Messages in this thread | | | Date | Tue, 12 Dec 2017 11:36:02 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] ARM: CPU hotplug: Delegate complete() to surviving CPU |
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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@gmail.com> > > Reported-by: Russell King - ARM Linux <linux@armlinux.org.uk> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> > > Tested-by: Tested-by: Fabio Estevam <fabio.estevam@nxp.com> > > Cc: Russell King <linux@armlinux.org.uk> > > Cc: Ingo Molnar <mingo@kernel.org> > > Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org> > > Cc: Michal Hocko <mhocko@suse.com> > > Cc: Thomas Gleixner <tglx@linutronix.de> > > Cc: <linux-arm-kernel@lists.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
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