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    Subject[PATCH AUTOSEL 5.12 20/80] sched/core: Initialize the idle task with preemption disabled
    Date
    From: Valentin Schneider <valentin.schneider@arm.com>

    [ Upstream commit f1a0a376ca0c4ef1fc3d24e3e502acbb5b795674 ]

    As pointed out by commit

    de9b8f5dcbd9 ("sched: Fix crash trying to dequeue/enqueue the idle thread")

    init_idle() can and will be invoked more than once on the same idle
    task. At boot time, it is invoked for the boot CPU thread by
    sched_init(). Then smp_init() creates the threads for all the secondary
    CPUs and invokes init_idle() on them.

    As the hotplug machinery brings the secondaries to life, it will issue
    calls to idle_thread_get(), which itself invokes init_idle() yet again.
    In this case it's invoked twice more per secondary: at _cpu_up(), and at
    bringup_cpu().

    Given smp_init() already initializes the idle tasks for all *possible*
    CPUs, no further initialization should be required. Now, removing
    init_idle() from idle_thread_get() exposes some interesting expectations
    with regards to the idle task's preempt_count: the secondary startup always
    issues a preempt_disable(), requiring some reset of the preempt count to 0
    between hot-unplug and hotplug, which is currently served by
    idle_thread_get() -> idle_init().

    Given the idle task is supposed to have preemption disabled once and never
    see it re-enabled, it seems that what we actually want is to initialize its
    preempt_count to PREEMPT_DISABLED and leave it there. Do that, and remove
    init_idle() from idle_thread_get().

    Secondary startups were patched via coccinelle:

    @begone@
    @@

    -preempt_disable();
    ...
    cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);

    Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Acked-by: Peter Zijlstra <peterz@infradead.org>
    Link: https://lore.kernel.org/r/20210512094636.2958515-1-valentin.schneider@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    arch/alpha/kernel/smp.c | 1 -
    arch/arc/kernel/smp.c | 1 -
    arch/arm/kernel/smp.c | 1 -
    arch/arm64/include/asm/preempt.h | 2 +-
    arch/arm64/kernel/smp.c | 1 -
    arch/csky/kernel/smp.c | 1 -
    arch/ia64/kernel/smpboot.c | 1 -
    arch/mips/kernel/smp.c | 1 -
    arch/openrisc/kernel/smp.c | 2 --
    arch/parisc/kernel/smp.c | 1 -
    arch/powerpc/kernel/smp.c | 1 -
    arch/riscv/kernel/smpboot.c | 1 -
    arch/s390/include/asm/preempt.h | 4 ++--
    arch/s390/kernel/smp.c | 1 -
    arch/sh/kernel/smp.c | 2 --
    arch/sparc/kernel/smp_32.c | 1 -
    arch/sparc/kernel/smp_64.c | 3 ---
    arch/x86/include/asm/preempt.h | 2 +-
    arch/x86/kernel/smpboot.c | 1 -
    arch/xtensa/kernel/smp.c | 1 -
    include/asm-generic/preempt.h | 2 +-
    init/main.c | 6 +-----
    kernel/fork.c | 2 +-
    kernel/sched/core.c | 2 +-
    kernel/smpboot.c | 1 -
    25 files changed, 8 insertions(+), 34 deletions(-)

    diff --git a/arch/alpha/kernel/smp.c b/arch/alpha/kernel/smp.c
    index f4dd9f3f3001..4b2575f936d4 100644
    --- a/arch/alpha/kernel/smp.c
    +++ b/arch/alpha/kernel/smp.c
    @@ -166,7 +166,6 @@ smp_callin(void)
    DBGS(("smp_callin: commencing CPU %d current %p active_mm %p\n",
    cpuid, current, current->active_mm));

    - preempt_disable();
    cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
    }

    diff --git a/arch/arc/kernel/smp.c b/arch/arc/kernel/smp.c
    index 52906d314537..db0e104d6835 100644
    --- a/arch/arc/kernel/smp.c
    +++ b/arch/arc/kernel/smp.c
    @@ -189,7 +189,6 @@ void start_kernel_secondary(void)
    pr_info("## CPU%u LIVE ##: Executing Code...\n", cpu);

    local_irq_enable();
    - preempt_disable();
    cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
    }

    diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
    index 74679240a9d8..c7bb168b0d97 100644
    --- a/arch/arm/kernel/smp.c
    +++ b/arch/arm/kernel/smp.c
    @@ -432,7 +432,6 @@ asmlinkage void secondary_start_kernel(void)
    #endif
    pr_debug("CPU%u: Booted secondary processor\n", cpu);

