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
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