Re: [PATCH/RFC] Re: recvmmsg() timeout behavior strangeness [RESEND]

[Arnaldo Carvalho de Melo]

Em Thu, May 22, 2014 at 04:27:45PM +0200, Michael Kerrisk (man-pages) escreveu:
> Hi Arnaldo,
> 
> On 05/21/2014 11:05 PM, Arnaldo Carvalho de Melo wrote:
> > Em Mon, May 12, 2014 at 11:34:51AM -0300, Arnaldo Carvalho de Melo escreveu:
> >> Em Mon, May 12, 2014 at 12:15:25PM +0200, Michael Kerrisk (man-pages) escreveu:
> >>> Hi Arnaldo,
> >  
> >>> Ping!
> > 
> >> I acknowledge the problem, the timeout has to be passed to the
> >> underlying ->recvmsg() implementations that should return the time spent
> >> waiting for each packet, so that we can accrue that at recvmmsg level.
> >  
> >> We can do either passing an extra timeout parameter to the recvmsg
> >> implementations or using some struct sock member to specify that
> >> timeout.
> >  
> >> The first approach is intrusive, touches tons of files, so I'll try
> >> making it all mostly transparent by hooking into sock_rcvtimeo()
> >> somehow.
> > 
> > But after thinking a bit more, looks like we need to do that, please
> > take a look at the attached patch to see if it addresses the problem.
> > 
> > Mostly it adds a new timeop to the per protocol recvmsg()
> > implementations, that, if not NULL, should be used instead of
> > SO_RCVTIMEO.
> > 
> > since the underlying recvmsg implementations already check that timeout,
> > return what is remaining, that will then be used in subsequent recvmsg
> > calls, at the end we just convert it back to timespec format.
> > 
> > In most cases it is just passed to skb_recv_datagram, that will check
> > the pointer, use it and update if not NULL.
> > 
> > Should have no problems, but I only did a boot with a system with this
> > patch applied, no problems noticed on a normal desktop session, ssh,
> > etc.
> 
> Thanks! I applied this patch against 3.15-rc6.
> 
> recvmmsg() now (mostly) does what I expect: 
> * it waits until either the timeout expires or vlen messages 
>   have been received
> * If no message is received before timeout, it returns -1/EAGAIN.
> * If vlen messages are received before the timeout expires, then
>   the remaining time is returned in timeout.
> 
> One question: in the event that the call is interrupted by a signal 
> handler, it fails (as expected) with EINTR, but the 'timeout' value is 
> not updated with the remaining time on the timer. Would it be desirable 
> to emulate the behavior of select() (and other syscalls) in this 
> respect, and instead return the remaining time if interrupted by 
> a signal?

I think so, will check how to achieve that!
 
> Cheers,
> 
> Michael
> 
> -- 
> 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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