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[Qemu-devel] [PULL for-2.4 2/7] block: update bdrv_drain_all()/bdrv_drai
From: |
Stefan Hajnoczi |
Subject: |
[Qemu-devel] [PULL for-2.4 2/7] block: update bdrv_drain_all()/bdrv_drain() comments |
Date: |
Tue, 7 Jul 2015 13:08:19 +0100 |
The doc comments for bdrv_drain_all() and bdrv_drain() are outdated: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 <address@hidden>
Cc: Paolo Bonzini <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
Reviewed-by: Markus Armbruster <address@hidden>
Reviewed-by: Fam Zheng <address@hidden>
Message-id: address@hidden
---
block/io.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)
diff --git a/block/io.c b/block/io.c
index 305e0d9..d4bc83b 100644
--- 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;
--
2.4.3