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>
diff --git a/include/block/aio.h b/include/block/aio.h
index be3adfe..7df271d 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -53,18 +53,12 @@
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 @@
*/
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 @@
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 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 @@
* 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);
