Re: fcntl: F_SETLEASE/F_RDLCK question

["William A.(Andy) Adamson" <andros@citi.umich.edu>]

> Hi Michael,
> 
> On Tue, 3 May 2005 12:00:18 +0200 (MEST) "Michael Kerrisk" <mtk-lkml@gmx.net> wrote:
> >
> > Indeed the problem referred to is fixed, but it looks like another 
> > one may have been introduced.
> > 
> > It now appears (I tested on 2.6.11.6) that if a process opens 
> > a file O_RDWR and then tries to place a read lease via that
> > file descriptor, then the F_SETLEASE fails with EAGAIN, 
> > even though no other process has the file open for writing.  
> > (On the other hand, if the process opens the file 
> > O_WRONLY, then it can place either a read or a write lease.  
> > This is how I think things always worked, but it seems 
> > inconsistent with the aforementioned behaviour.)  
> > 
> > Some further testing showed the following (both open() 
> > and fcntl(F_SETLEASE) from same process):
> > 
> >  open()  |  lease requested
> >   flag   | F_RDLCK  | F_WRLCK
> > ---------+----------+----------
> > O_RDONLY | okay     |  okay
> > O_WRONLY | EAGAIN   |  okay
> > O_RDWR   | EAGAIN   |  okay
> > 
> > This seems strange (I imagine the caller should be excluded 
> > from the list of processes being checked to see if the file 
> > is opened for writing), and differs from earlier kernel
> > versions.  What is the intended behaviour here?
> 
> Thanks for the testing.  My expectation is that it shouldn't matter how
> the current process opened the file for either type of lease.  However,
> you are right (IMHO) that the current process should *not* be counted as a
> writer in the case of trying to obtain a F_RDLCK lease.

i believe the current implementation is correct. opening a file for write 
means that you can not have a read lease, caller included.


-->Andy


> 
> How does this (completely untested, not even compiled) patch look?
> 
> diff -ruNP linus/fs/locks.c linus-leases.1/fs/locks.c
> --- linus/fs/locks.c	2005-04-26 15:38:00.000000000 +1000
> +++ linus-leases.1/fs/locks.c	2005-05-03 23:00:14.000000000 +1000
> @@ -1288,7 +1288,8 @@
>  		goto out;
>  
>  	error = -EAGAIN;
> -	if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount) > 0))
> +	if ((arg == F_RDLCK) && (atomic_read(&inode->i_writecount)
> +				 > ((filp->f_mode & FMODE_WRITE) ? 1 : 0)))
>  		goto out;
>  	if ((arg == F_WRLCK)
>  	    && ((atomic_read(&dentry->d_count) > 1)
> 
> -- 
> Cheers,
> Stephen Rothwell                    sfr@canb.auug.org.au
> http://www.canb.auug.org.au/~sfr/


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