thread-pool: avoid per-thread-pool EventNotifier

EventNotifier is implemented using an eventfd or pipe.  It therefore
consumes file descriptors, which can be limited by rlimits and should
therefore be used sparingly.

Switch from EventNotifier to QEMUBH in thread-pool.c.  Originally
EventNotifier was used because qemu_bh_schedule() was not thread-safe
yet.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit c2e50e3)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

thread-pool.c

@@ -21,7 +21,6 @@
 #include "block/coroutine.h"
 #include "trace.h"
 #include "block/block_int.h"
-#include "qemu/event_notifier.h"
 #include "block/thread-pool.h"
 #include "qemu/main-loop.h"
 
@@ -57,8 +56,8 @@ struct ThreadPoolElement {
 };
 
 struct ThreadPool {
-    EventNotifier notifier;
     AioContext *ctx;
+    QEMUBH *completion_bh;
     QemuMutex lock;
     QemuCond check_cancel;
     QemuCond worker_stopped;
@@ -119,7 +118,7 @@ static void *worker_thread(void *opaque)
             qemu_cond_broadcast(&pool->check_cancel);
         }
 
-        event_notifier_set(&pool->notifier);
+        qemu_bh_schedule(pool->completion_bh);
     }
 
     pool->cur_threads--;
@@ -168,12 +167,11 @@ static void spawn_thread(ThreadPool *pool)
     }
 }
 
-static void event_notifier_ready(EventNotifier *notifier)
+static void thread_pool_completion_bh(void *opaque)
 {
-    ThreadPool *pool = container_of(notifier, ThreadPool, notifier);
+    ThreadPool *pool = opaque;
     ThreadPoolElement *elem, *next;
 
-    event_notifier_test_and_clear(notifier);
 restart:
     QLIST_FOREACH_SAFE(elem, &pool->head, all, next) {
         if (elem->state != THREAD_CANCELED && elem->state != THREAD_DONE) {
@@ -215,7 +213,7 @@ static void thread_pool_cancel(BlockDriverAIOCB *acb)
         qemu_sem_timedwait(&pool->sem, 0) == 0) {
         QTAILQ_REMOVE(&pool->request_list, elem, reqs);
         elem->state = THREAD_CANCELED;
-        event_notifier_set(&pool->notifier);
+        qemu_bh_schedule(pool->completion_bh);
     } else {
         pool->pending_cancellations++;
         while (elem->state != THREAD_CANCELED && elem->state != THREAD_DONE) {
@@ -224,7 +222,7 @@ static void thread_pool_cancel(BlockDriverAIOCB *acb)
         pool->pending_cancellations--;
     }
     qemu_mutex_unlock(&pool->lock);
-    event_notifier_ready(&pool->notifier);
+    thread_pool_completion_bh(pool);
 }
 
 static const AIOCBInfo thread_pool_aiocb_info = {
@@ -293,8 +291,8 @@ static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
     }
 
     memset(pool, 0, sizeof(*pool));
-    event_notifier_init(&pool->notifier, false);
     pool->ctx = ctx;
+    pool->completion_bh = aio_bh_new(ctx, thread_pool_completion_bh, pool);
     qemu_mutex_init(&pool->lock);
     qemu_cond_init(&pool->check_cancel);
     qemu_cond_init(&pool->worker_stopped);
@@ -304,8 +302,6 @@ static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
 
     QLIST_INIT(&pool->head);
     QTAILQ_INIT(&pool->request_list);
-
-    aio_set_event_notifier(ctx, &pool->notifier, event_notifier_ready);
 }
 
 ThreadPool *thread_pool_new(AioContext *ctx)
@@ -339,11 +335,10 @@ void thread_pool_free(ThreadPool *pool)
 
     qemu_mutex_unlock(&pool->lock);
 
-    aio_set_event_notifier(pool->ctx, &pool->notifier, NULL);
+    qemu_bh_delete(pool->completion_bh);
     qemu_sem_destroy(&pool->sem);
     qemu_cond_destroy(&pool->check_cancel);
     qemu_cond_destroy(&pool->worker_stopped);
     qemu_mutex_destroy(&pool->lock);
-    event_notifier_cleanup(&pool->notifier);
     g_free(pool);
 }