linux-kernel - Re: [PATCH v3] CPU hotplug: active_writer not woken up in some cases

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20141209172427.4bde12a7@thinkpad-w530>
Date:	Tue, 9 Dec 2014 17:24:27 +0100
From:	David Hildenbrand <dahi@...ux.vnet.ibm.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, heiko.carstens@...ibm.com,
	borntraeger@...ibm.com, rafael.j.wysocki@...el.com,
	peterz@...radead.org, oleg@...hat.com, bp@...e.de, jkosina@...e.cz
Subject: Re: [PATCH v3] CPU hotplug: active_writer not woken up in some
 cases - deadlock

> On Tue, Dec 09, 2014 at 01:23:31PM +0100, David Hildenbrand wrote:
> > Commit b2c4623dcd07 ("rcu: More on deadlock between CPU hotplug and expedited
> > grace periods") introduced another problem that can easily be reproduced by
> > starting/stopping cpus in a loop.
> > 
> > E.g.:
> >   for i in `seq 5000`; do
> >       echo 1 > /sys/devices/system/cpu/cpu1/online
> >       echo 0 > /sys/devices/system/cpu/cpu1/online
> >   done
> > 
> > Will result in:
> >   INFO: task /cpu_start_stop:1 blocked for more than 120 seconds.
> >   Call Trace:
> >   ([<00000000006a028e>] __schedule+0x406/0x91c)
> >    [<0000000000130f60>] cpu_hotplug_begin+0xd0/0xd4
> >    [<0000000000130ff6>] _cpu_up+0x3e/0x1c4
> >    [<0000000000131232>] cpu_up+0xb6/0xd4
> >    [<00000000004a5720>] device_online+0x80/0xc0
> >    [<00000000004a57f0>] online_store+0x90/0xb0
> >   ...
> > 
> > And a deadlock.
> > 
> > Problem is that if the last ref in put_online_cpus() can't get the
> > cpu_hotplug.lock the puts_pending count is incremented, but a sleeping
> > active_writer might never be woken up, therefore never exiting the loop in
> > cpu_hotplug_begin().
> > 
> > This fix wakes up the active_writer proactively. The writer already goes back to
> > sleep if the ref count isn't already down to 0, so this should be fine.
> > 
> > In order to avoid many potential races, we have to:
> > - Protect current_writer by a spin lock. When holding this lock we can be sure
> >   that the writer won't vainsh or change. (use-after-free)
> > - Increment the cpu_hotplug.puts_pending count before we test for an
> >   active_writer. (otherwise a wakeup might get lost)
> > - Move setting of TASK_UNINTERRUPTIBLE in cpu_hotplug_begin() above the
> >   condition check. (otherwise a wakeup might get lost)
> > 
> > Can't reproduce it with this fix.
> 
> Would wait_event()/wake_up() work for the wakeup-writer case?
> 
> 							Thanx, Paul
> 

Thanks! Was also thinking about wait queues. Will investigate if that can help
to beautify this :)

David

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