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

[Thomas Gleixner]

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

Thanks,

	tglx