Re: [Qemu-devel] [PATCH v3 05/10] tcg: Clarify thread safety check in tb_add_jump()

["Alex Bennée" <alex.bennee@linaro.org>]

Sergey Fedorov <sergey.fedorov@linaro.org> writes:

> From: Sergey Fedorov <serge.fdrv@gmail.com>
>
> The check is to make sure that another thread hasn't already done the
> same while we were outside of tb_lock. Mention this in a comment.
>
> Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
> Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
> ---
>
> Changes in v2:
>  * Typo fixed in the commit title
>  * Complete rewrite of the commit body and the patch based on Paolo's comments
>
>  include/exec/exec-all.h | 31 +++++++++++++++++--------------
>  1 file changed, 17 insertions(+), 14 deletions(-)
>
> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
> index b055716ed690..8e81ef5fb2c2 100644
> --- a/include/exec/exec-all.h
> +++ b/include/exec/exec-all.h
> @@ -391,21 +391,24 @@ static inline void tb_set_jmp_target(TranslationBlock *tb,
>  static inline void tb_add_jump(TranslationBlock *tb, int n,
>                                 TranslationBlock *tb_next)
>  {
> -    /* NOTE: this test is only needed for thread safety */
> -    if (!tb->jmp_list_next[n]) {
> -        qemu_log_mask_and_addr(CPU_LOG_EXEC, tb->pc,
> -                               "Linking TBs %p [" TARGET_FMT_lx
> -                               "] index %d -> %p [" TARGET_FMT_lx "]\n",
> -                               tb->tc_ptr, tb->pc, n,
> -                               tb_next->tc_ptr, tb_next->pc);
> -        /* patch the native jump address */
> -        tb_set_jmp_target(tb, n, (uintptr_t)tb_next->tc_ptr);
> -
> -        /* add in TB jmp circular list */
> -        tb->jmp_list_next[n] = tb_next->jmp_list_first;
> -        assert(((uintptr_t)tb & 3) == 0);
> -        tb_next->jmp_list_first = (uintptr_t)tb | n;
> +    if (tb->jmp_list_next[n]) {
> +        /* Another thread has already done this while we were
> +         * outside of the lock; nothing to do in this case */
> +        return;
>      }
> +    qemu_log_mask_and_addr(CPU_LOG_EXEC, tb->pc,
> +                           "Linking TBs %p [" TARGET_FMT_lx
> +                           "] index %d -> %p [" TARGET_FMT_lx "]\n",
> +                           tb->tc_ptr, tb->pc, n,
> +                           tb_next->tc_ptr, tb_next->pc);
> +
> +    /* patch the native jump address */
> +    tb_set_jmp_target(tb, n, (uintptr_t)tb_next->tc_ptr);
> +
> +    /* add in TB jmp circular list */
> +    tb->jmp_list_next[n] = tb_next->jmp_list_first;
> +    assert(((uintptr_t)tb & 3) == 0);

I think this assert can be dropped. The only call explicitly masks with
TB_EXIT_MASK (which would be a better choice than the number 3 anyway)
so something really strange would have had to happen in the intervening
few lines.

Otherwise:

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

> +    tb_next->jmp_list_first = (uintptr_t)tb | n;
>  }
>
>  /* GETRA is the true target of the return instruction that we'll execute,


--
Alex Bennée