Re: [PATCH v2 1/2] ramlist: Make dirty bitmap blocks of ramlist resizable

[Stefan Hajnoczi]

On Mon, Nov 30, 2020 at 09:11:03PM +0800, Keqian Zhu wrote:
> @@ -1839,15 +1841,26 @@ static void dirty_memory_extend(ram_addr_t 
> old_ram_size,
>          new_blocks = g_malloc(sizeof(*new_blocks) +
>                                sizeof(new_blocks->blocks[0]) * 
> new_num_blocks);
>  
> -        if (old_num_blocks) {
> +        if (cpy_num_blocks) {
>              memcpy(new_blocks->blocks, old_blocks->blocks,
> -                   old_num_blocks * sizeof(old_blocks->blocks[0]));
> +                   cpy_num_blocks * sizeof(old_blocks->blocks[0]));
>          }
>  
> -        for (j = old_num_blocks; j < new_num_blocks; j++) {
> -            new_blocks->blocks[j] = bitmap_new(DIRTY_MEMORY_BLOCK_SIZE);
> +        if (extend) {
> +            for (j = cpy_num_blocks; j < new_num_blocks; j++) {
> +                new_blocks->blocks[j] = bitmap_new(DIRTY_MEMORY_BLOCK_SIZE);
> +            }
> +        } else {
> +            for (j = cpy_num_blocks; j < old_num_blocks; j++) {
> +                /* We are safe to free it, for that it is out-of-use */
> +                g_free(old_blocks->blocks[j]);

This looks unsafe because this code uses Read Copy Update (RCU):

  old_blocks = qatomic_rcu_read(&ram_list.dirty_memory[i]);

Other threads may still be accessing old_blocks so we cannot modify it
immediately. Changes need to be deferred until the next RCU period.
g_free_rcu() needs to be used to do this.

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