linux-kernel - Re: linux-next ppc64: RCU mods cause __might

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Message-ID: <CANsGZ6bw4JwMRgUriooDvFB=g-HkpyL8XJa9s47RPfOEPCMcpw@mail.gmail.com>
Date:	Wed, 2 May 2012 15:54:24 -0700
From:	Hugh Dickins <hughd@...gle.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"Paul E. McKenney" <paul.mckenney@...aro.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: linux-next ppc64: RCU mods cause __might_sleep BUGs

On Wed, May 2, 2012 at 2:54 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
> On Thu, May 03, 2012 at 07:20:15AM +1000, Benjamin Herrenschmidt wrote:
>> On Wed, 2012-05-02 at 13:25 -0700, Hugh Dickins wrote:
>> > Got it at last.  Embarrassingly obvious.  __rcu_read_lock() and
>> > __rcu_read_unlock() are not safe to be using __this_cpu operations,
>> > the cpu may change in between the rmw's read and write: they should
>> > be using this_cpu operations (or, I put preempt_disable/enable in the
>> > __rcu_read_unlock below).  __this_cpus there work out fine on x86,
>> > which was given good instructions to use; but not so well on PowerPC.
>> >
>> > I've been running successfully for an hour now with the patch below;
>> > but I expect you'll want to consider the tradeoffs, and may choose a
>> > different solution.
>>
>> Didn't Linus recently rant about these __this_cpu vs this_cpu nonsense ?
>>
>> I thought that was going out..
>
> Linus did rant about __raw_get_cpu_var() because it cannot use the x86
> %fs segement overrides a bit more than a month ago.  The __this_cpu
> stuff is useful if you have preemption disabled -- avoids the extra
> layer of preempt_disable().
>
> Or was this a different rant?

https://lkml.org/lkml/2011/11/29/321

I think it ended up with Christoph removing the more egregious
variants, but this_cpu_that and __this_cpu_the_other remaining.

Hugh
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