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Subject[PATCH] membarrier: Update example to take TSO into account
Date
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.

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/
---
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);
}
--
2.11.0
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