About stack pointers in thread_base

[jaeyong yoo]

Thanks a lot!
It is very clear now.

Actually, the reason that I have confused is the following:
If you see the source code below, the type of stack is long [] and the type
of stack_base is addr_t.
Although they are both pointers, it confuses me that one is pure address
(addr_t) and the other is long [].
That's why I conjectured they could be different things.

Also, is there a particular reason that you position thread_base in between
stack and stack base?
If not, I think placing stack and stack_base right next to each other would
be better (just an opinion).

                                /**
                                 * Top of the stack
                                 */
                                long stack[];

                                /**
                                 * Pointer to corresponding 'Thread_base'
object
                                 */
                                Thread_base *thread_base;

                                /**
                                 * Virtual address of the start of the stack
                                 *
                                 * This address is pointing to the begin of
the dataspace used
                                 * for backing the thread context except
for the UTCB (which is
                                 * managed by the kernel).
                                 */
                                addr_t stack_base;
Best regards,
Jaeyong

On Sat, Dec 29, 2012 at 4:37 AM, Norman Feske
<norman.feske at ...1...>wrote:

> Dear Jaeyong,
>
> > While I was browsing source code, I found two confusing variables:
> > At genode/base/include/base/thread.h
> >         Thread_base::Context {
> >
> >                                 long stack[];
> >
> >                                 addr_t stack_base;
> >         }
> >
> > Although, they are commented a bit, it is hard to understand what are
> they
> > doing and what's the difference.
> > Could you tell me about them more?
>
> please let me first refer you to the brief description of the concept
> behind the code:
>
>
>
> http://genode.org/documentation/release-notes/10.02#New_thread-context_management
>
> Each 'Thread_base' object is accompanied by a 'Thread_base::Context'
> object, which holds the thread's stack and information about
> thread-local storage (i.e., the UTCB) and similar meta data. In contrast
> to 'Thread_base' objects, which are plain C++ objects that can reside
> anywhere (on BSS, heap, or even on a stack), thread context objects are
> placed within the so-called thread-context area, which is a region of
> the process' virtual address space managed by the process itself. Each
> thread owns a 1MB portion of the context area. Only a tiny bit of this
> portion is actually used for the thread context. The gaps between the
> thread contexts are not populated with memory. This way, stack overruns
> will result in an unresolvable page fault (similar to a segmentation
> fault) rather than corrupted memory.
>
> Each thread context consists of two parts, meta data expressed by the
> 'Thread_base::Context' type and the stack. Whereas the meta data has the
> same size and layout for each thread (well, the main thread is an
> exception here), the stack size differs from thread to thread. The
> metadata is always positioned such that it starts after the stack.
> Because the stack always grows from higher to lower addresses, an
> overrunning stack won't overwrite its meta data. The 'stack_base'
> variable points to the "other end" of the stack - the lower one which
> would get crossed if the stack overruns.
>
> I hope this description could be of help. Do you have a suggestion about
> a comment that would have helped you to get a better understanding in
> the first place? I'd like to make the situation easier for future
> readers of the code.
>
> Best regards
> Norman
>
> --
> Dr.-Ing. Norman Feske
> Genode Labs
>
> http://www.genode-labs.com · http://genode.org
>
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