Date: Tue, 9 Dec 2014 11:21:08 +0100
From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: David Hildenbrand <dahi@linux.vnet.ibm.com>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, borntraeger@de.ibm.com,
        rafael.j.wysocki@intel.com, peterz@infradead.org, oleg@redhat.com,
        bp@suse.de, jkosina@suse.cz
Subject: Re: [PATCH v2] CPU hotplug: active_writer not woken up in some cases
 - deadlock
Message-ID: <20141209102108.GE4362@osiris>
References: <1418070082-13512-1-git-send-email-dahi@linux.vnet.ibm.com>
 <20141208212236.GU25340@linux.vnet.ibm.com>
 <20141209085930.6b831850@thinkpad-w530>
 <20141209091447.GD4362@osiris>
 <20141209111101.201e3544@thinkpad-w530>
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On Tue, Dec 09, 2014 at 11:11:01AM +0100, David Hildenbrand wrote:
> > > Therefore we have to move the condition check inside the 
> > >   __set_current_state(TASK_UNINTERRUPTIBLE) -> schedule();
> > > section to not miss any wake ups when the condition is satisfied.
> > > 
> > > So wake_up_process() will either see TASK_RUNNING and do nothing or see
> > > TASK_UNINTERRUPTIBLE and set it to TASK_RUNNING, so schedule() will in
> > > fact be woken up again.
> > 
> > Or the third alternative would be that 'active_writer' which was running
> > on CPU2 already terminated and wake_up_process() has a non-NULL pointer to
> > task_struct which is already dead.
> > Or is there anything that prevents this use-after-free race?
> 
> Hmmm ... I think that is also a valid scenario.
> That would mean we need soemthing like this:
> 
>  void put_online_cpus(void)
>  {
> + struct task_struct *awr;
> +
>         if (cpu_hotplug.active_writer == current)
>                 return;
>         if (!mutex_trylock(&cpu_hotplug.lock)) {
> +         awr = ACCESS_ONCE(cpu_hotplug.active_writer);
> +         if (unlikely(awr))
> +                 get_task_struct(awr);

How would this solve the problem?

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