block: update bdrv_drain_all()/bdrv_drain() comments

The doc comments for bdrv_drain_all() and bdrv_drain() are outdated:

 * The bdrv_drain() comment is a poor man's bdrv_lock()/bdrv_unlock()
   which Fam Zheng is currently developing.  Unfortunately this warning
   was never really enough because devices keep submitting I/O and op
   blockers don't prevent that.

 * The bdrv_drain_all() comment is still partially correct but reflects
   the nature of the implementation rather than API documentation.

Do make it clear that bdrv_drain() is only appropriate within an
AioContext.  For anything spanning AioContexts you need
bdrv_drain_all().

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Fam Zheng <famz@redhat.com>
Message-id: 1435854281-6078-1-git-send-email-stefanha@redhat.com

diff --git a/block/io.c b/block/io.c
index 305e0d9..d4bc83b 100644 (file)
--- a/block/io.c
+++ b/block/io.c
@@ -236,12 +236,12 @@ static bool bdrv_requests_pending(BlockDriverState *bs)
 /*
  * Wait for pending requests to complete on a single BlockDriverState subtree
  *
- * See the warning in bdrv_drain_all().  This function can only be called if
- * you are sure nothing can generate I/O because you have op blockers
- * installed.
- *
  * Note that unlike bdrv_drain_all(), the caller must hold the BlockDriverState
  * AioContext.
+ *
+ * Only this BlockDriverState's AioContext is run, so in-flight requests must
+ * not depend on events in other AioContexts.  In that case, use
+ * bdrv_drain_all() instead.
  */
 void bdrv_drain(BlockDriverState *bs)
 {
@@ -260,12 +260,6 @@ void bdrv_drain(BlockDriverState *bs)
  *
  * This function does not flush data to disk, use bdrv_flush_all() for that
  * after calling this function.
- *
- * Note that completion of an asynchronous I/O operation can trigger any
- * number of other I/O operations on other devices---for example a coroutine
- * can be arbitrarily complex and a constant flow of I/O can come until the
- * coroutine is complete.  Because of this, it is not possible to have a
- * function to drain a single device's I/O queue.
  */
 void bdrv_drain_all(void)
 {
@@ -288,6 +282,12 @@ void bdrv_drain_all(void)
         }
     }
 
+    /* Note that completion of an asynchronous I/O operation can trigger any
+     * number of other I/O operations on other devices---for example a
+     * coroutine can submit an I/O request to another device in response to
+     * request completion.  Therefore we must keep looping until there was no
+     * more activity rather than simply draining each device independently.
+     */
     while (busy) {
         busy = false;