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    SubjectPowerPC BUG: using smp_processor_id() in preemptible code
    With recent 2.6.37-rc, with CONFIG_PREEMPT=y CONFIG_DEBUG_PREEMPT=y
    on the PowerPC G5, I get spammed by BUG warnings each time I swapoff:

    BUG: using smp_processor_id() in preemptible [00000000] code: swapoff/3974
    caller is .hpte_need_flush+0x4c/0x2e8
    Call Trace:
    [c0000001b4a3f830] [c00000000000f3cc] .show_stack+0x6c/0x16c (unreliable)
    [c0000001b4a3f8e0] [c00000000023eda0] .debug_smp_processor_id+0xe4/0x11c
    [c0000001b4a3f970] [c00000000002f2f4] .hpte_need_flush+0x4c/0x2e8
    [c0000001b4a3fa30] [c0000000000e7ef8] .vunmap_pud_range+0x148/0x200
    [c0000001b4a3fb10] [c0000000000e8058] .vunmap_page_range+0xa8/0xd4
    [c0000001b4a3fbb0] [c0000000000e80a4] .free_unmap_vmap_area+0x20/0x38
    [c0000001b4a3fc40] [c0000000000e8138] .remove_vm_area+0x7c/0xb4
    [c0000001b4a3fcd0] [c0000000000e8308] .__vunmap+0x50/0x104
    [c0000001b4a3fd60] [c0000000000ef3fc] .SyS_swapoff+0x59c/0x6a8
    [c0000001b4a3fe30] [c0000000000075a8] syscall_exit+0x0/0x40

    I notice hpte_need_flush() itself acknowledges
    * Must be called from within some kind of spinlock/non-preempt region...

    Though I didn't actually bisect, I believe this is since Jeremy's
    64141da587241301ce8638cc945f8b67853156ec "vmalloc: eagerly clear ptes
    on vunmap", which moves a call to vunmap_page_range() from one place
    (which happened to be inside a spinlock) to another (where it's not).

    I guess my warnings would be easily silenced by moving that call to
    vunmap_page_range() down just inside the spinlock below it; but I'm
    dubious that that's the right fix - it looked as if there are other
    paths through vmalloc.c where vunmap_page_range() has been getting
    called without preemption disabled, long before Jeremy's change,
    just paths that I never happen to go down in my limited testing.

    For the moment I'm using the obvious patch below to keep it quiet;
    but I doubt that this is the right patch either. I'm hoping that
    ye who understand the importance of hpte_need_flush() will be best
    able to judge what to do. Or might there be other architectures
    expecting to be unpreemptible there?


    --- a/mm/vmalloc.c
    +++ b/mm/vmalloc.c
    @@ -37,11 +37,13 @@ static void vunmap_pte_range(pmd_t *pmd,
    pte_t *pte;

    + preempt_disable(); /* Stop __vunmap() triggering smp_processor_id() in preemptible from hpte_need_flush() */
    pte = pte_offset_kernel(pmd, addr);
    do {
    pte_t ptent = ptep_get_and_clear(&init_mm, addr, pte);
    WARN_ON(!pte_none(ptent) && !pte_present(ptent));
    } while (pte++, addr += PAGE_SIZE, addr != end);
    + preempt_enable();

    static void vunmap_pmd_range(pud_t *pud, unsigned long addr, unsigned long end)

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      Last update: 2010-12-29 23:57    [W:0.022 / U:0.064 seconds]
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