Re: [Xen-devel] [PATCH] x86, microcode: Add local mutex to not hit a deadlock.

[Borislav Petkov <bp@xxxxxxxxx>]

On Wed, May 08, 2013 at 12:13:03PM -0400, Konrad Rzeszutek Wilk wrote:
> This can easily be triggered if a new CPU is added (via
> ACPI hotplug mechanism) and from user-space do:
> 
> echo 1 > /sys/devices/system/cpu/cpu3/online
> 
> (or wait for UDEV to do it) on a newly appeared CPU.
> 
> The deadlock is that the "store_online" in drivers/base/cpu.c
> takes the cpu_hotplug_driver_lock() lock, then calls "cpu_up".
> "cpu_up" eventually ends up calling "save_mc_for_early"
> which also takes the cpu_hotplug_driver_lock() lock.
> 
> And here is that kernel thinks of it:
> 
> smpboot: Stack at about ffff880075c39f44
> smpboot: CPU3: has booted.
> microcode: CPU3 sig=0x206a7, pf=0x2, revision=0x25
> 
> =============================================
> [ INFO: possible recursive locking detected ]
> 3.9.0upstream-10129-g167af0e #1 Not tainted
> ---------------------------------------------
> sh/2487 is trying to acquire lock:
>  (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] 
> cpu_hotplug_driver_lock+0x12/0x20
> 
> but task is already holding lock:
>  (x86_cpu_hotplug_driver_mutex){+.+.+.}, at: [<ffffffff81075512>] 
> cpu_hotplug_driver_lock+0x12/0x20
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(x86_cpu_hotplug_driver_mutex);
>   lock(x86_cpu_hotplug_driver_mutex);
> 
>  *** DEADLOCK ***

Ok, just for my own understanding: is this something which can actually
happen now?

Judging by the presence of traces, it can be triggered in a guest,
correct?

Thanks.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
--

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