    - preempt_disable();
    trace_hardirqs_off();

    /*
    diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
    index 80e946b2abee..e83f0982b99c 100644
    --- a/arch/arm64/include/asm/preempt.h
    +++ b/arch/arm64/include/asm/preempt.h
    @@ -23,7 +23,7 @@ static inline void preempt_count_set(u64 pc)
    } while (0)

    #define init_idle_preempt_count(p, cpu) do { \
    - task_thread_info(p)->preempt_count = PREEMPT_ENABLED; \
    + task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
    } while (0)

    static inline void set_preempt_need_resched(void)
    diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
    index 357590beaabb..48fd89256739 100644
    --- a/arch/arm64/kernel/smp.c
    +++ b/arch/arm64/kernel/smp.c
    @@ -223,7 +223,6 @@ asmlinkage notrace void secondary_start_kernel(void)
    init_gic_priority_masking();

    rcu_cpu_starting(cpu);
    - preempt_disable();
    trace_hardirqs_off();

    /*
    diff --git a/arch/csky/kernel/smp.c b/arch/csky/kernel/smp.c
    index 0f9f5eef9338..e2993539af8e 100644
    --- a/arch/csky/kernel/smp.c
    +++ b/arch/csky/kernel/smp.c
    @@ -281,7 +281,6 @@ void csky_start_secondary(void)
    pr_info("CPU%u Online: %s...\n", cpu, __func__);

    local_irq_enable();
    - preempt_disable();
    cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
    }

    diff --git a/arch/ia64/kernel/smpboot.c b/arch/ia64/kernel/smpboot.c
    index 49b488580939..d10f780c13b9 100644
    --- a/arch/ia64/kernel/smpboot.c
    +++ b/arch/ia64/kernel/smpboot.c
    @@ -441,7 +441,6 @@ start_secondary (void *unused)
    #endif
    efi_map_pal_code();
    cpu_init();
    - preempt_disable();
    smp_callin();

    cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
    diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
    index ef86fbad8546..d542fb7af3ba 100644
    --- a/arch/mips/kernel/smp.c
    +++ b/arch/mips/kernel/smp.c
    @@ -348,7 +348,6 @@ asmlinkage void start_secondary(void)
    */

    calibrate_delay();
    - preempt_disable();
    cpu = smp_processor_id();
    cpu_data[cpu].udelay_val = loops_per_jiffy;

    diff --git a/arch/openrisc/kernel/smp.c b/arch/openrisc/kernel/smp.c
    index 48e1092a64de..415e209732a3 100644
    --- a/arch/openrisc/kernel/smp.c
    +++ b/arch/openrisc/kernel/smp.c
    @@ -145,8 +145,6 @@ asmlinkage __init void secondary_start_kernel(void)
    set_cpu_online(cpu, true);

    local_irq_enable();
    -
    - preempt_disable();
    /*
    * OK, it's off to the idle thread for us
    */
    diff --git a/arch/parisc/kernel/smp.c b/arch/parisc/kernel/smp.c
    index 10227f667c8a..1405b603b91b 100644
    --- a/arch/parisc/kernel/smp.c
    +++ b/arch/parisc/kernel/smp.c
    @@ -302,7 +302,6 @@ void __init smp_callin(unsigned long pdce_proc)
    #endif

    smp_cpu_init(slave_id);
    - preempt_disable();

    flush_cache_all_local(); /* start with known state */
    flush_tlb_all_local(NULL);
    diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
    index c2473e20f5f5..1d7daef1c3b6 100644
    --- a/arch/powerpc/kernel/smp.c
    +++ b/arch/powerpc/kernel/smp.c
    @@ -1530,7 +1530,6 @@ void start_secondary(void *unused)
    smp_store_cpu_info(cpu);
    set_dec(tb_ticks_per_jiffy);
    rcu_cpu_starting(cpu);
    - preempt_disable();
    cpu_callin_map[cpu] = 1;

