linux-kernel - Re: [PATCH] ARM: CPU hotplug: Delegate complete() to surviving CPU

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Date:   Tue, 12 Dec 2017 17:37:59 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Peng Fan <van.freenix@...il.com>,
        Fabio Estevam <fabio.estevam@....com>,
        Ingo Molnar <mingo@...nel.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] ARM: CPU hotplug: Delegate complete() to surviving CPU

On Tue, Dec 12, 2017 at 09:20:59AM -0800, Paul E. McKenney wrote:
> The ARM implementation of arch_cpu_idle_dead() invokes complete(), but
> does so after RCU has stopped watching the outgoing CPU, which results
> in lockdep complaints because complete() invokes functions containing RCU
> readers.  This patch therefore uses Thomas Gleixner's trick of delegating
> the complete() call to a surviving CPU via smp_call_function_single().
> 
> Reported-by: Peng Fan <van.freenix@...il.com>
> Reported-by: Russell King - ARM Linux <linux@...linux.org.uk>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Tested-by: Tested-by: Fabio Estevam <fabio.estevam@....com>
> Cc: Russell King <linux@...linux.org.uk>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: <linux-arm-kernel@...ts.infradead.org>

As I just described in response to Fabio's testing, this doesn't solve
anything if CONFIG_BL_SWITCHER is enabled.  We could lose the unlock of
a spinlock in the GIC code for sending the IPI.  As I already said
previously in our discussion (but I guess you just don't believe me):

"2. there's some optional locking in the GIC driver that cause problems
   for the cpu dying path.

The concensus last time around was that the IPI solution is a non-
starter, so the seven year proven-reliable solution (disregarding the
RCU warning) persists because I don't think anyone came up with a
better solution."

Using smp_call_function_single() invokes the IPI paths.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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