Re: [PATCH] cpu hotplug: Prevent Page Fault in cpuhp_remove_callbacks()

From: Thomas Gleixner
Date: Thu Jul 20 2017 - 10:05:34 EST


On Wed, 19 Jul 2017, Ethan Barnes wrote:
> There is a page fault in cpu hotplug when removing the callbacks for the
> last dynamic state.
> The last dynamic state will not be removed, causing a page fault when
> hotplug thinks the callbacks still exists and calls it.
>
> The problem is as follows:
> - __cpuhp_remove_state() eventually calls cpuhp_store_callbacks() with
> NULLs for most params.
> - If state is the last state (i.e. CPUHP_AP_ONLINE_DYN or
> CPUHP_BP_PREPARE_STATE) then cpuhp_store_callbacks() calls
> cpuhp_reserve_state(), which returns the *next* available DYN state.
> - The NULLs are stored in that *next* state (i.e. AP_ONLINE_DYN + 1 or
> BP_PREPARE_DYN + 1) instead of in AP_ONLINE_DYN or BP_PREPARE_DYN. Thus,
> the last state is never cleared. The callbacks now point to invalid memory.
> - When the cpu is onlined again and/or offlined, then the invalid
> callback is invoked, causing a page fault.

Indeed.

> The patch above solves this by detecting when a state is being removed,
> and not calling cpuhp_reserve_state().
>
> I have brought this up before, but hoping to get some traction on it
> this time:
> https://lkml.org/lkml/2017/7/5/574

Escaped my attention.

> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index eee0331..f7fda16 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1252,7 +1252,8 @@ static int cpuhp_store_callbacks(enum cpuhp_state
> state, const char *name,

That patch is corrupted by your mail-client or mail-server or both.

> struct cpuhp_step *sp;
> int ret = 0;
>
> - if (state == CPUHP_AP_ONLINE_DYN || state == CPUHP_BP_PREPARE_DYN) {
> + if (name &&
> + (state == CPUHP_AP_ONLINE_DYN || state ==
> CPUHP_BP_PREPARE_DYN)) {
> ret = cpuhp_reserve_state(state);
> if (ret < 0)
> return ret;
> Western Digital Corporation (and its subsidiaries) E-mail Confidentiality Notice & Disclaimer:
>

Can you please make sure that there is a blank line before that
automatically inserted disclaimer?

Thanks,

tglx