Re: [glibc PATCH v2] fcntl-linux.h: add new definitions and manual updates for open file description locks

[Jeff Layton <jlayton at redhat dot com>]

On Thu, 24 Apr 2014 14:31:43 +0200
"Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com> wrote:

> Jeff,
> 
> Did you receive my mail with comments on the previous patch?
> (I got no reply.) It looks like some of those comments that needed to
> be addressed were not.
> 
> Cheers,
> 
> Michael
> 

I did get the mail and thought I had addressed them all. The only thing
I didn't change based on your comments was the one about the "return 0"
in main() being unnecessary in the example program. That's not true
since main is an int return function. I suppose we could turn it into a
void return, but does it really matter here?

> On 04/24/2014 02:15 PM, Jeff Layton wrote:
> > Open file description locks have been merged into the Linux kernel for
> > v3.15.  Add the appropriate command-value definitions and an update to
> > the manual that describes their usage.
> > 
> > ChangeLog:
> > 
> > 2014-04-24  Jeff Layton  <jlayton@redhat.com>
> > 
> > 	[BZ#16839]
> > 	* manual/llio.texi: add section about open file description locks
> > 
> > 	* sysdeps/unix/sysv/linux/bits/fcntl-linux.h:
> > 	  (F_OFD_GETLK, F_OFD_SETLK, F_OFD_SETLKW): New macros.
> > ---
> >  manual/examples/ofdlocks.c                 |  77 +++++++++
> >  manual/llio.texi                           | 241 ++++++++++++++++++++++++++++-
> >  sysdeps/unix/sysv/linux/bits/fcntl-linux.h |  17 ++
> >  3 files changed, 332 insertions(+), 3 deletions(-)
> >  create mode 100644 manual/examples/ofdlocks.c
> > 
> > diff --git a/manual/examples/ofdlocks.c b/manual/examples/ofdlocks.c
> > new file mode 100644
> > index 000000000000..85e193cdabe6
> > --- /dev/null
> > +++ b/manual/examples/ofdlocks.c
> > @@ -0,0 +1,77 @@
> > +/* Open File Description Locks Usage Example
> > +   Copyright (C) 1991-2014 Free Software Foundation, Inc.
> > +
> > +   This program is free software; you can redistribute it and/or
> > +   modify it under the terms of the GNU General Public License
> > +   as published by the Free Software Foundation; either version 2
> > +   of the License, or (at your option) any later version.
> > +
> > +   This program is distributed in the hope that it will be useful,
> > +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> > +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> > +   GNU General Public License for more details.
> > +
> > +   You should have received a copy of the GNU General Public License
> > +   along with this program; if not, see <http://www.gnu.org/licenses/>.
> > +*/
> > +
> > +#define _GNU_SOURCE
> > +#include <stdio.h>
> > +#include <sys/types.h>
> > +#include <sys/stat.h>
> > +#include <unistd.h>
> > +#include <fcntl.h>
> > +#include <pthread.h>
> > +
> > +#define FILENAME	"/tmp/foo"
> > +#define NUM_THREADS	3
> > +#define ITERATIONS	5
> > +
> > +void *
> > +thread_start (void *arg)
> > +{
> > +  int i, fd, len;
> > +  long tid = (long) arg;
> > +  char buf[256];
> > +  struct flock lck = {
> > +    .l_whence = SEEK_SET,
> > +    .l_start = 0,
> > +    .l_len = 1,
> > +  };
> > +
> > +  fd = open ("/tmp/foo", O_RDWR | O_CREAT, 0666);
> > +
> > +  for (i = 0; i < ITERATIONS; i++)
> > +    {
> > +      lck.l_type = F_WRLCK;
> > +      fcntl (fd, F_OFD_SETLKW, &lck);
> > +
> > +      len = sprintf (buf, "%d: tid=%ld fd=%d\n", i, tid, fd);
> > +
> > +      lseek (fd, 0, SEEK_END);
> > +      write (fd, buf, len);
> > +      fsync (fd);
> > +
> > +      lck.l_type = F_UNLCK;
> > +      fcntl (fd, F_OFD_SETLK, &lck);
> > +
> > +      /* sleep to ensure lock is yielded to another thread */
> > +      usleep (1);
> > +    }
> > +  pthread_exit (NULL);
> > +}
> > +
> > +int
> > +main (int argc, char **argv)
> > +{
> > +  long i;
> > +  pthread_t threads[NUM_THREADS];
> > +
> > +  truncate (FILENAME, 0);
> > +
> > +  for (i = 0; i < NUM_THREADS; i++)
> > +    pthread_create (&threads[i], NULL, thread_start, (void *) i);
> > +
> > +  pthread_exit (NULL);
> > +  return 0;
> > +}
> > diff --git a/manual/llio.texi b/manual/llio.texi
> > index 6f8adfc607d7..764f67d469cc 100644
> > --- a/manual/llio.texi
> > +++ b/manual/llio.texi
> > @@ -57,6 +57,10 @@ directly.)
