Re: [Qemu-devel] [Qemu-block] [PATCH] blk: postpone request execution on a context protected with "drained section"

[Kevin Wolf]

Am 26.06.2019 um 10:46 hat Denis Plotnikov geschrieben:
> On 24.06.2019 12:46, Denis Plotnikov wrote:
> > On 21.06.2019 12:59, Vladimir Sementsov-Ogievskiy wrote:
> >> 21.06.2019 12:16, Kevin Wolf wrote:
> >>> Am 09.04.2019 um 12:01 hat Kevin Wolf geschrieben:
> >>>> Am 02.04.2019 um 10:35 hat Denis Plotnikov geschrieben:
> >>>>> On 13.03.2019 19:04, Kevin Wolf wrote:
> >>>>>> Am 14.12.2018 um 12:54 hat Denis Plotnikov geschrieben:
> >>>>>>> On 13.12.2018 15:20, Kevin Wolf wrote:
> >>>>>>>> Am 13.12.2018 um 12:07 hat Denis Plotnikov geschrieben:
> >>>>>>>>> Sounds it should be so, but it doesn't work that way and that's why:
> >>>>>>>>> when doing mirror we may resume postponed coroutines too early when 
> >>>>>>>>> the
> >>>>>>>>> underlying bs is protected from writing at and thus we encounter the
> >>>>>>>>> assert on a write request execution at bdrv_co_write_req_prepare 
> >>>>>>>>> when
> >>>>>>>>> resuming the postponed coroutines.
> >>>>>>>>>
> >>>>>>>>> The thing is that the bs is protected for writing before execution 
> >>>>>>>>> of
> >>>>>>>>> bdrv_replace_node at mirror_exit_common and bdrv_replace_node calls
> >>>>>>>>> bdrv_replace_child_noperm which, in turn, calls 
> >>>>>>>>> child->role->drained_end
> >>>>>>>>> where one of the callbacks is blk_root_drained_end which check
> >>>>>>>>> if(--blk->quiesce_counter == 0) and runs the postponed requests
> >>>>>>>>> (coroutines) if the coundition is true.
> >>>>>>>>
> >>>>>>>> Hm, so something is messed up with the drain sections in the mirror
> >>>>>>>> driver. We have:
> >>>>>>>>
> >>>>>>>>          bdrv_drained_begin(target_bs);
> >>>>>>>>          bdrv_replace_node(to_replace, target_bs, &local_err);
> >>>>>>>>          bdrv_drained_end(target_bs);
> >>>>>>>>
> >>>>>>>> Obviously, the intention was to keep the BlockBackend drained during
> >>>>>>>> bdrv_replace_node(). So how could blk->quiesce_counter ever get to 0
> >>>>>>>> inside bdrv_replace_node() when target_bs is drained?
> >>>>>>>>
> >>>>>>>> Looking at bdrv_replace_child_noperm(), it seems that the function 
> >>>>>>>> has
> >>>>>>>> a bug: Even if old_bs and new_bs are both drained, the 
> >>>>>>>> quiesce_counter
> >>>>>>>> for the parent reaches 0 for a moment because we call .drained_end 
> >>>>>>>> for
> >>>>>>>> the old child first and .drained_begin for the new one later.
> >>>>>>>>
> >>>>>>>> So it seems the fix would be to reverse the order and first call
> >>>>>>>> .drained_begin for the new child and then .drained_end for the old
> >>>>>>>> child. Sounds like a good new testcase for tests/test-bdrv-drain.c, 
> >>>>>>>> too.
> >>>>>>> Yes, it's true, but it's not enough...
> >>>>>>
> >>>>>> Did you ever implement the changes suggested so far, so that we could
> >>>>>> continue from there? Or should I try and come up with something myself?
> >>>>>
> >>>>> Sorry for the late reply...
> >>>>> Yes, I did ...
> >>>>
> >>>> If there are more question or problems, can you post the patches in
> >>>> their current shape (as an RFC) or a git URL so I can play with it a
> >>>> bit? If you could include a failing test case, too, that would be ideal.
> >>>
> >>> Denis? Please?
> >>>
> >>> We really should get this fixed and I would be willing to lend a hand,
> >>> but if you keep your patches secret, I can't really do so and would have
> >>> to duplicate your work.
> >>>
> >>> Also, please see my old answer from April below for the last problem you
> >>> had with implementing the correct approach.
> >>>
> >>> Kevin
> > 
> > Hi Kevin,
> > I'm sorry for not replying for so long. Please, give me some time (a day
> > or two) so I could refresh everything and send the current state of the
> > patches as well as the test case checking the issue
> 
> Hi Kevin,
> The current state of the patches is available at 
> https://github.com/denis-plotnikov/qemu/tree/postponed-request

Are you sure you pushed the correct version?

I don't see any of the things we discussed above in this branch, i.e.
using blk_root_drained_begin/end, fixing bdrv_replace_child_noperm(),
fixing the drain calls in mirror etc.

> I didn't manage to create an automatic reproducer but one of the patches 
> contains a step-by-step description of how to reproduce the problem.

Ok, I'll try whether I can reproduce this.

Kevin