| Message ID | 1435854281-6078-1-git-send-email-stefanha@redhat.com |
|---|---|
| State | New |
| Headers | show |
Stefan Hajnoczi <stefanha@redhat.com> writes: > The doc comments for bdrv_drain_all() and bdrv_drain() are outdated: 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> For what it's worth (I'm not deep into our AIO): Reviewed-by: Markus Armbruster <armbru@redhat.com>
On Thu, 07/02 17:24, Stefan Hajnoczi wrote: > The doc comments for bdrv_drain_all() and bdrv_drain() are outdated: 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: Fam Zheng <famz@redhat.com>
On Thu, Jul 02, 2015 at 05:24:41PM +0100, Stefan Hajnoczi wrote: > The doc comments for bdrv_drain_all() and bdrv_drain() are outdated: 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> > --- > block/io.c | 20 ++++++++++---------- > 1 file changed, 10 insertions(+), 10 deletions(-) Thanks, applied to my block tree: https://github.com/stefanha/qemu/commits/block Stefan
diff --git a/block/io.c b/block/io.c index e295992..6f5704f 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;
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> --- block/io.c | 20 ++++++++++---------- 1 file changed, 10 insertions(+), 10 deletions(-)