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 2396fcf1b149894f0fc5172bbbd3353197d80965..44cb09bfaa8c342c8b496dd93d195ceb8861cb35 100644 (file)
--- 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 04f6a0c54682f6bbb30a528ed1133acf87a07cc7..4daee1bd405e7cfb23b7ca1c6aec6c900889d4d7 100644 (file)
--- 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 b71563b7c90c5f3136d4e2517e4de95bb8576202..13d2b1a0a9c73904563e2b9b46d40d19e3316686 100644 (file)
--- 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