linux-kernel - Re: [PATCH] cpu/hotplug: Serialize callback invocations proper

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Date:   Tue, 14 Mar 2017 18:43:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Bart Van Assche <Bart.VanAssche@...disk.com>
cc:     "bigeasy@...utronix.de" <bigeasy@...utronix.de>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH] cpu/hotplug: Serialize callback invocations proper

On Tue, 14 Mar 2017, Bart Van Assche wrote:
> On Tue, 2017-03-14 at 16:06 +0100, Sebastian Andrzej Siewior wrote:
> > The setup/remove_state/instance() functions in the hotplug core code are
> > serialized against concurrent CPU hotplug, but unfortunately not serialized
> > against themself.
> > 
> > As a consequence a concurrent invocation of these function results in
> > corruption of the callback machinery because two instances try to invoke
> > callbacks on remote cpus at the same time. This results in missing callback
> > invocations and initiator threads waiting forever on the completion.
> > 
> > The obvious solution to replace get_cpu_online() with cpu_hotplug_begin()
> > is not possible because at least one callsite calls into these functions
> > from a get_online_cpu() locked region.
> > 
> > Extend the protection scope of the cpuhp_state_mutex from solely protecting
> > the state arrays to cover the callback invocation machinery as well.
> > 
> > Reported-by: Bart Van Assche <Bart.VanAssche@...disk.com>
> > Fixes: 5b7aa87e0482 ("cpu/hotplug: Implement setup/removal interface")
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> Tested-by: Bart Van Assche <Bart.VanAssche@...disk.com>
> 
> So this regression was introduced in kernel v4.6? Anyway, thanks for the patch!

And it's very timing sensitive ....

Thanks for trying to bisect this!

       tglx