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Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease i
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback |
Date: |
Mon, 17 Sep 2018 17:59:19 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 |
On 17/09/2018 14:53, Kevin Wolf wrote:
>>> I think I can drop the ref/unref pair, but not the whole patch (whose
>>> main point is reordering dec_in_flight vs. the AIO callback).
>>
>> You're right, though I think I did that on purpose back in the day.
>> IIRC it was related to bdrv_drain, which might never complete if called
>> from an AIO callback.
>
> Hm... This seems to become a common pattern, it's the same as for the
> job completion callbacks (only improved enough for the bug at hand to
> disappear instead of properly fixed in "blockjob: Lie better in
> child_job_drained_poll()").
>
> Either you say there is no activity even though there is still a
> callback pending, then bdrv_drain() called from elsewhere will return
> too early and we get a bug. Or you say there is activity, then any
> nested drain inside that callback will deadlock and we get a bug, too.
>
> So I suppose we need some way to know which activities to ignore during
> drain, depending on who is the caller? :-/
Some alternatives:
1) anything that needs to do invoke I/O for callbacks must use inc/dec
in flight manually. Simplest but hard to assert though. At least SCSI
IDE are broken.
2) callbacks cannot indirectly result in bdrv_drain. Sounds easier
since there are not many AIO callers anymore - plus it also seems easier
to add debugging code for.
3) we provide device callbacks for "am I busy" and invoke them from
bdrv_drain's poll loop.
Just off the top of my head. Not sure which is best.
Paolo
- [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, (continued)
- [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/13
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Paolo Bonzini, 2018/09/13
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/13
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Fam Zheng, 2018/09/14
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Paolo Bonzini, 2018/09/14
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/14
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Paolo Bonzini, 2018/09/14
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/17
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Paolo Bonzini, 2018/09/17
- Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/17
- Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback,
Paolo Bonzini <=
- Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/17
- Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Paolo Bonzini, 2018/09/17
- Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/18
- Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Paolo Bonzini, 2018/09/18
- Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Kevin Wolf, 2018/09/18
- Re: [Qemu-devel] [Qemu-block] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Paolo Bonzini, 2018/09/19
Re: [Qemu-devel] [PATCH v2 11/17] block-backend: Decrease in_flight only after callback, Max Reitz, 2018/09/13
[Qemu-devel] [PATCH v2 12/17] mirror: Fix potential use-after-free in active commit, Kevin Wolf, 2018/09/13