    if (smp_ops->setup_cpu)
    diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
    index 5e276c25646f..1941a6ce86a1 100644
    --- a/arch/riscv/kernel/smpboot.c
    +++ b/arch/riscv/kernel/smpboot.c
    @@ -176,7 +176,6 @@ asmlinkage __visible void smp_callin(void)
    * Disable preemption before enabling interrupts, so we don't try to
    * schedule a CPU that hasn't actually started yet.
    */
    - preempt_disable();
    local_irq_enable();
    cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
    }
    diff --git a/arch/s390/include/asm/preempt.h b/arch/s390/include/asm/preempt.h
    index b49e0492842c..23ff51be7e29 100644
    --- a/arch/s390/include/asm/preempt.h
    +++ b/arch/s390/include/asm/preempt.h
    @@ -32,7 +32,7 @@ static inline void preempt_count_set(int pc)
    #define init_task_preempt_count(p) do { } while (0)

    #define init_idle_preempt_count(p, cpu) do { \
    - S390_lowcore.preempt_count = PREEMPT_ENABLED; \
    + S390_lowcore.preempt_count = PREEMPT_DISABLED; \
    } while (0)

    static inline void set_preempt_need_resched(void)
    @@ -91,7 +91,7 @@ static inline void preempt_count_set(int pc)
    #define init_task_preempt_count(p) do { } while (0)

    #define init_idle_preempt_count(p, cpu) do { \
    - S390_lowcore.preempt_count = PREEMPT_ENABLED; \
    + S390_lowcore.preempt_count = PREEMPT_DISABLED; \
    } while (0)

    static inline void set_preempt_need_resched(void)
    diff --git a/arch/s390/kernel/smp.c b/arch/s390/kernel/smp.c
    index 58c8afa3da65..d60c7374d807 100644
    --- a/arch/s390/kernel/smp.c
    +++ b/arch/s390/kernel/smp.c
    @@ -877,7 +877,6 @@ static void smp_init_secondary(void)
    restore_access_regs(S390_lowcore.access_regs_save_area);
    cpu_init();
    rcu_cpu_starting(cpu);
    - preempt_disable();
    init_cpu_timer();
    vtime_init();
    vdso_getcpu_init();
    diff --git a/arch/sh/kernel/smp.c b/arch/sh/kernel/smp.c
    index 372acdc9033e..65924d9ec245 100644
    --- a/arch/sh/kernel/smp.c
    +++ b/arch/sh/kernel/smp.c
    @@ -186,8 +186,6 @@ asmlinkage void start_secondary(void)

    per_cpu_trap_init();

    - preempt_disable();
    -
    notify_cpu_starting(cpu);

    local_irq_enable();
    diff --git a/arch/sparc/kernel/smp_32.c b/arch/sparc/kernel/smp_32.c
    index 50c127ab46d5..22b148e5a5f8 100644
    --- a/arch/sparc/kernel/smp_32.c
    +++ b/arch/sparc/kernel/smp_32.c
    @@ -348,7 +348,6 @@ static void sparc_start_secondary(void *arg)
    */
    arch_cpu_pre_starting(arg);

    - preempt_disable();
    cpu = smp_processor_id();

    notify_cpu_starting(cpu);
    diff --git a/arch/sparc/kernel/smp_64.c b/arch/sparc/kernel/smp_64.c
    index e38d8bf454e8..ae5faa1d989d 100644
    --- a/arch/sparc/kernel/smp_64.c
    +++ b/arch/sparc/kernel/smp_64.c
    @@ -138,9 +138,6 @@ void smp_callin(void)

    set_cpu_online(cpuid, true);

    - /* idle thread is expected to have preempt disabled */
    - preempt_disable();
    -
    local_irq_enable();

    cpu_startup_entry(CPUHP_AP_ONLINE_IDLE);
    diff --git a/arch/x86/include/asm/preempt.h b/arch/x86/include/asm/preempt.h
    index f8cb8af4de5c..fe5efbcba824 100644
    --- a/arch/x86/include/asm/preempt.h
    +++ b/arch/x86/include/asm/preempt.h
    @@ -44,7 +44,7 @@ static __always_inline void preempt_count_set(int pc)
    #define init_task_preempt_count(p) do { } while (0)

