[src/trunk]: src/lib/librumpuser Fix a race condition in the worker thread ca...

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

details:   https://anonhg.NetBSD.org/src/rev/de5db814ae7b
branches:  trunk
changeset: 761735:de5db814ae7b
user:      pooka <pooka%NetBSD.org@localhost>
date:      Sun Feb 06 18:25:48 2011 +0000

description:
Fix a race condition in the worker thread caching logic: if we got
two or more syscall requests before any worker thread ran, we might
not have enough threads to handle the requests.  In some scenarios
this could lead to a deadlock.

diffstat:

 lib/librumpuser/rumpuser_sp.c |  17 +++++++++--------
 1 files changed, 9 insertions(+), 8 deletions(-)

diffs (61 lines):

diff -r d20933f240e8 -r de5db814ae7b lib/librumpuser/rumpuser_sp.c
--- a/lib/librumpuser/rumpuser_sp.c     Sun Feb 06 16:23:00 2011 +0000
+++ b/lib/librumpuser/rumpuser_sp.c     Sun Feb 06 18:25:48 2011 +0000
@@ -1,4 +1,4 @@
-/*      $NetBSD: rumpuser_sp.c,v 1.38 2011/01/28 19:21:28 pooka Exp $  */
+/*      $NetBSD: rumpuser_sp.c,v 1.39 2011/02/06 18:25:48 pooka Exp $  */
 
 /*
  * Copyright (c) 2010, 2011 Antti Kantee.  All Rights Reserved.
@@ -35,7 +35,7 @@
  */
 
 #include <sys/cdefs.h>
-__RCSID("$NetBSD: rumpuser_sp.c,v 1.38 2011/01/28 19:21:28 pooka Exp $");
+__RCSID("$NetBSD: rumpuser_sp.c,v 1.39 2011/02/06 18:25:48 pooka Exp $");
 
 #include <sys/types.h>
 #include <sys/atomic.h>
@@ -615,19 +615,18 @@
 
        for (;;) {
                pthread_mutex_lock(&sbamtx);
-               if (idleworker >= rumpsp_idleworker) {
+               if (__predict_false(idleworker >= rumpsp_idleworker)) {
                        nworker--;
                        pthread_mutex_unlock(&sbamtx);
                        break;
                }
-               idleworker++;
-               while (TAILQ_EMPTY(&syslist)) {
+               if (TAILQ_EMPTY(&syslist))
+                       idleworker++;
+               while (TAILQ_EMPTY(&syslist))
                        pthread_cond_wait(&sbacv, &sbamtx);
-               }
 
                sba = TAILQ_FIRST(&syslist);
                TAILQ_REMOVE(&syslist, sba, sba_entries);
-               idleworker--;
                pthread_mutex_unlock(&sbamtx);
 
                serv_handlesyscall(sba->sba_spc,
@@ -952,6 +951,7 @@
        if (idleworker > 0) {
                /* do we have a daemon's tool (i.e. idle threads)? */
                pthread_cond_signal(&sbacv);
+               idleworker--;
        } else if (nworker < rumpsp_maxworker) {
                /*
                 * Else, need to create one
@@ -959,8 +959,9 @@
                 * worker to pick up the syscall)
                 */
                if (pthread_create(&pt, &pattr_detached,
-                   serv_syscallbouncer, NULL) == 0)
+                   serv_syscallbouncer, NULL) == 0) {
                        nworker++;
+               }
        }
        pthread_mutex_unlock(&sbamtx);
 }