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    Subject[PATCH v2] tracing/ftrace: don't trace on early stage of a secondary cpu boot
    Impact: fix a crash/hard-reboot while enabling cpu on runtime

    On some archs, the boot of a secondary cpu can have an early fragile state.
    On x86-64, the pda is not initialized on the first stage of a cpu boot but
    it is needed to get the cpu number and the current task pointer. These datas
    are needed during tracing. As they were dereferenced at this stage, we got a
    crash while turning on a cpu on runtime while tracing.

    Some other archs like ia64 can have such kind of issue too.

    Changes on v2:

    We drop the previous solution of a per-arch called function to guess the current state
    of a cpu. That could make slow the tracing.
    This patch just drop the -pg flag on arch/x86/kernel/cpu/common.c where
    live the low level cpu boot functions, and on start_secondary() and a helper
    function used at this stage.

    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    ---
    arch/x86/include/asm/msr.h | 3 ++-
    arch/x86/kernel/cpu/Makefile | 5 +++++
    arch/x86/kernel/smpboot.c | 2 +-
    3 files changed, 8 insertions(+), 2 deletions(-)

    diff --git a/arch/x86/include/asm/msr.h b/arch/x86/include/asm/msr.h
    index 4640ddd..638bf62 100644
    --- a/arch/x86/include/asm/msr.h
    +++ b/arch/x86/include/asm/msr.h
    @@ -85,7 +85,8 @@ static inline void native_write_msr(unsigned int msr,
    asm volatile("wrmsr" : : "c" (msr), "a"(low), "d" (high) : "memory");
    }

    -static inline int native_write_msr_safe(unsigned int msr,
    +/* Can be uninlined because referenced by paravirt */
    +notrace static inline int native_write_msr_safe(unsigned int msr,
    unsigned low, unsigned high)
    {
    int err;
    diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
    index fc99173..c381330 100644
    --- a/arch/x86/kernel/cpu/Makefile
    +++ b/arch/x86/kernel/cpu/Makefile
    @@ -2,6 +2,11 @@
    # Makefile for x86-compatible CPU details, features and quirks
    #

    +# Don't trace early stages of a secondary CPU boot
    +ifdef CONFIG_FUNCTION_TRACER
    +CFLAGS_REMOVE_common.o = -pg
    +endif
    +
    obj-y := intel_cacheinfo.o addon_cpuid_features.o
    obj-y += proc.o capflags.o powerflags.o common.o
    obj-y += vmware.o hypervisor.o
    diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
    index b1d571b..31869bf 100644
    --- a/arch/x86/kernel/smpboot.c
    +++ b/arch/x86/kernel/smpboot.c
    @@ -282,7 +282,7 @@ static int __cpuinitdata unsafe_smp;
    /*
    * Activate a secondary processor.
    */
    -static void __cpuinit start_secondary(void *unused)
    +notrace static void __cpuinit start_secondary(void *unused)
    {
    /*
    * Don't put *anything* before cpu_init(), SMP booting is too
    --
    1.6.0.4


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