From: "Ara.T.Howard" <ahoward@...>
Date: 2003-10-07T08:03:08+09:00
Subject: Re: bug in open/flock ?

On Tue, 7 Oct 2003, Hal Fulton wrote:

> Ara.T.Howard wrote:
> > in the code below, i am able to obtain TWO exclusive locks on files using the
> > following logic:
> >
> >   iff flock(LOCK_EX | LOCK_NB) fails, then open again and retry
> >
> > this should not be possible should it??
> >
> > note that the following code SHOULD deadlock - but does not.
>
> [snip]
>

> Aren't locks on a per-process basis?

no.knotice that this will produce monotonically increasing timestamps:


~/eg/ruby > cat ./flock.rb

#!/usr/local/ruby-1.8.0/bin/ruby
require 'ftools'
require 'tempfile'

def compete who, f
  5.times do |i|
    f.flock File::LOCK_EX
    f.puts format("%s:%d @ %f\n",who, i, Time.now.to_f)
    f.flock File::LOCK_UN
  end
end

path = format(__FILE__ + '.out')
fd = open(path, File::WRONLY | File::TRUNC | File::CREAT)

if fork
  compete 'PARENT', fd

  Process.wait rescue nil
  open(path){|f| puts f.read}
  fd.close
  File.rm_f path if File.exist? path

else
  compete 'CHILD', fd
end


~/eg/ruby > ./flock.rb

PARENT:0 @ 1065472160.300986
PARENT:1 @ 1065472160.301536
CHILD:0 @ 1065472160.301911
CHILD:1 @ 1065472160.302190
CHILD:2 @ 1065472160.302303
CHILD:3 @ 1065472160.302411
CHILD:4 @ 1065472160.302520
PARENT:2 @ 1065472160.303144
PARENT:3 @ 1065472160.305026
PARENT:4 @ 1065472160.305172


flocks are durable across processes.  i think they are held in the kernel open
file table.  on my systems, they even work on nfs between machines!  (this is
not normal)



> Ruby threads all run in the same process.

which is an even stronger argument for why no two threads (let alone
processes) should EVER be able to obtain an exclusive lock at the same time.

-a
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  | Ara Howard
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