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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [day] [month] [year] [list]

Date:   Tue, 12 Dec 2017 11:36:02 -0800
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
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 05:37:59PM +0000, Russell King - ARM Linux wrote:
> 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):

Sorry, Russell, but most days I don't even believe myself.  So it is
nothing personal, just one of the occupational hazards of being 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.

OK, another approach is to have the dying CPU simply set an in-memory
flag, which a surviving CPU polls for.  There are of course any number
of ways of doing the polling loop.

So what bad thing happens when you use that approach?

							Thanx, Paul