aio: document locking

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 20170112180800.21085-10-pbonzini@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 16 insertions, 16 deletions

diff --git a/include/block/aio.h b/include/block/aio.h
index be3adfec43..7df271d2b9 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -53,18 +53,12 @@ struct LinuxAioState;
 struct AioContext {
     GSource source;
 
-    /* Protects all fields from multi-threaded access */
+    /* Used by AioContext users to protect from multi-threaded access.  */
     QemuRecMutex lock;
 
-    /* The list of registered AIO handlers */
+    /* The list of registered AIO handlers.  Protected by ctx->list_lock. */
     QLIST_HEAD(, AioHandler) aio_handlers;
 
-    /* This is a simple lock used to protect the aio_handlers list.
-     * Specifically, it's used to ensure that no callbacks are removed while
-     * we're walking and dispatching callbacks.
-     */
-    int walking_handlers;
-
     /* Used to avoid unnecessary event_notifier_set calls in aio_notify;
      * accessed with atomic primitives.  If this field is 0, everything
      * (file descriptors, bottom halves, timers) will be re-evaluated
@@ -90,9 +84,9 @@ struct AioContext {
      */
     uint32_t notify_me;
 
-    /* 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.
+    /* A lock to protect between QEMUBH and AioHandler adders and deleter,
+     * and to ensure that no callbacks are removed while we're walking and
+     * dispatching them.
      */
     QemuLockCnt list_lock;
 
@@ -114,7 +108,9 @@ struct AioContext {
     bool notified;
     EventNotifier notifier;
 
-    /* Thread pool for performing work and receiving completion callbacks */
+    /* Thread pool for performing work and receiving completion callbacks.
+     * Has its own locking.
+     */
     struct ThreadPool *thread_pool;
 
 #ifdef CONFIG_LINUX_AIO
@@ -124,7 +120,9 @@ struct AioContext {
     struct LinuxAioState *linux_aio;
 #endif
 
-    /* TimerLists for calling timers - one per clock type */
+    /* TimerLists for calling timers - one per clock type.  Has its own
+     * locking.
+     */
     QEMUTimerListGroup tlg;
 
     int external_disable_cnt;
@@ -178,9 +176,11 @@ void aio_context_unref(AioContext *ctx);
  * automatically takes care of calling aio_context_acquire and
  * aio_context_release.
  *
- * Access to timers and BHs from a thread that has not acquired AioContext
- * is possible.  Access to callbacks for now must be done while the AioContext
- * is owned by the thread (FIXME).
+ * Note that this is separate from bdrv_drained_begin/bdrv_drained_end.  A
+ * thread still has to call those to avoid being interrupted by the guest.
+ *
+ * Bottom halves, timers and callbacks can be created or removed without
+ * acquiring the AioContext.
  */
 void aio_context_acquire(AioContext *ctx);