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Re: [Qemu-devel] [PATCH v2 5/6] coroutine: Explicitly specify AioContext
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v2 5/6] coroutine: Explicitly specify AioContext when entering coroutine |
Date: |
Fri, 7 Apr 2017 14:26:42 +0100 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
On Fri, Apr 07, 2017 at 02:54:13PM +0800, Fam Zheng wrote:
> Coroutine in block layer should always be waken up in bs->aio_context
> rather than the "current" context where it is entered. They differ when
> the main loop is doing QMP tasks.
>
> Race conditions happen without this patch, because the wrong context is
> acquired in co_schedule_bh_cb, while the entered coroutine works on a
> different one:
>
> main loop iothread
> -----------------------------------------------------------------------
> blockdev_snapshot
> aio_context_acquire(bs->ctx)
> bdrv_flush(bs)
> bdrv_co_flush(bs)
> ...
> qemu_coroutine_yield(co)
> BDRV_POLL_WHILE()
> aio_context_release(bs->ctx)
> aio_context_acquire(bs->ctx)
> ...
> aio_co_wake(co)
> aio_poll(qemu_aio_context) ...
> co_schedule_bh_cb() ...
> qemu_coroutine_enter(co) ...
> /* (A) bdrv_co_flush(bs) /* (B) I/O on bs */
> continues... */
> aio_context_release(bs->ctx)
>
> Both (A) and (B) can access resources protected by bs->ctx, but (A) is
> not thread-safe.
>
> Make the block layer explicitly specify a desired context for the
> entered coroutine. For the rest callers, stick to the old behavior,
> qemu_get_aio_context() or qemu_get_current_aio_context().
I wasn't expecting the patch to touch so many places. If a coroutine
can be permanently associated with an AioContext then there's no need to
keep assigning co->ctx on each qemu_coroutine_enter().
I don't know about this one. Paolo is the best person to review it
since he's most familiar with the recent Coroutine/AioContext changes.
Stefan
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- [Qemu-devel] [PATCH v2 3/6] mirror: Fix aio context of mirror_top_bs, (continued)
[Qemu-devel] [PATCH v2 5/6] coroutine: Explicitly specify AioContext when entering coroutine, Fam Zheng, 2017/04/07
[Qemu-devel] [PATCH v2 6/6] tests/block-job-txn: Don't start block job before adding to txn, Fam Zheng, 2017/04/07
Re: [Qemu-devel] [PATCH v2 0/6] block: Fixes regarding dataplane and management operations, Kevin Wolf, 2017/04/07