linux-kernel - Re: [PATCH] pvcalls-back: do not return error on inet

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Date:   Mon, 5 Feb 2018 10:01:32 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>, jgross@...e.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pvcalls-back: do not return error on inet_accept
 EAGAIN

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@...reto.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."

https://xenbits.xen.org/docs/unstable/misc/pvcalls.html

Look under the "Accept" sub-chapter.

Cheers,

Stefano