Re: [Patch] x86_64: TASK_SIZE fixes for compatibility mode processes

[Andrew Morton <akpm@osdl.org>]

"Siddha, Suresh B" <suresh.b.siddha@intel.com> wrote:
>
> +#define TASK_SIZE_OF(child) 	((test_tsk_thread_flag(child, TIF_IA32)) ? IA32_PAGE_OFFSET : TASK_SIZE64)

The task size is an attribute of the task's mm_struct, not of the task.

The place where this tends to come unstuck is when a 32-bit task holds a
reference on a 64-bit tasks's task_struct via a read of a /proc file.  If
the 64-bit task exits then it is the 32-bit task who does the final freeing
of the 64-bit tasks's task_struct and mm_struct.  (and all vice-versa, of
course).  Will your patch handle this race scenario correctly?

I don't have much confidence that we'll get all this stuff right until we
move the task-size info into the mm_struct proper.  Testing the "size" of a
task would then be done via:

	if (task->mm->mm_size == MM_SIZE_32)

or whatever.  Although I suspect the number of places where we need to work
out the "size" of a task would fall markedly.