Re: [BUG] Quit and "(running)" problem

[Pedro Alves <pedro@codesourcery.com>]

A Friday 16 January 2009 08:54:21, Pierre Muller wrote:
>   I can confirm that your patch fixed the simple
> test case that I submitted in PR9747.

Thank you.

>   Reading the patch, I was wondering about the
> utility of the old_chain cleanup in fetch_inferior_event
> function. But this is probably due to my
> lack of comprehension of the cleanup chain mechanisms.
> 
>   Is it really possible to reach
>   do_cleanups (old_chain)
> with something else that old_chain
> as the top item on the cleanup list?

Yes, there's a make_cleanup_restore_current_thread call
there that adds a new cleanup to the chain.

>   I thought that all the cleanups where stored
> as local variables, so that all cleanups
> that were set in functions called while running any
> code called from within the fetch_inferior_event
> would be invalid data anyhow at that point,
> as the stack might have been overwritten by calls to other functions.

Some confusion here.  Instead of trying to explain the basic mechanism
and doing a lousy job at it, I suggest taking a look at the Cleanups section
in internals manual, if you haven't already, which I think explains
it quite nicelly:

 http://sourceware.org/gdb/current/onlinedocs/gdbint_15.html#SEC118

Another way to really understand cleanups is to step through
the make_cleanup, do_cleanups and discard_cleanups functions, it looks
scarrier than it is.


In this particular case, we have:

old_chain +----+- <null_cleanup>
               |
               +- <restore_current_thread> (always run this, wether leaving with an exception or leaving succesfully)
               |
ts_old_chain +-+- <finish_thread_state> (only run if there's an exception)
               
                  [do something that can throw]
             
                  [ if we got pass it sucessfully, discard the <finish_thread_state> cleanup chain ]

                  /* No error, don't finish the thread states yet.  */
                  discard_cleanups (ts_old_chain);

So, at this point we have something like:

old_chain +----+- <null_cleanup>
               |
               +- <restore_current_thread> (always run this, wether leaving with an exception or leaving succesfully)

ts_old_chain (invalid, dangling pointer)

                  /* Revert thread and frame.  */
                  do_cleanups (old_chain);
                    
                  This statement runs the <restore_current_thread> cleanup.

( I mentioned doing a lousy job explaining it.  )

-- 
Pedro Alves