On Wed, Nov 16, 2011 at 10:25:46AM -0500, Mike Frysinger wrote:
> On Wednesday 16 November 2011 08:13:21 Karel Zak wrote:
> > On Mon, Nov 14, 2011 at 01:23:38AM -0500, Mike Frysinger wrote:
> > > i've been using a trick with flock to add locking to all of my shell
> > > scripts.
> > >
> > > basically, i take a lock on the shell script itself:
> > > flock -eon ./test.sh ./test.sh
> > >
> > > with <=util-linux-2.20, this has worked fine. but starting with 2.20.1,
> > > i now
> > >
> > > get -ETXTBSY (on ext4, but i doubt that matters):
> > > $ echo '#!/bin/sh' > test.sh
> > > $ chmod a+rx test.sh
> > > $ ./flock -eon ./test.sh ./test.sh
> > > ./flock: ./test.sh: Text file busy
> > >
> > > the only commit made to flock.c between 2.20 and 2.20.1 is this:
> > > commit 75aaee08f06b92d119ed827c53d1af5474eb16ff
> > > flock: make flock(1) work on NFSv4
> > >
> > > and indeed, reverting that made my life happy again. reading the small
> > > patch shows the obvious flaw: you can't open a file for O_RDWR and
> > > attempt to execute it at the same time.
> >
> > Hmm... maybe we can add a new --read-write command line option for
> > NFS guys rather than be smart with access(). The regression is
> > unacceptable.
>
> i wonder what happens if you attempt to take a read-only lock on nfs. do you
> get back an error ?
yes, EIO
> if the former, one solution might be to open the file, attempt to grab the
> lock, and if the flock() fails, retry the whole thing but with O_RDWR instead.
>
> int rw_flags = O_RDONLY;
> ...
> retry:
> ... current open logic which uses new rw_flags var ...
>
> ... if flock fails, and rw_flags == O_RDONLY, then set rw_flags to O_RDWR
> and jump back to retry ...
Good idea, implemented and tested (git pull to get the change). It
seems it works with your example as well as with NFSv4.
Karel
--
Karel Zak <kzak@xxxxxxxxxx>
http://karelzak.blogspot.com
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