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Subject: Re: [PATCH] pvcalls-back: do not return error on inet_accept EAGAIN
To:     Stefano Stabellini <sstabellini@kernel.org>
Cc:     jgross@suse.com, xen-devel@lists.xenproject.org,
        linux-kernel@vger.kernel.org
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From:   Boris Ostrovsky <boris.ostrovsky@oracle.com>
Message-ID: <abe7aeba-ad27-467d-3da0-1026f5966a43@oracle.com>
Date:   Mon, 5 Feb 2018 13:34:13 -0500
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On 02/05/2018 01:01 PM, Stefano Stabellini wrote:
> On Sun, 4 Feb 2018, Boris Ostrovsky wrote:
>> On 02/02/2018 08:34 PM, Stefano Stabellini wrote:
>>> When the client sends a regular blocking accept request, the backend is
>>> expected to return only when the accept is completed, simulating a
>>> blocking behavior, or return an error.
>>>
>>> Specifically, on EAGAIN from inet_accept, the backend shouldn't return
>>> "EAGAIN" to the client. Instead, it should simply continue the wait.
>>> Otherwise, the client will send another accept request, which will cause
>>> another EAGAIN to be sent back, which is a waste of resources and not
>>> conforming to the expected behavior. Change the behavior by turning the
>>> "goto error" into a return.
>>>
>>> Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
>>
>> I am looking at SYSCALL_DEFINE4(accept4) and sock->ops->accept (which *I
>> think* is inet_accept, at least in some cases) passes all errors (including
>> EAGAIN)  back to the caller. Is this a different case?
> Hi Boris,
>
> I didn't explain myself well. You are right that inet_accept passes all
> errors back to the caller, but this is different: not only it would be a
> waste of resources to do so, but the different behavior is specified in
> the PVCalls spec:
>
> "The backend will reply to the request only when a new connection is
> successfully accepted, i.e. the backend does not return EAGAIN or
> EWOULDBLOCK."

Got it, thanks.

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>


-boris


>
> https://xenbits.xen.org/docs/unstable/misc/pvcalls.html
>
> Look under the "Accept" sub-chapter.
>
> Cheers,
>
> Stefano