Re: [PATCH v5 21/45] block: add bdrv_try_set_aio_context_tran transaction action

[Hanna Reitz]

On 20.06.22 22:57, Vladimir Sementsov-Ogievskiy wrote:
> On 6/13/22 10:46, Hanna Reitz wrote:
> > On 30.03.22 23:28, Vladimir Sementsov-Ogievskiy wrote:
> > > To be used in further commit.
> > >
> > > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@openvz.org>
> > > ---
> > >   block.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
> > >   1 file changed, 48 insertions(+)
> > >
> > > diff --git a/block.c b/block.c
> > > index be19964f89..1900cdf277 100644
> > > --- a/block.c
> > > +++ b/block.c
> > > @@ -2907,6 +2907,54 @@ static void bdrv_child_free(BdrvChild *child)
> > >       g_free(child);
> > >   }
> > > +typedef struct BdrvTrySetAioContextState {
> > > +    BlockDriverState *bs;
> > > +    AioContext *old_ctx;
> > > +} BdrvTrySetAioContextState;
> > > +
> > > +static void bdrv_try_set_aio_context_abort(void *opaque)
> > > +{
> > > +    BdrvTrySetAioContextState *s = opaque;
> > > +
> > > +    if (bdrv_get_aio_context(s->bs) != s->old_ctx) {
> > > +        bdrv_try_set_aio_context(s->bs, s->old_ctx, &error_abort);
> >
> > As far as I understand, users of this transaction will need to do a 
> > bit of AioContext lock shuffling: To set the context, they need to 
> > hold old_ctx, but not new_ctx; but in case of abort, they need to 
> > release old_ctx and acquire new_ctx before the abort handlers are 
> > called.  (Due to the constraints on bdrv_set_aio_context_ignore().)
> >
> > If that’s true, I think that should be documented somewhere.
>
> Hmm.. Actually, I think that bdrv_try_set_aio_context_abort() should 
> do this shuffling by it self. The only hope to correctly rollback a 
> transaction, is operation in assumption that on .abort() we are 
> exactly on the same state as on .prepare(), regardless of other 
> actions. And this means that old_ctx is acquired and new_ctx is not.

But if old_ctx is acquired and new_ctx is not, you cannot invoke 
bdrv_try_set_aio_context(bs, old_ctx), because that requires the current 
context (bdrv_get_aio_context(bs)) to be held, but not old_ctx (the 
“new” context for this call).