Re: [RFC][PATCH -mm 1/5] PM: Make freeze_processes SMP-safe

Rafael J. Wysocki
2006-11-28T23:40:37Z

Hi,

On Monday, 27 November 2006 00:09, Rafael J. Wysocki wrote:
> On Sunday, 26 November 2006 20:48, Pavel Machek wrote:
<--snip-->
> > > @@ -128,6 +135,21 @@ static unsigned int try_to_freeze_tasks(
> > > } while_each_thread(g, p);
> > > read_unlock(&tasklist_lock);
> > > yield(); /* Yield is okay here */
> > > + if (!todo) {
> > > + /* Make sure that none of the stopped processes has
> > > + * received the continuation signal after we checked
> > > + * last time.
> > > + */
> >
> > I do not like the counting idea; it should be simpler to just check if
> > all the processes are still stopped.
>
> I thought about that but didn't invent anything reasonable enough.
>
> > But I'm not sure if this is enough. What if signal is being delivered
> > on another CPU while freezing, still being delivered while this second
> > check runs, and then SIGCONT is delivered?
>
> Hm, is this possible in practice? I mean, if todo is 0 and nr_stopped doesn't
> change, then there are no processes that can send the SIGCONT (unless someone
> creates a kernel thread with PF_NOFREEZE that will do just that).
>
> Anyway, for now I've no idea how to fix this properly. Will think about it
> tomorrow.

As far as this particular problem is concerned, I think there are two possible
solutions.

One of them would be do disable the delivery of continuation signals before
we start freezing processes, but I don't know how to do this exactly so that
it's not racy. Also it would be quite intrusive.

The other one may be something along with the lines of the appended patch.

Greetings,
Rafael

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
kernel/power/process.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)

Index: linux-2.6.19-rc6-mm2/kernel/power/process.c
===================================================================
--- linux-2.6.19-rc6-mm2.orig/kernel/power/process.c 2006-11-28 22:49:30.000000000 +0100
+++ linux-2.6.19-rc6-mm2/kernel/power/process.c 2006-11-29 00:10:05.000000000 +0100
@@ -28,8 +28,7 @@ static inline int freezeable(struct task
if ((p == current) ||
(p->flags & PF_NOFREEZE) ||
(p->exit_state == EXIT_ZOMBIE) ||
- (p->exit_state == EXIT_DEAD) ||
- (p->state == TASK_STOPPED))
+ (p->exit_state == EXIT_DEAD))
return 0;
return 1;
}
@@ -103,9 +102,13 @@ static unsigned int try_to_freeze_tasks(
if (frozen(p))
continue;

+ if (p->state == TASK_STOPPED && freezing(p))
+ continue;
+
if (p->state == TASK_TRACED &&
(frozen(p->parent) ||
- p->parent->state == TASK_STOPPED)) {
+ (p->parent->state == TASK_STOPPED &&
+ freezing(p->parent)))) {
cancel_freezing(p);
continue;
}
@@ -185,6 +188,18 @@ int freeze_processes(void)
return 0;
}

+static void release_stopped_tasks(void)
+{
+ struct task_struct *g, *p;
+
+ read_lock(&tasklist_lock);
+ do_each_thread(g, p) {
+ if (p->state == TASK_STOPPED)
+ cancel_freezing(p);
+ } while_each_thread(g, p);
+ read_unlock(&tasklist_lock);
+}
+
static void thaw_tasks(int thaw_user_space)
{
struct task_struct *g, *p;
@@ -207,6 +222,7 @@ static void thaw_tasks(int thaw_user_spa
void thaw_processes(void)
{
printk("Restarting tasks ... ");
+ release_stopped_tasks();
thaw_tasks(FREEZER_KERNEL_THREADS);
thaw_tasks(FREEZER_USER_SPACE);
schedule();