[src/trunk]: src Retry enabling spin mutexes. We should be able to avoid pok...

[pooka <pooka%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/781c1b074a54
branches:  trunk
changeset: 786585:781c1b074a54
user:      pooka <pooka%NetBSD.org@localhost>
date:      Thu May 02 20:33:54 2013 +0000

description:
Retry enabling spin mutexes.  We should be able to avoid poking the
scheduler by just making wakeup from cv_wait() honor the same locking
order as when a spin mutex is acquired though mutex_enter().
*fingers crossed*

diffstat:

 lib/librumpuser/rumpuser_pth.c    |  55 +++++++++++++++++++++++++++++++-------
 sys/rump/librump/rumpkern/locks.c |   8 ++---
 2 files changed, 48 insertions(+), 15 deletions(-)

diffs (136 lines):

diff -r 807643a2799d -r 781c1b074a54 lib/librumpuser/rumpuser_pth.c
--- a/lib/librumpuser/rumpuser_pth.c    Thu May 02 19:14:59 2013 +0000
+++ b/lib/librumpuser/rumpuser_pth.c    Thu May 02 20:33:54 2013 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: rumpuser_pth.c,v 1.23 2013/05/02 19:14:59 pooka Exp $  */
+/*     $NetBSD: rumpuser_pth.c,v 1.24 2013/05/02 20:33:54 pooka Exp $  */
 
 /*
  * Copyright (c) 2007-2010 Antti Kantee.  All Rights Reserved.
@@ -28,7 +28,7 @@
 #include "rumpuser_port.h"
 
 #if !defined(lint)
-__RCSID("$NetBSD: rumpuser_pth.c,v 1.23 2013/05/02 19:14:59 pooka Exp $");
+__RCSID("$NetBSD: rumpuser_pth.c,v 1.24 2013/05/02 20:33:54 pooka Exp $");
 #endif /* !lint */
 
 #include <sys/queue.h>
@@ -378,17 +378,53 @@
        free(cv);
 }
 
+static void
+cv_unschedule(struct rumpuser_mtx *mtx, int *nlocks)
+{
+
+       rumpkern_unsched(nlocks, mtx);
+       mtxexit(mtx);
+}
+
+static void
+cv_reschedule(struct rumpuser_mtx *mtx, int nlocks)
+{
+
+       /*
+        * If the cv interlock is a spin mutex, we must first release
+        * the mutex that was reacquired by pthread_cond_wait(),
+        * acquire the CPU context and only then relock the mutex.
+        * This is to preserve resource allocation order so that
+        * we don't deadlock.  Non-spinning mutexes don't have this
+        * problem since they don't use a hold-and-wait approach
+        * to acquiring the mutex wrt the rump kernel CPU context.
+        *
+        * The more optimal solution would be to rework rumpkern_sched()
+        * so that it's possible to tell the scheduler
+        * "if you need to block, drop this lock first", but I'm not
+        * going poking there without some numbers on how often this
+        * path is taken for spin mutexes.
+        */
+       if ((mtx->flags & (RUMPUSER_MTX_SPIN | RUMPUSER_MTX_KMUTEX)) ==
+           (RUMPUSER_MTX_SPIN | RUMPUSER_MTX_KMUTEX)) {
+               NOFAIL_ERRNO(pthread_mutex_unlock(&mtx->pthmtx));
+               rumpkern_sched(nlocks, mtx);
+               rumpuser_mutex_enter_nowrap(mtx);
+       } else {
+               mtxenter(mtx);
+               rumpkern_sched(nlocks, mtx);
+       }
+}
+
 void
 rumpuser_cv_wait(struct rumpuser_cv *cv, struct rumpuser_mtx *mtx)
 {
        int nlocks;
 
        cv->nwaiters++;
-       rumpkern_unsched(&nlocks, mtx);
-       mtxexit(mtx);
+       cv_unschedule(mtx, &nlocks);
        NOFAIL_ERRNO(pthread_cond_wait(&cv->pthcv, &mtx->pthmtx));
-       mtxenter(mtx);
-       rumpkern_sched(nlocks, mtx);
+       cv_reschedule(mtx, nlocks);
        cv->nwaiters--;
 }
 
@@ -420,8 +456,7 @@
        clock_gettime(CLOCK_REALTIME, &ts);
 
        cv->nwaiters++;
-       rumpkern_unsched(&nlocks, mtx);
-       mtxexit(mtx);
+       cv_unschedule(mtx, &nlocks);
 
        ts.tv_sec += sec;
        ts.tv_nsec += nsec;
@@ -430,8 +465,8 @@
                ts.tv_nsec -= 1000*1000*1000;
        }
        rv = pthread_cond_timedwait(&cv->pthcv, &mtx->pthmtx, &ts);
-       mtxenter(mtx);
-       rumpkern_sched(nlocks, mtx);
+
+       cv_reschedule(mtx, nlocks);
        cv->nwaiters--;
 
        ET(rv);
diff -r 807643a2799d -r 781c1b074a54 sys/rump/librump/rumpkern/locks.c
--- a/sys/rump/librump/rumpkern/locks.c Thu May 02 19:14:59 2013 +0000
+++ b/sys/rump/librump/rumpkern/locks.c Thu May 02 20:33:54 2013 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: locks.c,v 1.60 2013/04/30 00:03:53 pooka Exp $ */
+/*     $NetBSD: locks.c,v 1.61 2013/05/02 20:33:54 pooka Exp $ */
 
 /*
  * Copyright (c) 2007-2011 Antti Kantee.  All Rights Reserved.
@@ -26,7 +26,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: locks.c,v 1.60 2013/04/30 00:03:53 pooka Exp $");
+__KERNEL_RCSID(0, "$NetBSD: locks.c,v 1.61 2013/05/02 20:33:54 pooka Exp $");
 
 #include <sys/param.h>
 #include <sys/kmem.h>
@@ -113,11 +113,9 @@
                isspin = 1;
        }
 
-#if 0
        /* spin mutex support needs some cpu scheduler rework  */
        if (isspin)
                ruflags |= RUMPUSER_MTX_SPIN;
-#endif
        rumpuser_mutex_init((struct rumpuser_mtx **)mtx, ruflags);
        ALLOCK(mtx, &mutex_lockops);
 }
@@ -144,7 +142,7 @@
 {
 
        WANTLOCK(mtx, false, false);
-       rumpuser_mutex_enter(RUMPMTX(mtx));
+       rumpuser_mutex_enter_nowrap(RUMPMTX(mtx));
        LOCKED(mtx, false);
 }