On Fri, Nov 18, 2016 at 02:11:58PM +0100, Thomas Gleixner wrote: > On Fri, 18 Nov 2016, Will Deacon wrote: > > On Thu, Nov 17, 2016 at 07:35:36PM +0100, Sebastian Andrzej Siewior wrote: > > > @@ -1082,15 +1077,18 @@ static int __init arch_hw_breakpoint_init(void) > > > register_undef_hook(&debug_reg_hook); > > > > > > /* > > > - * Reset the breakpoint resources. We assume that a halting > > > - * debugger will leave the world in a nice state for us. > > > + * Register CPU notifier which resets the breakpoint resources. We > > > + * assume that a halting debugger will leave the world in a nice state > > > + * for us. > > > */ > > > - on_each_cpu(reset_ctrl_regs, NULL, 1); > > > + ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "arm/hw_breakpoint:online", > > > + dbg_reset_online, NULL); > > > > I'm slightly unsure about this. The dbg_reset_online callback can execute > > undefined instructions (unfortunately, there's no way to probe the presence > > of some of the debug registers), so it absolutely has to run within the > > register_undef_hook/unregister_undef_hook calls that are in this function. > > > > With this patch, I worry that the callback can be postponed to ONLINE time > > for other CPUs, and then the kernel will panic. > > No. The flow is the following: > > register_undef_hook(&debug_reg_hook); > > ret = cpuhp_setup_state(.., dbg_reset_online, NULL); > { > for_each_online_cpu(cpu) { > ret = call_on_cpu(cpu, dbg_reset_online); > if (ret) > return ret: > } > } > > unregister_undef_hook(&debug_reg_hook); > > The only difference to the current code is that the call is not invoked via > a smp function call (on_each_cpu), it's pushed to the hotplug thread > context of each cpu and executed there. > > But it's guaranteed that cpuhp_setup_state() will not return before the > callback has been invoked on each online cpu.
Ok, that's good. > If cpus are not yet online when that code is invoked, then it's the same > behaviour as before. It will be invoked when the cpu comes online. Just to check, but what stops a CPU from coming online between the call to cpuhp_setup_state and the call to cpuhp_remove_state_nocalls in the case of failure (debug_err_mask isn't empty)? Will