powerpc/pseries/cpuidle: Fix preempt warning

[Michael Ellerman]

On Fri, 2018-11-23 at 16:30:11 UTC, Breno Leitao wrote:
> When booting a pseries kernel with PREEMPT enabled, it dumps the following
> warning:
> 
>    BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
>    caller is pseries_processor_idle_init+0x5c/0x22c
>    CPU: 13 PID: 1 Comm: swapper/0 Not tainted 4.20.0-rc3-00090-g12201a0128bc-dirty #828
>    Call Trace:
>    [c000000429437ab0] [c0000000009c8878] dump_stack+0xec/0x164 (unreliable)
>    [c000000429437b00] [c0000000005f2f24] check_preemption_disabled+0x154/0x160
>    [c000000429437b90] [c000000000cab8e8] pseries_processor_idle_init+0x5c/0x22c
>    [c000000429437c10] [c000000000010ed4] do_one_initcall+0x64/0x300
>    [c000000429437ce0] [c000000000c54500] kernel_init_freeable+0x3f0/0x500
>    [c000000429437db0] [c0000000000112dc] kernel_init+0x2c/0x160
>    [c000000429437e20] [c00000000000c1d0] ret_from_kernel_thread+0x5c/0x6c
> 
> This happens because the code calls get_lppaca() which calls get_paca() and
> it checks if preemption is disabled through check_preemption_disabled().
> 
> Preemption should be disabled because the per CPU variable may make no
> sense if there is a preemption (and a CPU switch) after it reads the per
> CPU data and when it is used.
> 
> At this device driver specifically, it is not a problem, because this code
> just need to have access to one lppaca struct, and it does not matter if it
> is the current per CPU lppaca struct or not (i.e. when there is a
> preemption and a CPU migration).
> 
> That said, the most appropriated fix seems to be related to avoiding the
> debug_smp_processor_id() call at get_paca(), instead of calling
> preempt_disable() before get_paca().
> 
> Signed-off-by: Breno Leitao <leitao at debian.org>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/2b038cbc5fcf12a7ee1cc9bfd5da1e

cheers