open on nfs server -> resource temporarily unavailable

[Andrey <thatisme@inp.nsk.su>]

Hello *,
  creating a file in the directory exported by the NFS server
    sometimes returns an error - "resource temporarily unavailable",
    when a client on importing computer is keeping reading that file.

serv:~# cat /etc/exports
/home/me/data-t	cli(ro,sync,no_subtree_check)

serv:~# systemctl restart nfs-kernel-server

cli:~# cat /etc/fstab
...
mhfpklytsserv:/home/me/data-t /home/me/dt nfs noauto,ro,noac,user 0 0

me@cli:~$ mount dt
me@cli:~$ mount
...
serv:/home/me/data-t on /home/me/dt type nfs4 (ro,nosuid,nodev,noexec,relatime,sync,vers=4.0,rsize=1048576,wsize=1048576,namlen=255,acregmin=0,acregmax=0,acdirmin=0,acdirmax=0,hard,noac,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=192.168.209.174,local_lock=none,addr=192.168.238.113,user=me)

It could be provoked in a few minutes by running on the server 
serv:~$ while : ; do date > data-t/now.txt; sleep 1; done

while on the client computer
me@cli:~$ while : ; do cat dt/now.txt; sleep 1; done

After a while (it's unpredictable might take up 10 minutes or a few seconds,
but in average about 2 minutes)
on the server I get:
data-t/now.txt: Resource temporarily unavailable

Lesser 'sleep', of 0.2 for ex., doesn't necessarily make the error appear quicker.
Also it doesn't depend whether a directory is exported read-only or read-write.
But without reading client the error never appears.

To narrow down the case a bit I made a test on 'C' for the server
which does the same thing as the script above - just writing down a date in the same file
- but checking an error on creating a file and writing to it.

Sometimes error appears just in a few seconds:
serv:~$ date;./tnfs-open data-t/now.txt ;date
Mon Aug  1 18:58:23 CEST 2016
open fd=-1 errno=11 -> Resource temporarily unavailable
Mon Aug  1 18:58:25 CEST 2016

serv:~$ errno 11
EAGAIN 11 Resource temporarily unavailable

I am at a loss :(

my system on the server and the client computes is the same Debian 8.5
:~$ dpkg -l 'nfs*'|grep '^ii'
ii  nfs-common        1:1.2.8-9    amd64        NFS support files common to client and server
ii  nfs-kernel-server 1:1.2.8-9    amd64        support for NFS kernel server

:~$ dpkg -l 'systemd*'|grep '^ii'
ii  systemd        215-17+deb8u4 amd64        system and service manager
ii  systemd-sysv   215-17+deb8u4 amd64        system and service manager - SysV links

:~$ dpkg -l 'linux-*'|grep '^ii'
ii  linux-base                          3.5                   all          Linux image base package
ii  linux-compiler-gcc-4.8-x86          3.16.7-ckt25-2+deb8u3 amd64        Compiler for Linux on x86 (meta-package)
ii  linux-headers-3.16.0-4-amd64        3.16.7-ckt25-2+deb8u3 amd64        Header files for Linux 3.16.0-4-amd64
ii  linux-headers-3.16.0-4-common       3.16.7-ckt25-2+deb8u3 amd64        Common header files for Linux 3.16.0-4
ii  linux-headers-amd64                 3.16+63               amd64        Header files for Linux amd64 configuration (meta-package)
ii  linux-image-3.16.0-4-amd64          3.16.7-ckt25-2+deb8u3 amd64        Linux 3.16 for 64-bit PCs
ii  linux-image-amd64                   3.16+63               amd64        Linux for 64-bit PCs (meta-package)
ii  linux-kbuild-3.16                   3.16.7-ckt20-1        amd64        Kbuild infrastructure for Linux 3.16
ii  linux-libc-dev:amd64                3.16.7-ckt25-2+deb8u3 amd64        Linux support headers for userspace development

Am I wrong with NFS options ?
But it was working on Debian 7 (and perhaps 6/5) and it began in May I upgraded to Debian 8.


Thanks,
   Andrey

PS:
I already posted the question to the list in June 
https://lists.debian.org/debian-user/2016/06/msg00738.html

But at that time I was looking for some kind of imposed limits.


PPS:
For those who would rather try that 'C' test above
cat tnfs-open.c
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <time.h>

int main (int argc, char *argv[])
{
  int fd,err,e,sec,ms;
  time_t tm;
  struct timespec tdly={.tv_sec=1, .tv_nsec=0};
  char *ept=NULL,*bf=NULL;

  if (argc < 2) {
    fprintf(stderr, "Usage: %s file-to-overwrite [milliseconds]\n", argv[0]);
    exit(1);
  }

  if (argc > 2) {
    ms=strtol(argv[2],&ept,10);
    if (ept==argv[2]) {
      fprintf(stderr, "interval should be integer number of milliseconds\n", ms);
      exit(1);
    } else {
      fprintf(stderr, "interval=%d ms\n", ms);
      sec=ms/1000;
      tdly.tv_sec=sec;
      tdly.tv_nsec=(ms-sec*1000)*1000000;
    }
  }
  for (;;) {
    tm=time(NULL);
    bf=ctime(&tm);
    fd=open(argv[1], O_WRONLY|O_CREAT|O_TRUNC, 0666);
    if (fd<0) {
      err=errno;
      fprintf(stderr, "open fd=%d errno=%d -> %s\n", fd, err,strerror(err));
      exit(2);
    }
    e=write(fd, bf, strlen(bf));
    if (e<0) {
      err=errno;
      fprintf(stderr, "write error=%d errno=%d\n", e, err,strerror(err));
      exit(3);
    }
    close(fd);
    nanosleep(&tdly,NULL);
  }
}