic are also shared by the processes? Or
> something else?
The same page mapped into the address space more than once. Of course
I don't mean different processes, as libatomic is *not* an IPC library.
Such a thing is easily constructable via mmap of a file, however.
> This make sense if the above understand of aliased pages is true. However,
> what if the memory is not mapped at page boundaries?
How do you suppose you'd map a page anywhere other than at a page boundary?
That's nonsensical. Virtual address spaces don't work that way.
> And this does not explain why a locked object is protected by multiple
> locks. If memory is always mapped at edge boundaries, then the offset of the
> object in the page will always be the same so one lock should work. If
> memory is not mapped at page boundaries, then if an object is mapped into
> two "non-overlapped" address space inside a page, multiple locks would still
> don't work.
...
> Besides aliased pages, does libatomic consider supporting nested locked
> atomic objects? For example, should the following work?
>
> typedef struct {
> _Atomic locked1_t obj1;
> /* other fields */
> } locked2_t;
>
> _Atomic locked2_t obj2;
Yes. The end address cannot be inferred from the start address. Thus we
lock every cacheline the object overlaps.
