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Re: [Qemu-block] [Qemu-devel] Question: an IO hang problem

From: Fam Zheng
Subject: Re: [Qemu-block] [Qemu-devel] Question: an IO hang problem
Date: Thu, 15 Mar 2018 14:37:46 +0800
User-agent: Mutt/1.9.2 (2017-12-15)

On Tue, 03/13 17:38, sochin.jiang wrote:
>  Hi, guys,
>  Recently, I encountered an IO hang problem in occasion which I cannot 
> reproduce it now.
>  I analyzed this problem carefully, the critical stack is as following:
> After reading the codes in linux-aio.c(see ioq_submit() function), I found 
> two situations could lead us here.
> 1) no AIOs are in flight(s->ioq.in_flight is 0) and another call to io_submit 
> returns -EAGAIN

So if there is no inflight I/O, why it would return -EAGAIN? The tricky thing
here is that since we're not expecting a completion, when should we retry?

> 2) no AIOs are in flight(s->ioq.in_flight is 0) and s->io_q.pending IOs reach 
> to MAX_EVENTS at once

I don't understand this case. We have,

        len = 0;
        QSIMPLEQ_FOREACH(aiocb, &s->io_q.pending, next) {
            iocbs[len++] = &aiocb->iocb;
            if (s->io_q.in_flight + len >= MAX_EVENTS) {

        ret = io_submit(s->ctx, len, iocbs);

If in_flight is 0, only (MAX_EVENTS - 1) requests can be added to iocbs, so
io_submit shouldn't return -EAGAIN.

> In both the two situations above, the do{...}while loop breaks out and set 
> s->io_q.blocked true.
> After that, AIO completion callback will never be called,  ioq_submit() 
> either, all pended requests will hang.
> Is there a proper way we can fix this while do not affect(stuck) the guest ?


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