linux-kernel - Re: Need advice how to fix an access to uninitialized per

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Date:	Fri, 3 Feb 2012 15:42:08 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm@...r.kernel.org,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	kosaki.motohiro@...fujitsu.com, tj@...nel.org, x86@...nel.org,
	hpa@...or.com, mingo@...hat.com, tglx@...utronix.de,
	Rik van Riel <riel@...hat.com>,
	"avi@...hat.com" <avi@...hat.com>
Subject: Re: Need advice how to fix an access to uninitialized per_cpu clock

On Thu, Feb 02, 2012 at 12:43:00PM +0100, Igor Mammedov wrote:
> While playing with kvm cpu hot-plug, I've probably stumbled on general
> kernel bug. So I'm lookng for advice on approach to fix it.
> 
> When kvm guest uses kvmclock, it may hang on cpu hot-plug at
> 
>     BSP:
>      smp_apic_timer_interrupt
>        ...
>          -> do_timer
>            -> update_wall_time
> 
> and a being on-lined CPU at waiting on sync with BSP at:
>      start_secondary:
>        while (!cpumask_test_cpu(smp_processor_id(), cpu_active_mask))
>                     cpu_relax();
> 
> because of unusually big value returned by clock->read(clock) in
> update_wall_time.
> This happens due to overflow in pvclock_get_nsec_offset
> 
>         u64 delta = tsc - shadow->tsc_timestamp;
> 
> when shadow->tsc_timestamp is bigger than tsc.
> And since pvclock_clocksource_read remembers and returns largest
> value of any clock that ever was returned, clock for affected guest
> virtually freezes.
> 
> Overflow happens due to reading undefined values from uninitialized
> per_cpu variable hv_clock. In case of cpu hot-plug, clock is read at
> least once on being on-lined cpu before it is initialized for this
> cpu:
> 
>       start_secondary
>         |-> smp_callin
>         |  ->  smp_store_cpu_info
>         |    -> identify_secondary_cpu
>         |      -> mtrr_ap_init
>         |        -> mtrr_restore
>         |          -> stop_machine_from_inactive_cpu
>         |            -> queue_stop_cpus_work
>         |              ...
>         |                -> kvm_clock_read
>         |...
>         |x86_cpuinit.setup_percpu_clockev
> 
> full call chain is below:
> [    0.002999]  [<ffffffff8102fc74>] pvclock_clocksource_read+0x9f/0x275
> [    0.002999]  [<ffffffff81071f9e>] ? check_preempt_wakeup+0x11c/0x1c2
> [    0.002999]  [<ffffffff8102f05d>] kvm_clock_read+0x21/0xd3
> [    0.002999]  [<ffffffff810154b1>] sched_clock+0x9/0xd
> [    0.002999]  [<ffffffff81070509>] sched_clock_local+0x12/0x75
> [    0.002999]  [<ffffffff8107065e>] sched_clock_cpu+0x84/0xc6
> [    0.002999]  [<ffffffff8106bfd3>] update_rq_clock+0x28/0x108
> [    0.002999]  [<ffffffff8106c15e>] enqueue_task+0x1d/0x64
> [    0.002999]  [<ffffffff8106c500>] activate_task+0x22/0x24
> [    0.002999]  [<ffffffff8106c90c>] ttwu_do_activate.constprop.39+0x32/0x61
> [    0.002999]  [<ffffffff8106cd0c>] try_to_wake_up+0x17e/0x1e1
> [    0.002999]  [<ffffffff8106cd98>] wake_up_process+0x15/0x17
> [    0.002999]  [<ffffffff8109ebbf>] cpu_stop_queue_work+0x3d/0x5f
> [    0.002999]  [<ffffffff8109ec6c>] queue_stop_cpus_work+0x8b/0xb6
> [    0.002999]  [<ffffffff8109f0e3>] stop_machine_from_inactive_cpu+0xb4/0xed
> [    0.002999]  [<ffffffff81023896>] ? mtrr_restore+0x4a/0x4a
> [    0.002999]  [<ffffffff81023eb3>] mtrr_ap_init+0x5a/0x5c
> [    0.002999]  [<ffffffff814b95f7>] identify_secondary_cpu+0x19/0x1b
> [    0.002999]  [<ffffffff814bc0d3>] smp_store_cpu_info+0x3c/0x3e
> [    0.002999]  [<ffffffff814bc4ed>] start_secondary+0x122/0x263
> 
> Looking at native_smp_prepare_cpus in arch/x86/kernel/smpboot.c
> I see that it unconditionally calls set_mtrr_aps_delayed_init which
> in turn effectively converts calls to mtrr_ap_init to nop ops.
> And later call to native_smp_cpus_done -> mtrr_aps_init completes
> mtrr initialization.
> 
> The same pattern might be noticed in suspend/hibernate handlers, that
> call enable_nonboot_cpus.
> 
>   enable_nonboot_cpus
>     -> arch_enable_nonboot_cpus_begin
>          -> set_mtrr_aps_delayed_init
> 
>     -> boot secondary cpus
> 
>     -> arch_enable_nonboot_cpus_end
>          -> mtrr_aps_init
> 
> So question is if the calling mtrr_ap_init so early in
> identify_secondary_cpu is really necessary?
> From current code it looks like it is never called at normal smp
> boot/resume time. And only path that triggers it is cpu hot-plug one.
> 
> I see following possible solutions:
> 
> 1. Could the call to mtrr_ap_init be just moved from identify_secondary_cpu
> to start_secondary right after x86_cpuinit.setup_percpu_clockev call?
>
> This will prevent an access to uninitialized per_cpu clock. Or move
> x86_cpuinit.setup_percpu_clockev before smp_callin?
> 
> 2. Another way to prevent access to uninitialized per_cpu clock is to
> introduce hook.early_percpu_clock_init that will be called before
> smp_callin or right before mtrr_ap_init.
> 
> I hope to see opinions about this matter from a more experienced people than me.

It would be more robust to introduce x86_cpuinit.early_percpu_clock_init
and use it for KVM, replacing setup_percpu_clockev hook.

setup_percpu_clockev initializes the APIC timer, but clearly sched_clock
should be functional before that, for example for ftrace (in addition to
sched_clock from scheduler as in the trace above).

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