Re: recvmmsg() timeout behavior strangeness [RESEND]

[Michael Kerrisk (man-pages)]

Hi Arnaldo,

Ping!

Cheers,

Michael


On Wed, Apr 30, 2014 at 3:59 PM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> Arnaldo,
>
> I raised this issue somewhat more than a year ago, here:
> http://thread.gmane.org/gmane.linux.man/3477
> but got no reply from you. (Chris Friesen in that thread agreed
> that there is a problem though.)
>
> Here, a slightly revised version of that mail, since I've just bumper
> into a related problem in a different context...
>
> As part of his attempt to better document the recvmmsg() syscall that
> you added in commit a2e2725541fad72416326798c2d7fa4dafb7d337, Elie de
> Brauwer alerted to me to some strangeness in the timeout behavior of
> the syscall. I suspect there's a bug that needs fixing, as detailed
> below.
>
> AFAICT, the timeout argument was added to this syscall as a result of
> the discussion here:
> http://markmail.org/message/m5l2ap4hiiimut6k#query:+page:1+mid:m5l2ap4hiiimut6k+state:results
> (20-21 May 2009, "[RFC 1/2] net: Introduce recvmmsg...")
>
> If I understand correctly, the *intended* purpose of the timeout
> argument is to set a limit on how long to wait for additional
> datagrams after the arrival of an initial datagram. However, the
> syscall behaves in quite a different way. Instead, it potentially
> blocks forever, regardless of the timeout. The way the timeout seems
> to work is as follows:
>
> 1. The timeout, T, is armed on receipt of first diagram, starting at time X.
> 2. After each further datagram is received, a check is made if we have
> reached time X+T. If we have reached that time, then the syscall
> returns.
>
> Since the timeout is only checked after the arrival of each datagram,
> we can have scenarios like the following:
>
> 0. Assume a timeout of 10 seconds, and that vlen is 5.
> 1. First datagram arrives at time X.
> 2. Second datagram arrives at time X+2 secs
> 3. No more datagrams arrive.
>
> In this case, the call blocks forever. Is that intended behavior?
> (Basically, if up to vlen-1 datagrams arrive before X+T, but then no
> more datagrams arrive, the call will remain blocked forever.) If it's
> intended behavior, could you elaborate the use case, since it would be
> good to add that to the man page. If not, a fix seems to be needed,
> since otherwise, it's hard to see how the recvmmsg() timeout argument
> can sanely be used.
>
> Thanks,
>
> Michael
>
> --
> Michael Kerrisk
> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
> Linux/UNIX System Programming Training: http://man7.org/training/



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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