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

[Paul E. McKenney]

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@xxxxxxxxx>
> > Reported-by: Russell King - ARM Linux <linux@xxxxxxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > Tested-by: Tested-by: Fabio Estevam <fabio.estevam@xxxxxxx>
> > Cc: Russell King <linux@xxxxxxxxxxxxxxx>
> > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > Cc: "Peter Zijlstra (Intel)" <peterz@xxxxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: <linux-arm-kernel@xxxxxxxxxxxxxxxxxxx>
> 
> 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