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    SubjectRe: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug
    Date
    On Sunday, 18 February 2007 12:32, Oleg Nesterov wrote:
    > On 02/18, Rafael J. Wysocki wrote:
    > >
    > > On Sunday, 18 February 2007 00:42, Oleg Nesterov wrote:
    > > > On 02/17, Rafael J. Wysocki wrote:
    > > > >
    > > > > On Saturday, 17 February 2007 22:34, Oleg Nesterov wrote:
    > > > > >
    > > > > > static inline int is_user_space(struct task_struct *p)
    > > > > > {
    > > > > > return p->mm && !(p->flags & PF_BORROWED_MM);
    > > > > > }
    > > > > >
    > > > > > This doesn't look right. First, an exiting task has ->mm == NULL after
    > > > > > do_exit()->exit_mm(). Probably not a problem. However, PF_BORROWED_MM
    > > > > > check is racy without task_lock(), so we can have a false positive as
    > > > > > well. Is it ok? We can freeze aio_wq prematurely.
    > > > >
    > > > > Right now aio_wq is not freezeable (PF_NOFREEZE).
    > > >
    > > > Right now yes, but we are going to change this?
    > >
    > > Well, is there any more reliable (and not racy) method of differentiating
    > > between kernel threads and user space processes?
    >
    > Not that I know of. At least, we can take task_lock() to really rule out
    > kernel threads at FREEZER_USER_SPACE stage.

    Something like this?

    ---
    kernel/power/process.c | 8 +++++++-
    1 file changed, 7 insertions(+), 1 deletion(-)

    Index: linux-2.6.20-mm2/kernel/power/process.c
    ===================================================================
    --- linux-2.6.20-mm2.orig/kernel/power/process.c
    +++ linux-2.6.20-mm2/kernel/power/process.c
    @@ -8,6 +8,7 @@

    #undef DEBUG

    +#include <linux/sched.h>
    #include <linux/smp_lock.h>
    #include <linux/interrupt.h>
    #include <linux/suspend.h>
    @@ -92,7 +93,12 @@ static void cancel_freezing(struct task_

    static inline int is_user_space(struct task_struct *p)
    {
    - return p->mm && !(p->flags & PF_BORROWED_MM);
    + int ret;
    +
    + task_lock(p);
    + ret = p->mm && !(p->flags & PF_BORROWED_MM);
    + task_unlock(p);
    + return ret;
    }

    static unsigned int try_to_freeze_tasks(int freeze_user_space)
    -
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