On 02/18, Rafael J. Wysocki wrote:
>
> 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?
I think yes, as a first step.
In the long term, I think it would be really nice to have CLONE_KERNEL_THREAD
(filtered out in sys_clone). This also allows us to cleanup copy_process().
For example, we can then introduce CLONE_UNHASHED (currently denoted as pid==0)
and kill the ugly "int pid" copy_process's parameter.
Oleg.
> ---
> 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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