flock in 2.20.1 can no longer lock the file it will execute

flock in 2.20.1 can no longer lock the file it will execute

[Mike Frysinger]

[-- Attachment #1: Type: text/plain, Size: 780 bytes --]

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.
-mike

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 836 bytes --]

Re: flock in 2.20.1 can no longer lock the file it will execute

[Karel Zak]

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.

    Karel

-- 
 Karel Zak  <kzak@redhat.com>
 http://karelzak.blogspot.com

Re: flock in 2.20.1 can no longer lock the file it will execute

[Mike Frysinger]

[-- Attachment #1: Type: Text/Plain, Size: 1675 bytes --]

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 ?  or does it fail silently at runtime in the nfs layers ?

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 ...
-mike

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 836 bytes --]

Re: flock in 2.20.1 can no longer lock the file it will execute

[Karel Zak]

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@redhat.com>
 http://karelzak.blogspot.com

Re: flock in 2.20.1 can no longer lock the file it will execute

[Mike Frysinger]

[-- Attachment #1: Type: Text/Plain, Size: 232 bytes --]

On Monday 21 November 2011 11:35:43 Karel Zak wrote:
>  Good idea, implemented and tested (git pull to get the change). It
>  seems it works with your example as well as with NFSv4.

latest git now works for my scripts too :)
-mike

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 836 bytes --]