> >                                           flags associated with open files.
> >  * File Locks::                          Fcntl commands for implementing
> >                                           file locking.
> > +* Open File Description Locks::         Fcntl commands for implementing
> > +                                         open file description locking.
> > +* Open File Description Locks Example:: An example of open file description lock
> > +                                         usage
> >  * Interrupt Input::                     Getting an asynchronous signal when
> >                                           input arrives.
> >  * IOCTLs::                              Generic I/O Control operations.
> > @@ -2890,7 +2894,7 @@ Get flags associated with the open file.  @xref{File Status Flags}.
> >  Set flags associated with the open file.  @xref{File Status Flags}.
> >  
> >  @item F_GETLK
> > -Get a file lock.  @xref{File Locks}.
> > +Test a file lock.  @xref{File Locks}.
> >  
> >  @item F_SETLK
> >  Set or clear a file lock.  @xref{File Locks}.
> > @@ -2898,6 +2902,18 @@ Set or clear a file lock.  @xref{File Locks}.
> >  @item F_SETLKW
> >  Like @code{F_SETLK}, but wait for completion.  @xref{File Locks}.
> >  
> > +@item F_OFD_GETLK
> > +Test a open file description lock.  @xref{Open File Description Locks}.
> > +Specific to Linux.
> > +
> > +@item F_OFD_SETLK
> > +Set or clear a file lock.  @xref{Open File Description Locks}.
> > +Specific to Linux.
> > +
> > +@item F_OFD_SETLKW
> > +Like @code{F_OFD_SETLK}, but wait for completion.
> > +@xref{Open File Description Locks}.  Specific to Linux.
> > +
> >  @item F_GETOWN
> >  Get process or process group ID to receive @code{SIGIO} signals.
> >  @xref{Interrupt Input}.
> > @@ -3576,6 +3592,10 @@ set_nonblock_flag (int desc, int value)
> >  
> >  @cindex file locks
> >  @cindex record locking
> > +This section describes record locks that are associated with the process.
> > +There is also a different type of record lock that is associated with the
> > +open file description instead of the process.  @xref{Open File Description Locks}.
> > +
> >  The remaining @code{fcntl} commands are used to support @dfn{record
> >  locking}, which permits multiple cooperating programs to prevent each
> >  other from simultaneously accessing parts of a file in error-prone
> > @@ -3641,7 +3661,10 @@ the file.
> >  @item pid_t l_pid
> >  This field is the process ID (@pxref{Process Creation Concepts}) of the
> >  process holding the lock.  It is filled in by calling @code{fcntl} with
> > -the @code{F_GETLK} command, but is ignored when making a lock.
> > +the @code{F_GETLK} command, but is ignored when making a lock.  If the
> > +conflicting lock is an open file description lock
> > +(@pxref{Open File Description Locks}), then this field will be set to
> > +@math{-1}.
> >  @end table
> >  @end deftp
> >  
> > @@ -3813,10 +3836,222 @@ that part of the file for writing.
> >  
> >  @c ??? This section could use an example program.
> >  
> > -Remember that file locks are only a @emph{voluntary} protocol for
> > +Remember that file locks are only an @emph{advisory} protocol for
> >  controlling access to a file.  There is still potential for access to
> >  the file by programs that don't use the lock protocol.
> >  
> > +@node Open File Description Locks
> > +@section Open File Description Locks
> > +
> > +In contrast to process-associated record locks (@pxref{File Locks}),
> > +open file description record locks are associated with an open file
> > +description rather than a process.
> > +
> > +Using @code{fcntl} to apply a open file description lock on a region that
> > +already has an existing open file description lock that was created via the
> > +same file descriptor will never cause a lock conflict.
> > +
> > +Open file description locks are also inherited by child processes across
> > +@code{fork}, or @code{clone} with @code{CLONE_FILES} set
> > +(@pxref{Creating a Process}), along with the file descriptor.
> > +
> > +It is important to distinguish between the file @emph{description} (an
> > +instance of an open file, usually created by a call to @code{open}) and
> > +a file @emph{descriptor}, which is a numeric value that refers to the
> > +former.  The locks described here are associated with the open file
> > +@emph{description} and not the open file @emph{descriptor}.
> > +
> > +Using @code{dup} (@pxref{Duplicating Descriptors}) to copy a file
> > +descriptor does not give you a new file description, but rather copies a
> > +reference to an existing open file description and assigns it to a new
> > +file descriptor.  Thus, open file description locks set on a file
> > +descriptor cloned by @code{dup} will never conflict with open file
> > +description locks set on the original descriptor since they refer to the
> > +same open file description.  Depending on the range and type of lock
> > +involved, the original lock may be modified by a @code{F_OFD_SETLK} or
> > +@code{F_OFD_SETLKW} command in this situation however.
