Re: [PATCH] membarrier: Update example to take TSO into account

[Michael Kerrisk (man-pages)]

On 09/18/2017 08:59 PM, Mathieu Desnoyers wrote:
> The example given specifically states that it focus on x86 (TSO memory
> model), but gives a read-read vs write-write ordering example, even
> though this scenario does not require explicit barriers on TSO.
> 
> So either we change the example architecture to a weakly-ordered
> architecture, or we change the example to a scenario requiring barriers
> on x86.
> 
> Let's stay on x86, but provide a Dekker as example instead.

Thanks for the patch, Mathieu. Applied. (And thanks for the links below.)

Cheers,

Michael

> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> CC: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> CC: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> Link: https://stackoverflow.com/questions/45970525/is-the-example-in-the-membarrier-man-page-pointless-in-x86
> Link: https://lwn.net/Articles/573436/
> ---
>  man2/membarrier.2 | 66 ++++++++++++++++++++++++++++++-------------------------
>  1 file changed, 36 insertions(+), 30 deletions(-)
> 
> diff --git a/man2/membarrier.2 b/man2/membarrier.2
> index 658dfa5d1..bbf611e10 100644
> --- a/man2/membarrier.2
> +++ b/man2/membarrier.2
> @@ -192,39 +192,42 @@ following code (x86) can be transformed using
>  static volatile int a, b;
>  
>  static void
> -fast_path(void)
> +fast_path(int *read_b)
>  {
> -    int read_a, read_b;
> -
> -    read_b = b;
> +    a = 1;
>      asm volatile ("mfence" : : : "memory");
> -    read_a = a;
> -
> -    /* read_b == 1 implies read_a == 1. */
> -
> -    if (read_b == 1 && read_a == 0)
> -        abort();
> +    *read_b = b;
>  }
>  
>  static void
> -slow_path(void)
> +slow_path(int *read_a)
>  {
> -    a = 1;
> -    asm volatile ("mfence" : : : "memory");
>      b = 1;
> +    asm volatile ("mfence" : : : "memory");
> +    *read_a = a;
>  }
>  
>  int
>  main(int argc, char **argv)
>  {
> +    int read_a, read_b;
> +
>      /*
>       * Real applications would call fast_path() and slow_path()
>       * from different threads. Call those from main() to keep
>       * this example short.
>       */
>  
> -    slow_path();
> -    fast_path();
> +    slow_path(&read_a);
> +    fast_path(&read_b);
> +
> +    /*
> +     * read_b == 0 implies read_a == 1 and
> +     * read_a == 0 implies read_b == 1.
> +     */
> +
> +    if (read_b == 0 && read_a == 0)
> +        abort();
>  
>      exit(EXIT_SUCCESS);
>  }
> @@ -275,31 +278,26 @@ init_membarrier(void)
>  }
>  
>  static void
> -fast_path(void)
> +fast_path(int *read_b)
>  {
> -    int read_a, read_b;
> -
> -    read_b = b;
> +    a = 1;
>      asm volatile ("" : : : "memory");
> -    read_a = a;
> -
> -    /* read_b == 1 implies read_a == 1. */
> -
> -    if (read_b == 1 && read_a == 0)
> -        abort();
> +    *read_b = b;
>  }
>  
>  static void
> -slow_path(void)
> +slow_path(int *read_a)
>  {
> -    a = 1;
> -    membarrier(MEMBARRIER_CMD_SHARED, 0);
>      b = 1;
> +    membarrier(MEMBARRIER_CMD_SHARED, 0);
> +    *read_a = a;
>  }
>  
>  int
>  main(int argc, char **argv)
>  {
> +    int read_a, read_b;
> +
>      if (init_membarrier())
>          exit(EXIT_FAILURE);
>  
> @@ -309,8 +307,16 @@ main(int argc, char **argv)
>       * this example short.
>       */
>  
> -    slow_path();
> -    fast_path();
> +    slow_path(&read_a);
> +    fast_path(&read_b);
> +
> +    /*
> +     * read_b == 0 implies read_a == 1 and
> +     * read_a == 0 implies read_b == 1.
> +     */
> +
> +    if (read_b == 0 && read_a == 0)
> +        abort();
>  
>      exit(EXIT_SUCCESS);
>  }
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/