Messages in this thread | | | Date | Mon, 25 Jun 2007 14:43:32 +0400 | From | Oleg Nesterov <> | Subject | Re: [PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks |
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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().
Second, call_usermodehelper's path should first increment the counter, and only then check usermodehelper_disabled, 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.
Oleg.
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