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Date: Thu, 22 Jun 2017 23:17:29 +0200
From: Jens Gustedt <Jens.Gustedt@...ia.fr>
To: musl@...ts.openwall.com
Subject: [PATCH 7/8] implement __unlock_requeue
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 | 16 ++++++++++++++++
3 files changed, 26 insertions(+)
diff --git a/src/internal/__lock.h b/src/internal/__lock.h
index c1f07fc0..a96d159f 100644
--- a/src/internal/__lock.h
+++ b/src/internal/__lock.h
@@ -20,3 +20,11 @@ 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 e612c6f9..6516e2e1 100644
--- a/src/thread/__lock.c
+++ b/src/thread/__lock.c
@@ -4,6 +4,22 @@
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);
+}
+
void __lock_slow(volatile int *l, int current)
{
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