From: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Date: Fri, 11 Sep 2015 14:33:43 +0000 (-0400)
Subject: Fix: dynamic fallback to compat futex on sys_futex ENOSYS
X-Git-Tag: v0.7.16~6
X-Git-Url: http://git.liburcu.org/?p=userspace-rcu.git;a=commitdiff_plain;h=4dcb7afe15ca3403eabccc73b0a353b253900441

Fix: dynamic fallback to compat futex on sys_futex ENOSYS

Some MIPS processors (e.g. Cavium Octeon II) dynamically check if the
CPU supports ll/sc within sys_futex, and return a ENOSYS errno if they
don't, even though the architecture implements sys_futex.

Handle this situation by always building the sys_futex compatibility
layer, and fall-back on it if sys_futex return a ENOSYS errno. This is
a tiny compat layer which adds very little space overhead.

This adds an unlikely branch on return from sys_futex, which should
not be an issue performance-wise (we've already taken a system call).

Since this is a fall-back mode, don't try to be clever, and don't cache
the result, so that the common cases (architectures with a properly
working sys_futex) don't get two conditional branches, just one.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Michael Jeanson <mjeanson@efficios.com>
CC: Jon Bernard <jbernard@debian.org>
---

diff --git a/Makefile.am b/Makefile.am
index 2396fcf..44cb09b 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -37,9 +37,7 @@ else
 COMPAT=
 endif
 
-if COMPAT_FUTEX
 COMPAT+=compat_futex.c
-endif
 
 RCULFHASH = rculfhash.c rculfhash-mm-order.c rculfhash-mm-chunk.c \
 		rculfhash-mm-mmap.c
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 04f6a0c..4daee1b 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -25,9 +25,7 @@ else
 COMPAT=
 endif
 
-if COMPAT_FUTEX
 COMPAT+=$(top_srcdir)/compat_futex.c
-endif
 
 URCU=$(top_srcdir)/urcu.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
 URCU_QSBR=$(top_srcdir)/urcu-qsbr.c $(top_srcdir)/urcu-pointer.c $(top_srcdir)/wfqueue.c $(COMPAT)
diff --git a/urcu/futex.h b/urcu/futex.h
index b71563b..13d2b1a 100644
--- a/urcu/futex.h
+++ b/urcu/futex.h
@@ -47,22 +47,66 @@ extern "C" {
  * (returns EINTR).
  */
 
-#ifdef CONFIG_RCU_HAVE_FUTEX
-#include <sys/syscall.h>
-#define futex(...)	syscall(__NR_futex, __VA_ARGS__)
-#define futex_noasync(uaddr, op, val, timeout, uaddr2, val3)	\
-		futex(uaddr, op, val, timeout, uaddr2, val3)
-#define futex_async(uaddr, op, val, timeout, uaddr2, val3)	\
-		futex(uaddr, op, val, timeout, uaddr2, val3)
-#else
 extern int compat_futex_noasync(int32_t *uaddr, int op, int32_t val,
-	const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
-#define futex_noasync(uaddr, op, val, timeout, uaddr2, val3)	\
-		compat_futex_noasync(uaddr, op, val, timeout, uaddr2, val3)
+		const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
 extern int compat_futex_async(int32_t *uaddr, int op, int32_t val,
-	const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
-#define futex_async(uaddr, op, val, timeout, uaddr2, val3)	\
-		compat_futex_async(uaddr, op, val, timeout, uaddr2, val3)
+		const struct timespec *timeout, int32_t *uaddr2, int32_t val3);
+
+#ifdef CONFIG_RCU_HAVE_FUTEX
+
+#include <unistd.h>
+#include <errno.h>
+#include <urcu/compiler.h>
+#include <urcu/syscall-compat.h>
+
+static inline int futex(int32_t *uaddr, int op, int32_t val,
+		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
+{
+	return syscall(__NR_futex, uaddr, op, val, timeout,
+			uaddr2, val3);
+}
+
+static inline int futex_noasync(int32_t *uaddr, int op, int32_t val,
+		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
+{
+	int ret;
+
+	ret = futex(uaddr, op, val, timeout, uaddr2, val3);
+	if (caa_unlikely(ret < 0 && errno == ENOSYS)) {
+		return compat_futex_noasync(uaddr, op, val, timeout,
+				uaddr2, val3);
+	}
+	return ret;
+
+}
+
+static inline int futex_async(int32_t *uaddr, int op, int32_t val,
+		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
+{
+	int ret;
+
+	ret = futex(uaddr, op, val, timeout, uaddr2, val3);
+	if (caa_unlikely(ret < 0 && errno == ENOSYS)) {
+		return compat_futex_async(uaddr, op, val, timeout,
+				uaddr2, val3);
+	}
+	return ret;
+}
+
+#else
+
+static inline int futex_noasync(int32_t *uaddr, int op, int32_t val,
+		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
+{
+	return compat_futex_noasync(uaddr, op, val, timeout, uaddr2, val3);
+}
+
+static inline int futex_async(int32_t *uaddr, int op, int32_t val,
+		const struct timespec *timeout, int32_t *uaddr2, int32_t val3)
+{
+	return compat_futex_async(uaddr, op, val, timeout, uaddr2, val3);
+}
+
 #endif
 
 #ifdef __cplusplus