linux-kernel - flock(2) cannot block other process which try to get same flock?

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Date:	Fri, 30 Sep 2011 20:09:38 +0900
From:	Minoru Usui <usui@....nes.nec.co.jp>
To:	linux-kernel@...r.kernel.org
Cc:	matthew@....cx, viro@...iv.linux.org.uk
Subject: flock(2) cannot block other process which try to get same flock?

Hi, 

I faced strange behaviour of flock(2) in upstream kernel(3.1.0-rc8+).
It seems flock cannot block other processes which try to get same flock.
If someone know the cause of this behaviour, please let me know.

My test is very very simple.
It tries to get flock against same file by 5 processes.
The result is following.

-----------------------------------------------------------------
# ./flock.sh
lock(pid:2384)
# unlock(pid:2384)
lock(pid:2385)   --- lock(a)
lock(pid:2384)   --- lock(b)
unlock(pid:2385) --- unlock(a)
unlock(pid:2384) --- unlock(b)
   :
-----------------------------------------------------------------

I setup auditd and check audit.log, it indicated same behaviour.

-----------------------------------------------------------------
type=SYSCALL msg=audit(1317375911.187:105): arch=c000003e syscall=73 success=yes exit=0 a0=0 a1=2 a2=0 a3=7fffe664f920 items=0 ppid=2382 pid=2384 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts1 ses=1 comm="flock" exe="/home/usui/devel/tp/flock" key=(null)

type=SYSCALL msg=audit(1317375912.187:106): arch=c000003e syscall=73 success=yes exit=0 a0=0 a1=8 a2=0 a3=8 items=0 ppid=1 pid=2384 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts1 ses=1 comm="flock" exe="/home/usui/devel/tp/flock" key=(null)

type=SYSCALL msg=audit(1317375911.187:107): arch=c000003e syscall=73 success=yes exit=0 a0=0 a1=2 a2=0 a3=7fffdc2e2c70 items=0 ppid=1 pid=2385 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts1 ses=1 comm="flock" exe="/home/usui/devel/tp/flock" key=(null)
  -> lock(a) (pid=2385, a1=2)

type=SYSCALL msg=audit(1317375912.187:108): arch=c000003e syscall=73 success=yes exit=0 a0=0 a1=2 a2=0 a3=7fffe664f920 items=0 ppid=1 pid=2384 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts1 ses=1 comm="flock" exe="/home/usui/devel/tp/flock" key=(null)
  -> lock(b) (pid=2384, a1=2)

type=SYSCALL msg=audit(1317375913.187:109): arch=c000003e syscall=73 success=yes exit=0 a0=0 a1=8 a2=0 a3=8 items=0 ppid=1 pid=2384 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts1 ses=1 comm="flock" exe="/home/usui/devel/tp/flock" key=(null)
  -> unlock(a) (pid=2385, a1=8)

type=SYSCALL msg=audit(1317375913.187:110): arch=c000003e syscall=73 success=yes exit=0 a0=0 a1=8 a2=0 a3=8 items=0 ppid=1 pid=2385 auid=0 uid=0 gid=0 euid=0 suid=0 fsuid=0 egid=0 sgid=0 fsgid=0 tty=pts1 ses=1 comm="flock" exe="/home/usui/devel/tp/flock" key=(null)
  -> unlock(b) (pid=2384, a1=8)
-----------------------------------------------------------------



Test programs are following.
-----------------------------------------------------------------
# cat -n flock.sh
     1  #! /bin/sh
     2  
     3  CNT=5
     4  
     5  if [ $# -ne 0 ]
     6  then
     7    CNT=$1
     8  fi
     9  
    10  for i in `seq 1 $CNT`
    11  do
    12          ./flock &
    13  done

# cat -n flock.c
     1  #include <stdio.h>
     2  #include <sys/file.h>
     3  #include <sys/types.h>
     4  #include <sys/stat.h>
     5  #include <fcntl.h>
     6  #include <errno.h>
     7  #include <unistd.h>
     8  #include <stdlib.h>
     9  
    10  #define LOCKFILE "/tmp/flocktest"
    11  int main(void)
    12  {
    13          int fd;
    14          int ret;
    15          int pid = getpid();
    16  
    17          while(1) {
    18                  if ((fd = open(LOCKFILE, O_RDONLY) < 0)) {
    19                                  perror("open()");
    20                                  exit(1);
    21                  }
    22  
    23                  if (ret = flock(fd, LOCK_EX) < 0) {
    24                          perror("flock(LOCK_EX)");
    25                          exit(1);
    26                  }
    27                  printf("lock(pid:%d)\n", pid);
    28                  fflush(stdout);
    29  
    30                  sleep(1);
    31  
    32                  printf("unlock(pid:%d)\n", pid);
    33                  fflush(stdout);
    34                  if (ret = flock(fd, LOCK_UN) < 0) {
    35                          perror("flock(LOCK_UN)");
    36                          exit(1);
    37                  }
    38  
    39                  if (close(fd) < 0) {
    40                          perror("flock(LOCK_UN)");
    41                          exit(1);
    42                  }
    43          }
    44  
    45          return 0;
    46  }
-----------------------------------------------------------------
-- 
Minoru Usui <usui@....nes.nec.co.jp>
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