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    Subject[RFC] should VM_BUG_ON(cond) really evaluate cond
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    In commit 4e60c86bd9e (gcc-4.6: mm: fix unused but set warnings)
    Andi forced VM_BUG_ON(cond) to evaluate cond, even if CONFIG_DEBUG_VM is
    not set :

    #ifdef CONFIG_DEBUG_VM
    #define VM_BUG_ON(cond) BUG_ON(cond)
    #else
    #define VM_BUG_ON(cond) do { (void)(cond); } while (0)
    #endif

    As a side effect, get_page()/put_page_testzero() are performing more bus
    transactions on contended cache line on some workloads (tcp on loopback
    for example, where a page is acting as a shared buffer)

    0,05 : ffffffff815e4775: je ffffffff815e4970 <tcp_sendmsg+0xc80>
    0,05 : ffffffff815e477b: mov 0x1c(%r9),%eax // useless
    3,32 : ffffffff815e477f: mov (%r9),%rax // useless
    0,51 : ffffffff815e4782: lock incl 0x1c(%r9)
    3,87 : ffffffff815e4787: mov (%r9),%rax
    0,00 : ffffffff815e478a: test $0x80,%ah
    0,00 : ffffffff815e478d: jne ffffffff815e49f2 <tcp_sendmsg+0xd02>

    Of course, we have to understand why

    (void) (atomic_read(&some_atomic) == 1);

    generates asm code...

    mov some_atomic,%eax

    Ah yes, this is because of the volatile...

    static inline int atomic_read(const atomic_t *v)
    {
    return (*(volatile int *)&(v)->counter);
    }

    So maybe a fix would be to introduce an atomic_read_stable() variant ?

    static inline int atomic_read_stable(const atomic_t *v)
    {
    return v->counter;
    }

    Thanks !




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