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block: update bdrv_drain_all()/bdrv_drain() comments

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The doc comments for bdrv_drain_all() and bdrv_drain() are outdated: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
This commit is contained in:
Stefan Hajnoczi 2015-07-02 17:24:41 +01:00
parent 1bd84ee717
commit 7a63f3cdc4
1 changed files with 10 additions and 10 deletions
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20
block/io.c
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@ -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;