From 7a63f3cdc44230109c91cdc0ee912c3cc7837141 Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Thu, 2 Jul 2015 17:24:41 +0100 Subject: [PATCH] block: update bdrv_drain_all()/bdrv_drain() comments The doc comments for bdrv_drain_all() and bdrv_drain() are outdated: * The bdrv_drain() comment is a poor man's bdrv_lock()/bdrv_unlock() which Fam Zheng is currently developing.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 Cc: Paolo Bonzini Signed-off-by: Stefan Hajnoczi Reviewed-by: Markus Armbruster Reviewed-by: Fam Zheng Message-id: 1435854281-6078-1-git-send-email-stefanha@redhat.com --- block/io.c | 20 ++++++++++---------- 1 file changed, 10 insertions(+), 10 deletions(-) diff --git a/block/io.c b/block/io.c index 305e0d952e..d4bc83b33b 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;