linux-kernel - Re: [PATCH] cpu/hotplug: Cache number of online CPUs

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Date:   Fri, 5 Jul 2019 23:00:32 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, Nadav Amit <namit@...are.com>,
        paulmck <paulmck@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH] cpu/hotplug: Cache number of online CPUs

On Fri, 5 Jul 2019, Thomas Gleixner wrote:
> On Fri, 5 Jul 2019, Mathieu Desnoyers wrote:
> > ----- On Jul 5, 2019, at 4:49 AM, Ingo Molnar mingo@...nel.org wrote:
> > > * Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> > >> The semantic I am looking for here is C11's relaxed atomics.
> > > 
> > > What does this mean?
> > 
> > C11 states:
> > 
> > "Atomic operations specifying memory_order_relaxed are  relaxed  only  with  respect
> > to memory ordering.  Implementations must still guarantee that any given atomic access
> > to a particular atomic object be indivisible with respect to all other atomic accesses
> > to that object."
> > 
> > So I am concerned that num_online_cpus() as proposed in this patch
> > try to access __num_online_cpus non-atomically, and without using
> > READ_ONCE().
> >
> > 
> > Similarly, the update-side should use WRITE_ONCE(). Protecting with a mutex
> > does not provide mutual exclusion against concurrent readers of that variable.
> 
> Again. This is nothing new. The current implementation of num_online_cpus()
> has no guarantees whatsoever. 
> 
> bitmap_hweight() can be hit by a concurrent update of the mask it is
> looking at.
> 
> num_online_cpus() gives you only the correct number if you invoke it inside
> a cpuhp_lock held section. So why do we need that fuzz about atomicity now?
> 
> It's racy and was racy forever and even if we add that READ/WRITE_ONCE muck
> then it still wont give you a reliable answer unless you hold cpuhp_lock at
> least for read. So fore me that READ/WRITE_ONCE is just a cosmetic and
> misleading reality distortion.

That said. If it makes everyone happy and feel better, I'm happy to add it
along with a bit fat comment which explains that it's just preventing a
theoretical store/load tearing issue and does not provide any guarantees
other than that.

Thanks,

	tglx