Re: [PATCH] BUG() in exec_mmap()

[Ernie Petrides <petrides@redhat.com>]

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On Thursday, 9-Oct-2003 at 17:47 -0300, Marcelo Tosatti wrote:

> On Thu, 9 Oct 2003, Mika Penttilä wrote:
>
> > Hmm.. you still need to mmput(old_mm) etc, just remove the mm_users == 1
> > optimization from the beginning of exec_mmap, so this patch is wrong!
>
> Right. Ill fix it up by hand.

Mika is correct that the exit_mmap(old_mm) still needs to happen on the
last use of the "mm_struct".  But whether it's called directly from
exec_mmap() or indirectly from mmput() still needs to depend on the
value of "mm_users".

The original logic avoided the mmdrop(active_mm) call if there was an
old_mm, so I'd infer that the mm_struct reference count is not bumped
twice for both references from the task_struct (mm and active_mm).  So
the patch would need to be reworked to avoid the double decrement, too.

Sorry I missed the discussion on the original changes.  Was there a
race condition with another cpu gaining a reference in proc_pid_status()
or access_process_vm() or something like that?  Is it possible to just
use down_read(&old_mm->mmap_sem) and up_read(&old_mm->mmap_sem) inside
exec_mmap() around the optimized call to exit_mmap() instead?

Cheers.  -ernie