Re: [bug?] mutual exclusion between posix and OFD locks

[Jan Stancek <jstancek@redhat.com>]

----- Original Message -----
> On Fri, 2018-06-15 at 13:11 +0200, Jan Stancek wrote:
> > Hi,
> > 
> > Attached is simplified reproducer (LTP fcntl36), where
> > 2 threads try to lock same region in a file. One is
> > using posix write lock, the other OFD read lock.
> > 
> > Observed problem: 2 threads obtain lock simultaneously.
> > 
> > --- strace excerpt ---
> > [pid 16853] 06:57:11 openat(AT_FDCWD, "tst_ofd_posix_locks", O_RDWR) = 3
> > [pid 16854] 06:57:11 openat(AT_FDCWD, "tst_ofd_posix_locks", O_RDWR) = 4
> > ...
> > [pid 16853] 06:57:12 fcntl(3, F_SETLKW, {l_type=F_WRLCK, l_whence=SEEK_SET,
> > l_start=0, l_len=4096} <unfinished ...>
> > [pid 16854] 06:57:12 fcntl(4, F_OFD_SETLKW, {l_type=F_RDLCK,
> > l_whence=SEEK_SET, l_start=0, l_len=4096} <unfinished ...>
> > [pid 16853] 06:57:12 <... fcntl resumed> ) = 0
> > [pid 16853] 06:57:12 nanosleep({tv_sec=0, tv_nsec=100000},  <unfinished
> > ...>
> > [pid 16854] 06:57:12 <... fcntl resumed> ) = 0
> > --- /strace excerpt ---
> > 
> > fcntl(2) says:
> >   Conflicting lock combinations (i.e., a read lock and a write
> >   lock or two write locks) where one lock is an open file
> >   description lock and the other is a traditional record lock
> >   conflict even when they are acquired by the same process on
> >   the same file descriptor.
> > 
> > Reproducible on x86_64 VM, with v4.17-11782-gbe779f03d563.
> > 
> > Thanks for having a look,
> > Jan
> > 
> 
> tl;dr: I think the test program is buggy. You're running afoul of one of
> the behaviors of traditional POSIX locks that caused us to add OFD locks
> in the first place. On any call to close() all traditional POSIX locks
> in the process are dropped.
> 
> Longer explanation: You have 3 thread pairs, and each one does a
> close(fd) at the end of the thread func. When you go to join the
> threads, it ends up calling close(fd), and that causes _all_ traditional
> POSIX locks to get released, even ones that might still be in use by
> other threads.
> 
> If you comment out the close(fd); calls in both thread funcs then the
> program seems to reliably run to completion.

Thanks Jeff. You're right, the problem goes away if I drop close().
I recall reading that part in man page, but this race eluded me.

Sorry for false alarm, we'll fix the test (fcntl36).

Regards,
Jan

> --
> Jeff Layton <jlayton@kernel.org>
>