[src/trunk]: src/sys/kern Simplify thread reference counting of the thread po...

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

details:   https://anonhg.NetBSD.org/src/rev/bc8a63197f6c
branches:  trunk
changeset: 446959:bc8a63197f6c
user:      thorpej <thorpej%NetBSD.org@localhost>
date:      Wed Dec 26 21:15:50 2018 +0000

description:
Simplify thread reference counting of the thread pool object.

diffstat:

 sys/kern/kern_threadpool.c |  61 +++++++++++++++------------------------------
 1 files changed, 21 insertions(+), 40 deletions(-)

diffs (153 lines):

diff -r da4bb72d8747 -r bc8a63197f6c sys/kern/kern_threadpool.c
--- a/sys/kern/kern_threadpool.c        Wed Dec 26 20:30:36 2018 +0000
+++ b/sys/kern/kern_threadpool.c        Wed Dec 26 21:15:50 2018 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: kern_threadpool.c,v 1.6 2018/12/26 20:30:36 thorpej Exp $      */
+/*     $NetBSD: kern_threadpool.c,v 1.7 2018/12/26 21:15:50 thorpej Exp $      */
 
 /*-
  * Copyright (c) 2014, 2018 The NetBSD Foundation, Inc.
@@ -81,7 +81,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: kern_threadpool.c,v 1.6 2018/12/26 20:30:36 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: kern_threadpool.c,v 1.7 2018/12/26 21:15:50 thorpej Exp $");
 
 #include <sys/types.h>
 #include <sys/param.h>
@@ -127,14 +127,14 @@
        struct threadpool_thread        tp_overseer;
        struct job_head                 tp_jobs;
        struct thread_head              tp_idle_threads;
-       unsigned int                    tp_refcnt;
+       uint64_t                        tp_refcnt;
        int                             tp_flags;
 #define        THREADPOOL_DYING        0x01
        struct cpu_info                 *tp_cpu;
        pri_t                           tp_pri;
 };
 
-static int     threadpool_hold(struct threadpool *);
+static void    threadpool_hold(struct threadpool *);
 static void    threadpool_rele(struct threadpool *);
 
 static int     threadpool_percpu_create(struct threadpool_percpu **, pri_t);
@@ -274,13 +274,11 @@
        /* XXX overseer */
        TAILQ_INIT(&pool->tp_jobs);
        TAILQ_INIT(&pool->tp_idle_threads);
-       pool->tp_refcnt = 0;
+       pool->tp_refcnt = 1;            /* overseer's reference */
        pool->tp_flags = 0;
        pool->tp_cpu = ci;
        pool->tp_pri = pri;
 
-       error = threadpool_hold(pool);
-       KASSERT(error == 0);
        pool->tp_overseer.tpt_lwp = NULL;
        pool->tp_overseer.tpt_pool = pool;
        pool->tp_overseer.tpt_job = NULL;
@@ -348,40 +346,24 @@
        mutex_destroy(&pool->tp_lock);
 }
 
-static int
+static void
 threadpool_hold(struct threadpool *pool)
 {
-       unsigned int refcnt;
 
-       do {
-               refcnt = pool->tp_refcnt;
-               if (refcnt == UINT_MAX)
-                       return EBUSY;
-       } while (atomic_cas_uint(&pool->tp_refcnt, refcnt, (refcnt + 1))
-           != refcnt);
-
-       return 0;
+       KASSERT(mutex_owned(&pool->tp_lock));
+       pool->tp_refcnt++;
+       KASSERT(pool->tp_refcnt != 0);
 }
 
 static void
 threadpool_rele(struct threadpool *pool)
 {
-       unsigned int refcnt;
 
-       do {
-               refcnt = pool->tp_refcnt;
-               KASSERT(0 < refcnt);
-               if (refcnt == 1) {
-                       mutex_spin_enter(&pool->tp_lock);
-                       refcnt = atomic_dec_uint_nv(&pool->tp_refcnt);
-                       KASSERT(refcnt != UINT_MAX);
-                       if (refcnt == 0)
-                               cv_broadcast(&pool->tp_overseer.tpt_cv);
-                       mutex_spin_exit(&pool->tp_lock);
-                       return;
-               }
-       } while (atomic_cas_uint(&pool->tp_refcnt, refcnt, (refcnt - 1))
-           != refcnt);
+       KASSERT(mutex_owned(&pool->tp_lock));
+       KASSERT(0 < pool->tp_refcnt);
+       pool->tp_refcnt--;
+       if (pool->tp_refcnt == 0)
+               cv_broadcast(&pool->tp_overseer.tpt_cv);
 }
 
 /* Unbound thread pools */
@@ -876,12 +858,7 @@
                if (TAILQ_EMPTY(&pool->tp_idle_threads)) {
                        TP_LOG(("%s: Got a job, need to create a thread.\n",
                                __func__));
-                       error = threadpool_hold(pool);
-                       if (error) {
-                               (void)kpause("thrdplrf", false, hz,
-                                   &pool->tp_lock);
-                               continue;
-                       }
+                       threadpool_hold(pool);
                        mutex_spin_exit(&pool->tp_lock);
 
                        struct threadpool_thread *const thread =
@@ -910,6 +887,10 @@
                                    &pool->tp_lock);
                                continue;
                        }
+                       /*
+                        * New kthread now owns the reference to the pool
+                        * taken above.
+                        */
                        KASSERT(lwp != NULL);
                        TAILQ_INSERT_TAIL(&pool->tp_idle_threads, thread,
                            tpt_entry);
@@ -972,11 +953,11 @@
 
                mutex_spin_enter(&pool->tp_lock);
        }
+       threadpool_rele(pool);
        mutex_spin_exit(&pool->tp_lock);
 
        TP_LOG(("%s: exiting.\n", __func__));
 
-       threadpool_rele(pool);
        kthread_exit(0);
 }
 
@@ -1045,6 +1026,7 @@
                thread->tpt_job = NULL;
                TAILQ_INSERT_TAIL(&pool->tp_idle_threads, thread, tpt_entry);
        }
+       threadpool_rele(pool);
        mutex_spin_exit(&pool->tp_lock);
 
        TP_LOG(("%s: thread %p exiting.\n", __func__, thread));
@@ -1052,6 +1034,5 @@
        KASSERT(!cv_has_waiters(&thread->tpt_cv));
        cv_destroy(&thread->tpt_cv);
        pool_cache_put(threadpool_thread_pc, thread);
-       threadpool_rele(pool);
        kthread_exit(0);
 }