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SubjectRe: [PATCH 15/20] ARM/hw_breakpoint: Convert to hotplug state machine
On Thu, Nov 17, 2016 at 07:35:36PM +0100, Sebastian Andrzej Siewior wrote:
> Install the callbacks via the state machine and let the core invoke
> the callbacks on the already online CPUs.
>
> smp_call_function_single() has been removed because the function is already
> invoked on the target CPU.
>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Russell King <linux@armlinux.org.uk>
> Cc: linux-arm-kernel@lists.infradead.org
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> arch/arm/kernel/hw_breakpoint.c | 44 ++++++++++++++++++-----------------------
> 1 file changed, 19 insertions(+), 25 deletions(-)

[...]

> static int __init arch_hw_breakpoint_init(void)
> {
> + int ret;
> +
> debug_arch = get_debug_arch();
>
> if (!debug_arch_supported()) {
> @@ -1072,8 +1069,6 @@ static int __init arch_hw_breakpoint_init(void)
> core_num_brps = get_num_brps();
> core_num_wrps = get_num_wrps();
>
> - cpu_notifier_register_begin();
> -
> /*
> * We need to tread carefully here because DBGSWENABLE may be
> * driven low on this core and there isn't an architected way to
> @@ -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.

Will

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