From mboxrd@z Thu Jan 1 00:00:00 1970 From: paulmck@linux.vnet.ibm.com (Paul E. McKenney) Date: Tue, 12 Dec 2017 11:36:02 -0800 Subject: [PATCH] ARM: CPU hotplug: Delegate complete() to surviving CPU In-Reply-To: <20171212173759.GE10595@n2100.armlinux.org.uk> References: <20171212172059.GA11875@linux.vnet.ibm.com> <20171212173759.GE10595@n2100.armlinux.org.uk> Message-ID: <20171212193602.GG7829@linux.vnet.ibm.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org 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 > > Reported-by: Russell King - ARM Linux > > Signed-off-by: Paul E. McKenney > > Tested-by: Tested-by: Fabio Estevam > > Cc: Russell King > > Cc: Ingo Molnar > > Cc: "Peter Zijlstra (Intel)" > > Cc: Michal Hocko > > Cc: Thomas Gleixner > > Cc: > > 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