Re: [PATCH] sched: fix race between sched_move_domain() and vcpu_wake()

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

On 10.10.2013 19:29, David Vrabel wrote:
> From: David Vrabel <david.vrabel@citrix.com>
>
> sched_move_domain() changes v->processor for all the domain's VCPUs.
> If another domain, softirq etc. triggers a simultaneous call to
> vcpu_wake() (e.g., by setting an event channel as pending), then
> vcpu_wake() may lock one schedule lock and try to unlock another.
>
> vcpu_schedule_lock() attempts to handle this but only does so for the
> window between reading the schedule_lock from the per-CPU data and the
> spin_lock() call.  This does not help with sched_move_domain()
> changing v->processor between the calls to vcpu_schedule_lock() and
> vcpu_schedule_unlock().
>
> Fix the race by taking the schedule_lock for v->processor in
> sched_move_domain().
>
> Signed-off-by: David Vrabel <david.vrabel@citrix.com>
> Cc: George Dunlap <george.dunlap@eu.citrix.com>
> Cc: Juergen Gross <juergen.gross@ts.fujitsu.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
>
> Just taking the lock for the old processor seemed sufficient to me as
> anything seeing the new value would lock and unlock using the same new
> value.  But do we need to take the schedule_lock for the new processor
> as well (in the right order of course)?

I don't think it is necessary to take both locks. There can't be any
scheduler specific (e.g. credit) activity on the vcpu(s), as they are
removed from the source scheduler before and will be added to the target
scheduler after switching the processor.

BTW: good catch! I think this explains a problem I have been searching for
some time now...

Acked-by: Juergen Gross <juergen.gross@ts.fujitsu.com>

>
> This is reproducable by constantly migrating a domain between two CPU
> pools.
> 8<------------
> while true; do
>      xl cpupool-migrate $1 Pool-1
>      xl cpupool-migrate $1 Pool-0
> done
> ---
>   xen/common/schedule.c |    7 +++++++
>   1 files changed, 7 insertions(+), 0 deletions(-)
>
> diff --git a/xen/common/schedule.c b/xen/common/schedule.c
> index 1ddfb22..28e063e 100644
> --- a/xen/common/schedule.c
> +++ b/xen/common/schedule.c
> @@ -278,6 +278,9 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
>       new_p = cpumask_first(c->cpu_valid);
>       for_each_vcpu ( d, v )
>       {
> +        spinlock_t *schedule_lock = per_cpu(schedule_data,
> +                                            v->processor).schedule_lock;
> +
>           vcpudata = v->sched_priv;
>
>           migrate_timer(&v->periodic_timer, new_p);
> @@ -285,7 +288,11 @@ int sched_move_domain(struct domain *d, struct cpupool *c)
>           migrate_timer(&v->poll_timer, new_p);
>
>           cpumask_setall(v->cpu_affinity);
> +
> +        spin_lock_irq(schedule_lock);
>           v->processor = new_p;
> +        spin_unlock_irq(schedule_lock);
> +
>           v->sched_priv = vcpu_priv[v->vcpu_id];
>           evtchn_move_pirqs(v);
>
>


-- 
Juergen Gross                 Principal Developer Operating Systems
PBG PDG ES&S SWE OS6                   Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@ts.fujitsu.com
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