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xarray的实现在./lib/xarray.c中xarray 的解释如下：The XArray is an abstract data type which behaves like a very large arrayof pointers.  It meets many of the same needs as a hash or a conventionalresizable array.  Unlike a hash, it allows you to sensibly go to thenext or previous entry in a cache-efficient manner.  In contrast to aresizable array, there is no need to copy data or change MMU mappings inorder to grow the array.  It is more memory-efficient, parallelisableand cache friendly than a doubly-linked list.  It takes advantage ofRCU to perform lookups without locking.总结一下，就是比hash可更高效的前后搜索，比数组高效是在数组增长的时候不用改变mmu 映射等。用法很简单，文档中给出的例子如下：   void foo_init(struct foo *foo)    {        xa_init_flags(&foo->array, XA_FLAGS_LOCK_BH);    }    int foo_store(struct foo *foo, unsigned long index, void *entry)    {        int err;        xa_lock_bh(&foo->array);        err = xa_err(__xa_store(&foo->array, index, entry, GFP_KERNEL));        if (!err)            foo->count++;        xa_unlock_bh(&foo->array);        return err;    }    /* foo_erase() is only called from softirq context */    void foo_erase(struct foo *foo, unsigned long index)    {        xa_lock(&foo->array);        __xa_erase(&foo->array, index);        foo->count--;        xa_unlock(&foo->array);    }

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