> > +
> > +Open file description locks always conflict with process-associated locks,
> > +even if acquired by the same process or on the same open file
> > +descriptor.
> > +
> > +Open file description locks use the same @code{struct flock} as
> > +process-associated locks as an argument (@pxref{File Locks}) and the
> > +macros for the @code{cmd} values are also declared in the header file
> > +@file{fcntl.h}. To use them, the macro @code{_GNU_SOURCE} must be
> > +defined prior to including any header file.
> > +
> > +In contrast to process-associated locks, any @code{struct flock} used as
> > +an argument to open file description lock commands must have the @code{l_pid}
> > +value set to @math{0}.  Also, when returning information about an
> > +open file description lock in a @code{F_GETLK} or @code{F_OFD_GETLK} request,
> > +the @code{l_pid} field in @code{struct flock} will be set to @math{-1}
> > +to indicate that a lock is not associated with a process.
> > +
> > +When the same @code{struct flock} is reused as an argument to a
> > +@code{F_OFD_SETLK} or @code{F_OFD_SETLKW} request after being used for an
> > +@code{F_OFD_GETLK} request, it is necessary to inspect and reset the
> > +@code{l_pid} field to @math{0}.
> > +
> > +@pindex fcntl.h.
> > +
> > +@deftypevr Macro int F_OFD_GETLK
> > +This macro is used as the @var{command} argument to @code{fcntl}, to
> > +specify that it should get information about a lock.  This command
> > +requires a third argument of type @w{@code{struct flock *}} to be passed
> > +to @code{fcntl}, so that the form of the call is:
> > +
> > +@smallexample
> > +fcntl (@var{filedes}, F_OFD_GETLK, @var{lockp})
> > +@end smallexample
> > +
> > +If there is a lock already in place that would block the lock described
> > +by the @var{lockp} argument, information about that lock is written to
> > +@code{*@var{lockp}}.  Existing locks are not reported if they are
> > +compatible with making a new lock as specified.  Thus, you should
> > +specify a lock type of @code{F_WRLCK} if you want to find out about both
> > +read and write locks, or @code{F_RDLCK} if you want to find out about
> > +write locks only.
> > +
> > +There might be more than one lock affecting the region specified by the
> > +@var{lockp} argument, but @code{fcntl} only returns information about
> > +one of them. Which lock is returned in this situation is undefined.
> > +
> > +The @code{l_whence} member of the @var{lockp} structure are set to
> > +@code{SEEK_SET} and the @code{l_start} and @code{l_len} fields set to identify
> > +the locked region.
> > +
> > +If no conflicting lock exists, the only change to the @var{lockp} structure
> > +is to update the @code{l_type} field to the value @code{F_UNLCK}.
> > +
> > +The normal return value from @code{fcntl} with this command is either @math{0}
> > +on success or @math{-1}, which indicates an error. The following @code{errno}
> > +error conditions are defined for this command:
> > +
> > +@table @code
> > +@item EBADF
> > +The @var{filedes} argument is invalid.
> > +
> > +@item EINVAL
> > +Either the @var{lockp} argument doesn't specify valid lock information,
> > +the operating system kernel doesn't support open file description locks, or the file
> > +associated with @var{filedes} doesn't support locks.
> > +@end table
> > +@end deftypevr
> > +
> > +@comment fcntl.h
> > +@comment POSIX.1
> > +@deftypevr Macro int F_OFD_SETLK
> > +This macro is used as the @var{command} argument to @code{fcntl}, to
> > +specify that it should set or clear a lock.  This command requires a
> > +third argument of type @w{@code{struct flock *}} to be passed to
> > +@code{fcntl}, so that the form of the call is:
> > +
> > +@smallexample
> > +fcntl (@var{filedes}, F_OFD_SETLK, @var{lockp})
> > +@end smallexample
> > +
> > +If the open file already has a lock on any part of the
> > +region, the old lock on that part is replaced with the new lock.  You
> > +can remove a lock by specifying a lock type of @code{F_UNLCK}.
> > +
> > +If the lock cannot be set, @code{fcntl} returns immediately with a value
> > +of @math{-1}.  This function does not wait for other tasks
> > +to release locks.  If @code{fcntl} succeeds, it returns @math{0}.
> > +
> > +The following @code{errno} error conditions are defined for this
> > +command:
> > +
> > +@table @code
> > +@item EAGAIN
> > +The lock cannot be set because it is blocked by an existing lock on the
> > +file.
