Re: [Qemu-devel] [PATCH 5/8] tcg: Clarify "thread safaty" check in tb_add_jump()

[Sergey Fedorov <serge.fdrv@gmail.com>]

On 24/03/16 14:31, Paolo Bonzini wrote:
> On 24/03/2016 11:39, sergey.fedorov@linaro.org wrote:
>> +    /* FIXME: This test provides only some probablistic "thread safety" for
>> +     * user-mode emulation; appropriate synchronization/locking scheme should
>> +     * be implemented.
>> +     */
> There is appropriate locking.  This code:
>
>        if (next_tb != 0 && tb->page_addr[1] == -1
>            && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
>            tb_add_jump((TranslationBlock *)(next_tb & ~TB_EXIT_MASK),
>                        next_tb & TB_EXIT_MASK, tb);
>        }
>
> in cpu-exec.c runs under tb_lock.  However, two threads can decide to
> call tb_add_jump at the same time outside the lock, so we have to check
> inside the lock whether someone has already done the work.
>
> What the comment means is that, in single-threaded scenarios (current
> TCG and single-threaded user emulation), tb->jmp_list_next[n] is only
> set once.

Right, thanks for clarifying this. So I'm going to put mention this in
the comment, then.

Kind regards,
Sergey