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membarrier.2: Update example to take TSO into account 

The existing 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.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Michael Kerrisk <mtk.manpages@gmail.com>
CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Link: https://stackoverflow.com/questions/45970525/is-the-example-in-the-membarrier-man-page-pointless-in-x86
Link: https://lwn.net/Articles/573436/
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Mathieu Desnoyers 2017-09-18 14:59:39 -04:00 committed by Michael Kerrisk
parent f63588bd36
commit b9aff60ed3
1 changed files with 39 additions and 33 deletions
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72
man2/membarrier.2
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@ -192,39 +192,42 @@ following code (x86) can be transformed using
static volatile int a, b;
static void
fast_path(void)
{
int read_a, read_b;
read_b = b;
asm volatile ("mfence" : : : "memory");
read_a = a;
/* read_b == 1 implies read_a == 1. */
if (read_b == 1 && read_a == 0)
abort();
}
static void
slow_path(void)
fast_path(int *read_b)
{
a = 1;
asm volatile ("mfence" : : : "memory");
*read_b = b;
}
static void
slow_path(int *read_a)
{
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);
}