On 23.07.19 12:02, Kevin Wolf wrote: > Am 23.07.2019 um 11:41 hat Max Reitz geschrieben: >> 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 <mre...@redhat.com> >>>> --- >>>> 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... > > Nothing that should hold up your pull request, but I'd prefer to fix the > comment in a follow-up.
On second thought, does aio_wait_kick() wake up any context but the main context? I was under the impression that it doesn’t. If it doesn’t, I don’t know how bdrv_drained_end()’s AIO_WAIT_WHILE() will be woken up if it doesn’t run in the main context and it has to wait for yet another thread. Max > One thing where I could imagine it becoming relevant in the future is > cross-context block jobs. At the moment, we automatically pull the > target node into the AioContext of the source and fail if this isn't > possible, but that's really overly restrictive. > > Kevin
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