Messages in this thread | | | Date | Mon, 25 Jun 2007 07:58:51 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks |
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On Mon, Jun 25, 2007 at 02:43:32PM +0400, 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(). > > 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.
Interesting... So the thought is to have a synchronize_srcu_timeout() or something similar that waited for a grace period to elapse or for a timeout to expire, whichever comes first? It should not be too hard to arrange something, if needed.
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