Dear All,
Inside sys_ptrace, the function get_task_struct is invoked after
retrieving the child's task srtuct pointer. Why is it done? I have
tracked down the code and noticed that it is in fact an increment
on the (page?) counter. Can you help me understand it?
Is it necessary to call free_task_struct whenever its get counterpart
is called?
** arch/i386/kernel/ptrace.c
> read_lock(&tasklist_lock);
> child = find_task_by_pid(pid);
> if (child)
> get_task_struct(child);
> read_unlock(&tasklist_lock);
Thanks in advance,
Fabiano
On Sat, 2004-04-17 at 12:46 -0300, Fabiano Ramos wrote:
> Inside sys_ptrace, the function get_task_struct is invoked after
> retrieving the child's task srtuct pointer. Why is it done? I have
> tracked down the code and noticed that it is in fact an increment
> on the (page?) counter. Can you help me understand it?
Think of get_task_struct(foo) as a reference count on foo's task
structure. So long as the reference count is elevated, foo's task
structure cannot be deallocated.
get_task_struct() is called here because sys_ptrace() needs to ensure
that the child's task structure does not go away out from under this
function, but we do not want to hold the tasklist_lock.
> Is it necessary to call free_task_struct whenever its get counterpart
> is called?
Yes, although it is called "put_task_struct()". It is called at the end
of this function.
Robert Love