Re: [Xen-devel] [PATCH 2/2] xen/rcu: don't use stop_machine_run() for rcu_barrier()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 17, 2020 at 01:11:59PM +0100, Jürgen Groß wrote:
> On 17.02.20 12:49, Julien Grall wrote:
> > Hi Juergen,
> > 
> > On 17/02/2020 07:20, Juergen Gross wrote:
> > > +void rcu_barrier(void)
> > >   {
> > > -    atomic_t cpu_count = ATOMIC_INIT(0);
> > > -    return stop_machine_run(rcu_barrier_action, &cpu_count, NR_CPUS);
> > > +    if ( !atomic_cmpxchg(&cpu_count, 0, num_online_cpus()) )
> > 
> > What does prevent the cpu_online_map to change under your feet?
> > Shouldn't you grab the lock via get_cpu_maps()?
> 
> Oh, indeed.
> 
> This in turn will require a modification of the logic to detect parallel
> calls on multiple cpus.

If you pick my patch to turn that into a rw lock you shouldn't worry
about parallel calls I think, but the lock acquisition can still fail
if there's a CPU plug/unplug going on:

https://lists.xenproject.org/archives/html/xen-devel/2020-02/msg00940.html

Roger.

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