diff options
Diffstat (limited to 'include/block/aio.h')
| -rw-r--r-- | include/block/aio.h | 32 |
1 files changed, 31 insertions, 1 deletions
diff --git a/include/block/aio.h b/include/block/aio.h index be91e3f701..9dd32e0f13 100644 --- a/include/block/aio.h +++ b/include/block/aio.h @@ -99,7 +99,19 @@ struct AioContext { */ int walking_bh; - /* Used for aio_notify. */ + /* Used by aio_notify. + * + * "notified" is used to avoid expensive event_notifier_test_and_clear + * calls. When it is clear, the EventNotifier is clear, or one thread + * is going to clear "notified" before processing more events. False + * positives are possible, i.e. "notified" could be set even though the + * EventNotifier is clear. + * + * Note that event_notifier_set *cannot* be optimized the same way. For + * more information on the problem that would result, see "#ifdef BUG2" + * in the docs/aio_notify_accept.promela formal model. + */ + bool notified; EventNotifier notifier; /* Thread pool for performing work and receiving completion callbacks */ @@ -174,6 +186,24 @@ QEMUBH *aio_bh_new(AioContext *ctx, QEMUBHFunc *cb, void *opaque); void aio_notify(AioContext *ctx); /** + * aio_notify_accept: Acknowledge receiving an aio_notify. + * + * aio_notify() uses an EventNotifier in order to wake up a sleeping + * aio_poll() or g_main_context_iteration(). Calls to aio_notify() are + * usually rare, but the AioContext has to clear the EventNotifier on + * every aio_poll() or g_main_context_iteration() in order to avoid + * busy waiting. This event_notifier_test_and_clear() cannot be done + * using the usual aio_context_set_event_notifier(), because it must + * be done before processing all events (file descriptors, bottom halves, + * timers). + * + * aio_notify_accept() is an optimized event_notifier_test_and_clear() + * that is specific to an AioContext's notifier; it is used internally + * to clear the EventNotifier only if aio_notify() had been called. + */ +void aio_notify_accept(AioContext *ctx); + +/** * aio_bh_poll: Poll bottom halves for an AioContext. * * These are internal functions used by the QEMU main loop. |