Re: [Qemu-devel] [PATCH for-4.1 2/2] block: Only the main loop can change AioContexts

[Max Reitz]

On 23.07.19 10:52, Kevin Wolf wrote:
> Am 22.07.2019 um 15:30 hat Max Reitz geschrieben:
>> bdrv_set_aio_context_ignore() can only work in the main loop:
>> bdrv_drained_begin() only works in the main loop and the node's (old)
>> AioContext; and bdrv_drained_end() really only works in the main loop
>> and the node's (new) AioContext (contrary to its current comment, which
>> is just wrong).
>>
>> Consequentially, bdrv_set_aio_context_ignore() must be called from the
>> main loop.  Luckily, assuming that we can make block graph changes only
>> from the main loop as well, all its callers do that already.
>>
>> Note that changing a node's context in a sense is an operation that
>> changes the block graph, so it actually makes sense to require this
>> function to be called from the main loop.
>>
>> Also, fix bdrv_drained_end()'s description.  You can only use it from
>> the main loop or the node's AioContext, and in the latter case, the
>> whole subtree must be in the same context.
>>
>> Fixes: e037c09c78520cbdb6da7cfc6ad0256d5870b814
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  include/block/block.h |  8 +++-----
>>  block.c               | 13 ++++++++-----
>>  2 files changed, 11 insertions(+), 10 deletions(-)
>>
>> diff --git a/include/block/block.h b/include/block/block.h
>> index 60f00479e0..50a07c1c33 100644
>> --- a/include/block/block.h
>> +++ b/include/block/block.h
>> @@ -667,11 +667,9 @@ void bdrv_subtree_drained_begin(BlockDriverState *bs);
>>   *
>>   * This polls @bs's AioContext until all scheduled sub-drained_ends
>>   * have settled.  On one hand, that may result in graph changes.  On
>> - * the other, this requires that all involved nodes (@bs and all of
>> - * its parents) are in the same AioContext, and that the caller has
>> - * acquired it.
>> - * If there are any nodes that are in different contexts from @bs,
>> - * these contexts must not be acquired.
>> + * the other, this requires that the caller either runs in the main
>> + * loop; or that all involved nodes (@bs and all of its parents) are
>> + * in the caller's AioContext.
>>   */
>>  void bdrv_drained_end(BlockDriverState *bs);
> 
> I think you are right about the requirement that bdrv_drained_end() is
> only called from the main or the BDS AioContext, which is a requirement
> that directly comes from AIO_WAIT_WHILE().
> 
> However, I don't see why we have requirements on the AioContext of the
> parent nodes (or any other nodes), except possibly not holding their
> lock.  We don't poll their context, so it shouldn't matter in which
> context they are?

Hm.  I don’t know how I got confused there, you’re right.

I don’t think the (falsely given) restriction hurts, though, because a
subtree should be within a single context anyway (unless you’re in
bdrv_set_aio_context_ignore(), which needs to be in the main context).

So, hm, yes, I messed up this comment a bit now.  But now it’s just more
restrictive than it needs to be and I don’t think callers are going to
care, so...

Max

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