Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 12 May 2007 01:22:06 +0200
"Rafael J. Wysocki" <rjw@sisk.pl> wrote:

> On Saturday, 12 May 2007 00:56, Linus Torvalds wrote:
> > 
> > On Fri, 11 May 2007, Rafael J. Wysocki wrote:
> > > 
> > > For user space processes this condition is always true.
> > > 
> > > For kernel threads:
> > > (1) the change of tsk->mm from NULL to a nonzero value is only made in
> > > fs/aio.c:use_mm() along with the setting of PF_BORROWED_MM under
> > > the task_lock(),
> > > (2) the change of tsk->mm from a nonzero value to NULL is only made in
> > > fs/aio.c:unuse_mm() along with the resetting of PF_BORROWED_MM
> > > under the task_lock().
> > > Therefore, by taking the task_lock() here we make sure that the condition
> > > is alyways false when we check it for kernel threads.
> > 
> > Why *test* it then and return anything?
> > 
> > Why not just doa "task_lock(p); task_unlock(p);" with no return value? 
> > 
> > As it is, it sounds like either the code is buggy, or it's pointless.
> 
> I'm not sure what you mean.
> 
> We use this function (ie. kernel/power/process.c:is_user_space()) to
> distinguish kernel threads from user space processes.  Therefore we make it
> always return true for user space processes and always return false for kernel
> threads.  In the latter case we need to use the task_lock() to ensure that the
> result is as desired (ie. false), because otherwise it might be racing with
> either fs/aio.c:use_mm() or fs/aio.c:unuse_mm().
> 

ah, OK.

static void use_mm(struct mm_struct *mm)
{
	struct mm_struct *active_mm;
	struct task_struct *tsk = current;

	task_lock(tsk);
	tsk->flags |= PF_BORROWED_MM;
	active_mm = tsk->active_mm;
	atomic_inc(&mm->mm_count);
	tsk->mm = mm;
	tsk->active_mm = mm;
	/*
	 * Note that on UML this *requires* PF_BORROWED_MM to be set, otherwise
	 * it won't work. Update it accordingly if you change it here
	 */
	switch_mm(active_mm, mm, tsk);
	task_unlock(tsk);

So is_user_space() requires that the state of p->mm and p->flags be
consistent: it doesn't want to be looking at those two things in that
three-statement window above.

Good changelogging and commenting save quite a bit of time and email.