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Re: [PATCH v5 1/4] linux-aio: use LinuxAioState from the running thread

From: Kevin Wolf
Subject: Re: [PATCH v5 1/4] linux-aio: use LinuxAioState from the running thread
Date: Tue, 7 Mar 2023 13:17:17 +0100

Am 07.03.2023 um 11:58 hat Paolo Bonzini geschrieben:
> On 3/7/23 09:48, Kevin Wolf wrote:
> > You mean we have a device that has a separate iothread, but a request is
> > submitted from the main thread? This isn't even allowed today; if a node
> > is in an iothread, all I/O must be submitted from that iothread. Do you
> > know any code that does submit I/O from the main thread instead?
> I think it is allowed, you just have to take the AioContext lock around the
> bdrv_*?

Didn't we say at some point that we don't want to do this either? Though
maybe it's not strictly forbidden then.

> For example it could happen when you do block device migration.

As in migration/block.c? As far as I can tell, all of the requests made
there are actually processed in the iothread. (blk_aio_*() calls the
callback in the iothread even when it was called from the main thread
itself, which feels like a trap, but it shouldn't be affected by this
change lower in the stack.)

The potentially critical code would be coroutine_fns that call
blk_co_*() or bdrv_co_*() directly while running in a different thread.
Everything else schedules a new coroutine in the AioContext of the block


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