[src/trunk]: src/lib/librumpuser rumpuser_rwlock: replace internal pthread_sp...

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

details:   https://anonhg.NetBSD.org/src/rev/4b1c4f48fe98
branches:  trunk
changeset: 790191:4b1c4f48fe98
user:      rmind <rmind%NetBSD.org@localhost>
date:      Tue Sep 24 23:45:16 2013 +0000

description:
rumpuser_rwlock: replace internal pthread_spin_lock (for protecting reader
count) with atomic operations.  As pthread_spin_lock is not adaptive lock, it
can have hugely negative impact if contended here, especially with priority
inversions.  Now contended rwlock(9) no longer falls flat in RUMP kernels.

diffstat:

 lib/librumpuser/rumpuser_pth.c |  25 ++++++++++++++++++-------
 1 files changed, 18 insertions(+), 7 deletions(-)

diffs (89 lines):

diff -r 6b231a79e271 -r 4b1c4f48fe98 lib/librumpuser/rumpuser_pth.c
--- a/lib/librumpuser/rumpuser_pth.c    Tue Sep 24 22:52:14 2013 +0000
+++ b/lib/librumpuser/rumpuser_pth.c    Tue Sep 24 23:45:16 2013 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: rumpuser_pth.c,v 1.31 2013/09/23 10:35:20 pooka Exp $  */
+/*     $NetBSD: rumpuser_pth.c,v 1.32 2013/09/24 23:45:16 rmind Exp $  */
 
 /*
  * Copyright (c) 2007-2010 Antti Kantee.  All Rights Reserved.
@@ -28,10 +28,13 @@
 #include "rumpuser_port.h"
 
 #if !defined(lint)
-__RCSID("$NetBSD: rumpuser_pth.c,v 1.31 2013/09/23 10:35:20 pooka Exp $");
+__RCSID("$NetBSD: rumpuser_pth.c,v 1.32 2013/09/24 23:45:16 rmind Exp $");
 #endif /* !lint */
 
 #include <sys/queue.h>
+#if defined(__NetBSD__)
+#include <sys/atomic.h>
+#endif
 
 #include <assert.h>
 #include <errno.h>
@@ -242,7 +245,7 @@
 struct rumpuser_rw {
        pthread_rwlock_t pthrw;
        pthread_spinlock_t spin;
-       int readers;
+       unsigned int readers;
        struct lwp *writer;
        int downgrade; /* someone is downgrading (hopefully lock holder ;) */
 };
@@ -251,14 +254,15 @@
 rw_amwriter(struct rumpuser_rw *rw)
 {
 
-       return rw->writer == rumpuser_curlwp() && rw->readers == -1;
+       return rw->writer == rumpuser_curlwp() && rw->readers == (unsigned)-1;
 }
 
 static int
 rw_nreaders(struct rumpuser_rw *rw)
 {
+       unsigned nreaders = rw->readers;
 
-       return rw->readers > 0 ? rw->readers : 0;
+       return nreaders != (unsigned)-1 ? nreaders : 0;
 }
 
 static int
@@ -283,7 +287,7 @@
        }
        assert(rw->readers == 0);
        rw->writer = rumpuser_curlwp();
-       rw->readers = -1;
+       rw->readers = (unsigned)-1;
        return 0;
 }
 
@@ -300,20 +304,27 @@
 rw_readup(struct rumpuser_rw *rw)
 {
 
+#if defined(__NetBSD__)
+       atomic_inc_uint(&rw->readers);
+#else
        pthread_spin_lock(&rw->spin);
-       assert(rw->readers >= 0);
        ++rw->readers;
        pthread_spin_unlock(&rw->spin);
+#endif
 }
 
 static void
 rw_readdown(struct rumpuser_rw *rw)
 {
 
+#if defined(__NetBSD__)
+       atomic_dec_uint(&rw->readers);
+#else
        pthread_spin_lock(&rw->spin);
        assert(rw->readers > 0);
        --rw->readers;
        pthread_spin_unlock(&rw->spin);
+#endif
 }
 
 void