21.04.2021 11:38, Paolo Bonzini wrote:
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.
That would be great.
I have one more question in mind:
Is it effective to use CoMutex here? We are protecting only some fast
manipulations with data, not io path or something like that. Will simple
QemuMutex work better? Even if CoMutex doesn't have any overhead, I don't think
than if thread A wants to modify task list, but mutex is held by thread B (for
similar thing), there is a reason for thread A to yield and do some other
things: it can just wait several moments on mutex while B is modifying task
list..
--
Best regards,
Vladimir
