• Breno Leitao's avatar
    powerpc/pseries/cpuidle: Fix preempt warning · 2b038cbc
    Breno Leitao authored
    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.
    
    In this device driver specifically, it is not a problem, because this
    code just needs 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 appropriate 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: default avatarBreno Leitao <leitao@debian.org>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    2b038cbc
cpuidle-pseries.c 6.66 KB