[PATCH 6/7] implement __unlock_requeue

[Jens Gustedt <Jens.Gustedt@inria.fr>]

We need a special case of unlock when we might take advantage of the
futex requeue feature to move a thread from a condition queue to the one
of the correponding mutex
---
 src/internal/__lock.h |  8 ++++++++
 src/internal/libc.h   |  2 ++
 src/thread/__lock.c   | 17 +++++++++++++++++
 3 files changed, 27 insertions(+)

diff --git a/src/internal/__lock.h b/src/internal/__lock.h
index f16d6176..0c2e9bfb 100644
--- a/src/internal/__lock.h
+++ b/src/internal/__lock.h
@@ -19,3 +19,11 @@ static inline void __unlock_fast(volatile int *l)
 		}
 	}
 }
+
+static inline void __unlock_requeue_fast(volatile int *l, volatile int *r, int shared)
+{
+	extern void __unlock_requeue_slow(volatile int*, volatile int*, int);
+	/* We have to check if l[0] had been touched at all. */
+	if (l[0] < 0 && (a_fetch_add(l, -(INT_MIN + 1)) != (INT_MIN + 1)))
+		__unlock_requeue_slow(l, r, shared);
+}
diff --git a/src/internal/libc.h b/src/internal/libc.h
index a594d0c5..999b4cba 100644
--- a/src/internal/libc.h
+++ b/src/internal/libc.h
@@ -50,6 +50,8 @@ extern char *__progname, *__progname_full;
 void __lock_slow(volatile int *, int) ATTR_LIBC_VISIBILITY;
 void __lock(volatile int *) ATTR_LIBC_VISIBILITY;
 void __unlock(volatile int *) ATTR_LIBC_VISIBILITY;
+void __unlock_requeue(volatile int *, volatile int *, int) ATTR_LIBC_VISIBILITY;
+void __unlock_requeue_slow(volatile int *, volatile int *, int) ATTR_LIBC_VISIBILITY;
 int __lockfile(FILE *) ATTR_LIBC_VISIBILITY;
 void __unlockfile(FILE *) ATTR_LIBC_VISIBILITY;
 #define LOCK(x) __lock(x)
diff --git a/src/thread/__lock.c b/src/thread/__lock.c
index a3d8d4d0..b0b0c67c 100644
--- a/src/thread/__lock.c
+++ b/src/thread/__lock.c
@@ -19,6 +19,23 @@
 
 weak_alias(__lock_fast, __lock);
 weak_alias(__unlock_fast, __unlock);
+weak_alias(__unlock_requeue_fast, __unlock_requeue);
+
+/* __unlock_requeue handles the case where we have a second futex
+ * queue to which the a waiting thread should be scheduled if shared
+ * is false. This is e.g the case when l is the futex of a condition
+ * variable and r is the one of a private mutex: if the mutex is
+ * private, it is more efficient to just insert the thread in the
+ * queue of that mutex, instead of waking him up and put it back to
+ * sleep immediately when trying to lock the mutex. */
+
+void __unlock_requeue_slow(volatile int *l, volatile int *r, int shared)
+{
+	if (shared) __wake(l, 1, 1);
+	else __syscall(SYS_futex, l, FUTEX_REQUEUE|FUTEX_PRIVATE, 0, 1, r) != -ENOSYS
+		|| __syscall(SYS_futex, l, FUTEX_REQUEUE, 0, 1, r);
+}
+
 
 void __lock_slow(volatile int *l, int current)
 {
-- 
2.15.1