netdev - Re: [PATCH 1/3] net: sendmmsg should only return an error if no messages were sent

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Message-ID: <20110805144027.GA7560@ghostprotocols.net>
Date:	Fri, 5 Aug 2011 11:40:27 -0300
From:	Arnaldo Carvalho de Melo <acme@...hat.com>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	rdenis@...phalempin.com, netdev@...r.kernel.org
Subject: Re: [PATCH 1/3] net: sendmmsg should only return an error if no
	messages were sent

Em Fri, Aug 05, 2011 at 09:20:08AM +0100, Steven Whitehouse escreveu:
> On Fri, 2011-08-05 at 12:57 +0900, Tetsuo Handa wrote:
> > Anton Blanchard wrote:
> > > sendmmsg uses a similar error return strategy as recvmmsg but it
> > > turns out to be a confusing way to communicate errors.
> > > 
> > > The current code stores the error code away and returns it on the next
> > > sendmmsg call. This means a call with completely valid arguments could
> > > get an error from a previous call.
> > > 
> > > Change things so we only return an error if no datagrams could be sent.
> > > If less than the requested number of messages were sent, the application
> > > must retry starting at the first failed one and if the problem is
> > > persistent the error will be returned.
> > > 
> > > This matches the behaviour of other syscalls like read/write - it
> > > is not an error if less than the requested number of elements are sent.
> > 
> > OK. David S. Miller suggested this behavior and Anton Blanchard agreed with
> > this behavior.
> > 
> > Quoting from commit a2e27255 "net: Introduce recvmmsg socket syscall":
> > | . R?mi Denis-Courmont & Steven Whitehouse: If we receive N < vlen
> > |   datagrams and then recvmsg returns an error, recvmmsg will return
> > |   the successfully received datagrams, store the error and return it
> > |   in the next call.
> > 
> > R?mi Denis-Courmont, Steven Whitehouse and Arnaldo Carvalho de Melo, do you
> > want to change recvmmsg()'s behaviour as well?
> 
> Since I've joined this part way through it seems, I'm assuming that if
> something was sent/received then that will be returned and the error
> stored until the next call. If nothing was sent/received then the error
> can be returned immediately.
> 
> That is what I'd expect to be the case, since otherwise it is impossible
> to know how much has been successfully sent/received in the partial
> failure case, I think. Also it means that sendmmesg/recvmmsg matches
> sendmsg/recvmsg in terms of expected return values and thus the
> principle of least surprise.
> 
> So if thats what is being proposed, then it sounds good to me,

Sounds sane to me too.

- Arnaldo
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