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

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