lkml.org 
[lkml]   [2007]   [Jun]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks
Date
On Monday, 25 June 2007 12:43, Oleg Nesterov wrote:
> Rafael J. Wysocki wrote:
> >
> > static int usermodehelper_pm_callback(struct notifier_block *nfb,
> > unsigned long action,
> > void *ignored)
> > {
> > + long retval;
> > +
> > switch (action) {
> > case PM_HIBERNATION_PREPARE:
> > case PM_SUSPEND_PREPARE:
> > usermodehelper_disabled = 1;
> > - return NOTIFY_OK;
> > + /*
> > + * From now on call_usermodehelper_exec() won't start any new
> > + * helpers, so it is sufficient if running_helpers turns out to
> > + * be zero at one point (it may be increased later, but that
> > + * doesn't matter).
> > + */
> > + retval = wait_event_timeout(running_helpers_waitq,
> > + atomic_read(&running_helpers) == 0,
> > + RUNNING_HELPERS_TIMEOUT);
> > + if (retval) {
> > + return NOTIFY_OK;
> > + } else {
> > + usermodehelper_disabled = 0;
> > + return NOTIFY_BAD;
>
> I think this is racy. First, this needs smp_mb() between "usermodehelper_disabled = 1"
> and wait_event_timeout().

Yes ...

> Second, call_usermodehelper's path should first increment the counter, and only
> then check usermodehelper_disabled,

It does this already.

> and it needs an mb() in between too. Otherwise, the helper can see
> usermodehelper_disabled == 0, then PM_SUSPEND_PREPARE comes and
> returns NOTIFY_OK, then the helper increments the counter and starts application.
>
> Sadly, you can't use srcu/qrcu because it doesn't handle timeouts.

OK

If I understand that correctly, it should suffice to place smp_mb() after
usermodehelper_disabled = 1; in usermodehelper_pm_callback() and another
smp_mb() after atomic_inc(&running_helpers); in new_helper().

Is that correct?

Greetings,
Rafael


--
"Premature optimization is the root of all evil." - Donald Knuth
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2007-06-25 16:59    [W:0.453 / U:0.656 seconds]
©2003-2014 Jasper Spaans. Advertise on this site