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

[John Snow]

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

> 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(-)
>>
>