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Re: [RFC PATCH 0/3] block-copy: lock tasks and calls list

From: Paolo Bonzini
Subject: Re: [RFC PATCH 0/3] block-copy: lock tasks and calls list
Date: Wed, 21 Apr 2021 10:38:58 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.7.0

On 20/04/21 15:12, Vladimir Sementsov-Ogievskiy wrote:
20.04.2021 13:04, Emanuele Giuseppe Esposito wrote:
This serie of patches continues Paolo's series on making the
block layer thread safe. Add a CoMutex lock for both tasks and
calls list present in block/block-copy.c

I think, we need more information about what kind of thread-safety we want. Should the whole interface of block-copy be thread safe? Or only part of it? What is going to be shared between different threads? Which functions will be called concurrently from different threads? This should be documented in include/block/block-copy.h.

I guess all of it. So more state fields should be identified in BlockCopyState, especially in_flight_bytes. ProgressMeter and SharedResource should be made thread-safe on their own, just like the patch I posted for RateLimit.

What I see here, is that some things are protected by mutex.. Some things not. What became thread-safe?

For example, in block_copy_dirty_clusters(), we modify task fields without any mutex held:

  block_copy_task_shrink doesn't take mutex.
  task->zeroes is set without mutex as well

Agreed, these are bugs in the series.

Still all these accesses are done when task is already added to the list.

Looping in block_copy_common() is not thread safe as well.

That one should be mostly safe, because only one coroutine ever writes to all fields except cancelled. cancelled should be accessed with qatomic_read/qatomic_set, but there's also the problem that coroutine sleep/wake APIs are hard to use in a thread-safe manner (which affects block_copy_kick). This is a different topic and it is something I'm working on,

You also forget to protect QLIST_REMOVE() call in block_copy_task_end()..

Next, block-copy uses co-shared-resource API, which is not thread-safe (as it is directly noted in include/qemu/co-shared-resource.h).

Same thing is block/aio_task API, which is not thread-safe too.

So, we should bring thread-safety first to these smaller helper APIs.

Good point. Emanuele, can you work on ProgressMeter and SharedResource? AioTaskPool can also be converted to just use CoQueue instead of manually waking up coroutines.

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