> > +
> > +@item EBADF
> > +Either: the @var{filedes} argument is invalid; you requested a read lock
> > +but the @var{filedes} is not open for read access; or, you requested a
> > +write lock but the @var{filedes} is not open for write access.
> > +
> > +@item EINVAL
> > +Either the @var{lockp} argument doesn't specify valid lock information,
> > +the operating system kernel doesn't support open file description locks, or the
> > +file associated with @var{filedes} doesn't support locks.
> > +
> > +@item ENOLCK
> > +The system has run out of file lock resources; there are already too
> > +many file locks in place.
> > +
> > +Well-designed file systems never report this error, because they have no
> > +limitation on the number of locks.  However, you must still take account
> > +of the possibility of this error, as it could result from network access
> > +to a file system on another machine.
> > +@end table
> > +@end deftypevr
> > +
> > +@comment fcntl.h
> > +@comment POSIX.1
> > +@deftypevr Macro int F_OFD_SETLKW
> > +This macro is used as the @var{command} argument to @code{fcntl}, to
> > +specify that it should set or clear a lock.  It is just like the
> > +@code{F_OFD_SETLK} command, but causes the process to wait until the request
> > +can be completed.
> > +
> > +This command requires a third argument of type @code{struct flock *}, as
> > +for the @code{F_OFD_SETLK} command.
> > +
> > +The @code{fcntl} return values and errors are the same as for the
> > +@code{F_OFD_SETLK} command, but these additional @code{errno} error conditions
> > +are defined for this command:
> > +
> > +@table @code
> > +@item EINTR
> > +The function was interrupted by a signal while it was waiting.
> > +@xref{Interrupted Primitives}.
> > +
> > +@end table
> > +@end deftypevr
> > +
> > +Open file description locks are useful in the same sorts of situations as
> > +process-associated locks. They can also be used to synchronize file
> > +access between threads within the same process by having each thread perform
> > +its own @code{open} of the file, to obtain its own open file description.
> > +
> > +Because open file description locks are automatically freed only upon
> > +closing the last file descriptor that refers to the open file
> > +description, this locking mechanism avoids the possibility that locks
> > +are inadvertantly released due to a library routine opening and closing
> > +a file without the application being aware.
> > +
> > +As with process-associated locks, open file description locks are advisory.
> > +
> > +@node Open File Description Locks Example
> > +@section Open File Description Locks Example
> > +
> > +Here is an example of using open file description locks in a threaded
> > +program. If this program used process-associated locks, then it would be
> > +subject to data corruption because process-associated locks are shared
> > +by the threads inside a process, and thus cannot be used by one thread
> > +to lock out another thread in the same process.
> > +
> > +Proper error handling has been omitted in the following program for
> > +brevity.
> > +
> > +@smallexample
> > +@include ofdlocks.c.texi
> > +@end smallexample
> > +
> > +This example creates three threads each of which loops five times,
> > +appending to the file. Access to the file is serialized via open file
> > +description locks. If we compile and run the above program, we'll end up
> > +with /tmp/foo that has 15 lines in it.
> > +
> > +If we, however, were to replace the @code{F_OFD_SETLK} and
> > +@code{F_OFD_SETLKW} commands with their process-associated lock
> > +equivalents, the locking essentially becomes a noop since it is all done
> > +within the context of the same process. That leads to data corruption
> > +(typically manifested as missing lines) as some threads race in and
> > +overwrite the data written by others.
> > +
> >  @node Interrupt Input
> >  @section Interrupt-Driven Input
> >  
> > diff --git a/sysdeps/unix/sysv/linux/bits/fcntl-linux.h b/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
> > index 915eb3ede560..455389cd2c2a 100644
> > --- a/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
> > +++ b/sysdeps/unix/sysv/linux/bits/fcntl-linux.h
> > @@ -117,6 +117,23 @@
> >  # define F_SETLKW64	14	/* Set record locking info (blocking).	*/
> >  #endif
> >  
> > +/* open file description locks.
> > +
> > +   Usually record locks held by a process are released on *any* close and are
> > +   not inherited across a fork.
> > +
> > +   These cmd values will set locks that conflict with process-associated record
> > +   locks, but are "owned" by the opened file description, not the process.
> > +   This means that they are inherited across fork or clone with CLONE_FILES
> > +   like BSD (flock) locks, and they are only released automatically when the
> > +   last reference to the the file description against which they were acquired
> > +   is put. */
> > +#if __USE_GNU
> > +# define F_OFD_GETLK	36
> > +# define F_OFD_SETLK	37
> > +# define F_OFD_SETLKW	38
> > +#endif
> > +
> >  #ifdef __USE_LARGEFILE64
> >  # define O_LARGEFILE __O_LARGEFILE
> >  #endif
> > 
> 
> 


-- 
Jeff Layton <jlayton@redhat.com>