From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Sat, 29 Aug 2015 03:15:33 +0000 (-0400)
Subject: Update following changes to sys_membarrier ABI
X-Git-Tag: v0.9.0~44
X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=64f469e628fe45059f091f2808188fb52fa109c9;p=userspace-rcu.git

Update following changes to sys_membarrier ABI

sys_membarrier underwent changes between its original implementation and
its upcoming inclusion into the Linux kernel. Update its use to follow
those changes.

Should the prior user-space code be built against a kernel header that
defines SYS_membarrier, and executed against that kernel, the following
scenarios may happen:

- -1 will be returned with EINVAL errno if the 2nd argument (flags) is
  non-zero (the previous ABI expected a single argument),
- (MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY) defined as
  (1 << 0) | (1 << 16) will return -1 with EINVAL errno, because valid
  commands are now one-hot.

Therefore, should an incompatible user-space code try to use
sys_membarrier, it will simply think that the system does not have
membarrier support due to the negative return value upon query.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---

diff --git a/urcu.c b/urcu.c
index e773065..5ffeb79 100644
--- a/urcu.c
+++ b/urcu.c
@@ -77,9 +77,10 @@
 # define membarrier(...)		-ENOSYS
 #endif
 
-#define MEMBARRIER_EXPEDITED		(1 << 0)
-#define MEMBARRIER_DELAYED		(1 << 1)
-#define MEMBARRIER_QUERY		(1 << 16)
+enum membarrier_cmd {
+	MEMBARRIER_CMD_QUERY = 0,
+	MEMBARRIER_CMD_SHARED = (1 << 0),
+};
 
 #ifdef RCU_MEMBARRIER
 static int init_done;
@@ -167,7 +168,7 @@ static void mutex_unlock(pthread_mutex_t *mutex)
 static void smp_mb_master(int group)
 {
 	if (caa_likely(rcu_has_sys_membarrier))
-		(void) membarrier(MEMBARRIER_EXPEDITED);
+		(void) membarrier(MEMBARRIER_CMD_SHARED, 0);
 	else
 		cmm_smp_mb();
 }
@@ -514,11 +515,15 @@ void rcu_unregister_thread(void)
 #ifdef RCU_MEMBARRIER
 void rcu_init(void)
 {
+	int ret;
+
 	if (init_done)
 		return;
 	init_done = 1;
-	if (!membarrier(MEMBARRIER_EXPEDITED | MEMBARRIER_QUERY))
+	ret = membarrier(MEMBARRIER_CMD_QUERY, 0);
+	if (ret >= 0 && (ret & MEMBARRIER_CMD_SHARED)) {
 		rcu_has_sys_membarrier = 1;
+	}
 }
 #endif