[Qemu-devel] [PATCH 10/16] aio: make ctx->list_lock a QemuLockCnt, subsuming ctx->walking_bh

[Paolo Bonzini <pbonzini@redhat.com>]

This will make it possible to walk the list of bottom halves without
holding the AioContext lock---and in turn to call bottom half
handlers without holding the lock.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 async.c             | 31 ++++++++++++++-----------------
 include/block/aio.h | 12 +++++-------
 2 files changed, 19 insertions(+), 24 deletions(-)

diff --git a/async.c b/async.c
index fc4c173..bc7e142 100644
--- a/async.c
+++ b/async.c
@@ -51,12 +51,12 @@ QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
         .cb = cb,
         .opaque = opaque,
     };
-    qemu_mutex_lock(&ctx->list_lock);
+    qemu_lockcnt_lock(&ctx->list_lock);
     bh->next = ctx->first_bh;
     /* Make sure that the members are ready before putting bh into list */
     smp_wmb();
     ctx->first_bh = bh;
-    qemu_mutex_unlock(&ctx->list_lock);
+    qemu_lockcnt_unlock(&ctx->list_lock);
     return bh;
 }
 
@@ -71,13 +71,11 @@ int aio_bh_poll(AioContext *ctx)
     QEMUBH *bh, **bhp, *next;
     int ret;
 
-    ctx->walking_bh++;
+    qemu_lockcnt_inc(&ctx->list_lock);
 
     ret = 0;
-    for (bh = ctx->first_bh; bh; bh = next) {
-        /* Make sure that fetching bh happens before accessing its members */
-        smp_read_barrier_depends();
-        next = bh->next;
+    for (bh = atomic_rcu_read(&ctx->first_bh); bh; bh = next) {
+        next = atomic_rcu_read(&bh->next);
         /* The atomic_xchg is paired with the one in qemu_bh_schedule.  The
          * implicit memory barrier ensures that the callback sees all writes
          * done by the scheduling thread.  It also ensures that the scheduling
@@ -94,11 +92,8 @@ int aio_bh_poll(AioContext *ctx)
         }
     }
 
-    ctx->walking_bh--;
-
     /* remove deleted bhs */
-    if (!ctx->walking_bh) {
-        qemu_mutex_lock(&ctx->list_lock);
+    if (qemu_lockcnt_dec_and_lock(&ctx->list_lock)) {
         bhp = &ctx->first_bh;
         while (*bhp) {
             bh = *bhp;
@@ -109,7 +104,7 @@ int aio_bh_poll(AioContext *ctx)
                 bhp = &bh->next;
             }
         }
-        qemu_mutex_unlock(&ctx->list_lock);
+        qemu_lockcnt_unlock(&ctx->list_lock);
     }
 
     return ret;
@@ -165,7 +160,8 @@ aio_compute_timeout(AioContext *ctx)
     int timeout = -1;
     QEMUBH *bh;
 
-    for (bh = ctx->first_bh; bh; bh = bh->next) {
+    for (bh = atomic_rcu_read(&ctx->first_bh); bh;
+         bh = atomic_rcu_read(&bh->next)) {
         if (!bh->deleted && bh->scheduled) {
             if (bh->idle) {
                 /* idle bottom halves will be polled at least
@@ -240,7 +236,8 @@ aio_ctx_finalize(GSource     *source)
 
     thread_pool_free(ctx->thread_pool);
 
-    qemu_mutex_lock(&ctx->list_lock);
+    qemu_lockcnt_lock(&ctx->list_lock);
+    assert(!qemu_lockcnt_count(&ctx->list_lock));
     while (ctx->first_bh) {
         QEMUBH *next = ctx->first_bh->next;
 
@@ -250,12 +247,12 @@ aio_ctx_finalize(GSource     *source)
         g_free(ctx->first_bh);
         ctx->first_bh = next;
     }
-    qemu_mutex_unlock(&ctx->list_lock);
+    qemu_lockcnt_unlock(&ctx->list_lock);
 
     aio_set_event_notifier(ctx, &ctx->notifier, false, NULL);
     event_notifier_cleanup(&ctx->notifier);
     qemu_rec_mutex_destroy(&ctx->lock);
-    qemu_mutex_destroy(&ctx->list_lock);
+    qemu_lockcnt_destroy(&ctx->list_lock);
     timerlistgroup_deinit(&ctx->tlg);
 }
 
@@ -551,7 +548,7 @@ AioContext *aio_context_new(Error **errp)
                            (EventNotifierHandler *)
                            event_notifier_dummy_cb);
     ctx->thread_pool = NULL;
-    qemu_mutex_init(&ctx->list_lock);
+    qemu_lockcnt_init(&ctx->list_lock);
     qemu_rec_mutex_init(&ctx->lock);
     timerlistgroup_init(&ctx->tlg, aio_timerlist_notify, ctx);
 
diff --git a/include/block/aio.h b/include/block/aio.h
index 322a10e..fb0ff21 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -87,17 +87,15 @@ struct AioContext {
      */
     uint32_t notify_me;
 
-    /* lock to protect between bh's adders and deleter */
-    QemuMutex list_lock;
+    /* A lock to protect between bh's adders and deleter, and to ensure
+     * that no callbacks are removed while we're walking and dispatching
+     * them.
+     */
+    QemuLockCnt list_lock;
 
     /* Anchor of the list of Bottom Halves belonging to the context */
     struct QEMUBH *first_bh;
 
-    /* A simple lock used to protect the first_bh list, and ensure that
-     * no callbacks are removed while we're walking and dispatching callbacks.
-     */
-    int walking_bh;
-
     /* Used by aio_notify.
      *
      * "notified" is used to avoid expensive event_notifier_test_and_clear
-- 
2.5.0