linux-kernel - Re: [PATCH 15/20] ARM/hw_breakpoint: Convert to hotplug state machine

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Date:   Fri, 18 Nov 2016 14:42:15 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Will Deacon <will.deacon@....com>
cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, rt@...uxtronix.de,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 15/20] ARM/hw_breakpoint: Convert to hotplug state
 machine

On Fri, 18 Nov 2016, Will Deacon wrote:
> On Fri, Nov 18, 2016 at 02:11:58PM +0100, Thomas Gleixner wrote:
> > But it's guaranteed that cpuhp_setup_state() will not return before the
> > callback has been invoked on each online cpu.
> 
> Ok, that's good.
> 
> > If cpus are not yet online when that code is invoked, then it's the same
> > behaviour as before. It will be invoked when the cpu comes online.
> 
> Just to check, but what stops a CPU from coming online between the call
> to cpuhp_setup_state and the call to cpuhp_remove_state_nocalls in the
> case of failure (debug_err_mask isn't empty)?

Indeed! I missed that part. So we still need a get/put_online_cpus()
protection around all of this.

Just for curiosity sake. Wouldn't it be simpler and less error prone to
make the ARM_DBG_READ/WRITE macros use the exception table and handle that
in the undefined instruction handler to avoid this hook dance?

Thanks,

	tglx