Re: [Qemu-devel] [Qemu-block] [PATCH v0 0/2] Postponed actions

[Kevin Wolf <kwolf@redhat.com>]

Am 13.08.2018 um 10:32 hat Denis Plotnikov geschrieben:
> Ping ping!
> 
> On 16.07.2018 21:59, John Snow wrote:
> > 
> > 
> > On 07/16/2018 11:01 AM, Denis Plotnikov wrote:
> > > Ping!
> > > 
> > 
> > I never saw a reply to Stefan's question on July 2nd, did you reply
> > off-list?
> > 
> > --js
> Yes, I did. I talked to Stefan why the patch set appeared.

The rest of us still don't know the answer. I had the same question.

Kevin

> > > On 29.06.2018 15:40, Denis Plotnikov wrote:
> > > > There are cases when a request to a block driver state shouldn't have
> > > > appeared producing dangerous race conditions.
> > > > This misbehaviour is usually happens with storage devices emulated
> > > > without eventfd for guest to host notifications like IDE.
> > > > 
> > > > The issue arises when the context is in the "drained" section
> > > > and doesn't expect the request to come, but request comes from the
> > > > device not using iothread and which context is processed by the main
> > > > loop.
> > > > 
> > > > The main loop apart of the iothread event loop isn't blocked by the
> > > > "drained" section.
> > > > The request coming and processing while in "drained" section can spoil
> > > > the
> > > > block driver state consistency.
> > > > 
> > > > This behavior can be observed in the following KVM-based case:
> > > > 
> > > > 1. Setup a VM with an IDE disk.
> > > > 2. Inside a VM start a disk writing load for the IDE device
> > > >     e.g: dd if=<file> of=<file> bs=X count=Y oflag=direct
> > > > 3. On the host create a mirroring block job for the IDE device
> > > >     e.g: drive_mirror <your_IDE> <your_path>
> > > > 4. On the host finish the block job
> > > >     e.g: block_job_complete <your_IDE>
> > > >    Having done the 4th action, you could get an assert:
> > > > assert(QLIST_EMPTY(&bs->tracked_requests)) from mirror_run.
> > > > On my setup, the assert is 1/3 reproducible.
> > > > 
> > > > The patch series introduces the mechanism to postpone the requests
> > > > until the BDS leaves "drained" section for the devices not using
> > > > iothreads.
> > > > Also, it modifies the asynchronous block backend infrastructure to use
> > > > that mechanism to release the assert bug for IDE devices.
> > > > 
> > > > Denis Plotnikov (2):
> > > >     async: add infrastructure for postponed actions
> > > >     block: postpone the coroutine executing if the BDS's is drained
> > > > 
> > > >    block/block-backend.c | 58 ++++++++++++++++++++++++++++++---------
> > > >    include/block/aio.h   | 63 +++++++++++++++++++++++++++++++++++++++++++
> > > >    util/async.c          | 33 +++++++++++++++++++++++
> > > >    3 files changed, 142 insertions(+), 12 deletions(-)
> > > > 
> > > 
> 
> -- 
> Best,
> Denis