    #define init_idle_preempt_count(p, cpu) do { \
    - per_cpu(__preempt_count, (cpu)) = PREEMPT_ENABLED; \
    + per_cpu(__preempt_count, (cpu)) = PREEMPT_DISABLED; \
    } while (0)

    /*
    diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
    index 363b36bbd791..ebc4b13b74a4 100644
    --- a/arch/x86/kernel/smpboot.c
    +++ b/arch/x86/kernel/smpboot.c
    @@ -236,7 +236,6 @@ static void notrace start_secondary(void *unused)
    cpu_init();
    rcu_cpu_starting(raw_smp_processor_id());
    x86_cpuinit.early_percpu_clock_init();
    - preempt_disable();
    smp_callin();

    enable_start_cpu0 = 0;
    diff --git a/arch/xtensa/kernel/smp.c b/arch/xtensa/kernel/smp.c
    index cd85a7a2722b..1254da07ead1 100644
    --- a/arch/xtensa/kernel/smp.c
    +++ b/arch/xtensa/kernel/smp.c
    @@ -145,7 +145,6 @@ void secondary_start_kernel(void)
    cpumask_set_cpu(cpu, mm_cpumask(mm));
    enter_lazy_tlb(mm, current);

    - preempt_disable();
    trace_hardirqs_off();

    calibrate_delay();
    diff --git a/include/asm-generic/preempt.h b/include/asm-generic/preempt.h
    index d683f5e6d791..b4d43a4af5f7 100644
    --- a/include/asm-generic/preempt.h
    +++ b/include/asm-generic/preempt.h
    @@ -29,7 +29,7 @@ static __always_inline void preempt_count_set(int pc)
    } while (0)

    #define init_idle_preempt_count(p, cpu) do { \
    - task_thread_info(p)->preempt_count = PREEMPT_ENABLED; \
    + task_thread_info(p)->preempt_count = PREEMPT_DISABLED; \
    } while (0)

    static __always_inline void set_preempt_need_resched(void)
    diff --git a/init/main.c b/init/main.c
    index 5bd1a25f1d6f..c97d3c0247a1 100644
    --- a/init/main.c
    +++ b/init/main.c
    @@ -918,11 +918,7 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
    * time - but meanwhile we still have a functioning scheduler.
    */
    sched_init();
    - /*
    - * Disable preemption - early bootup scheduling is extremely
    - * fragile until we cpu_idle() for the first time.
    - */
    - preempt_disable();
    +
    if (WARN(!irqs_disabled(),
    "Interrupts were enabled *very* early, fixing it\n"))
    local_irq_disable();
    diff --git a/kernel/fork.c b/kernel/fork.c
    index 321a5e31d817..2654a5a9e5df 100644
    --- a/kernel/fork.c
    +++ b/kernel/fork.c
    @@ -2405,7 +2405,7 @@ static inline void init_idle_pids(struct task_struct *idle)
    }
    }

    -struct task_struct *fork_idle(int cpu)
    +struct task_struct * __init fork_idle(int cpu)
    {
    struct task_struct *task;
    struct kernel_clone_args args = {
    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 814200541f8f..e25b2d8ec18d 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -7428,7 +7428,7 @@ void show_state_filter(unsigned long state_filter)
    * NOTE: this function does not set the idle thread's NEED_RESCHED
    * flag, to make booting more robust.
    */
    -void init_idle(struct task_struct *idle, int cpu)
    +void __init init_idle(struct task_struct *idle, int cpu)
    {
    struct rq *rq = cpu_rq(cpu);
    unsigned long flags;
    diff --git a/kernel/smpboot.c b/kernel/smpboot.c
    index f25208e8df83..e4163042c4d6 100644
    --- a/kernel/smpboot.c
    +++ b/kernel/smpboot.c
    @@ -33,7 +33,6 @@ struct task_struct *idle_thread_get(unsigned int cpu)

    if (!tsk)
    return ERR_PTR(-ENOMEM);
    - init_idle(tsk, cpu);
    return tsk;
    }

    --
    2.30.2
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