Re: [PATCH resend #4] fcntl-linux.h: add new definitions and manual updates for open file description locks

[Michael Kerrisk (man-pages)]

On 06/26/2014 09:57 PM, Jeff Layton wrote:
> From: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
> 
> 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.

What's the hold-up here? Jeff's patches have been reviewed by at least
me and Carlos. It'd be nice if they made it in before the 2.20 code freeze...

Cheers,

Michael


> ChangeLog:
> 
> 2014-04-24  Jeff Layton  <jlayton@xxxxxxxxxxxxxxx>
> 
> 	[BZ#16839]
> 	* manual/llio.texi: add section about open file description locks
> 
> 	* manual/examples/ofdlocks.c:
> 	  example of open file description lock usage
> 
> 	* 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..864060dc7140 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 an open file description lock.  @xref{Open File Description Locks}.
> +Specific to Linux.
> +
> +@item F_OFD_SETLK
> +Set or clear an open file description lock.  @xref{Open File Description Locks}.
> +Specific to Linux.
> +
> +@item F_OFD_SETLKW
> +Like @code{F_OFD_SETLK}, but block until lock is acquired.
> +@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 an 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 open file @emph{description} (an
> +instance of an open file, usually created by a call to @code{open}) and
> +an open file @emph{descriptor}, which is a numeric value that refers to the
> +open file description.  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 open 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{command} 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 the 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 are 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 command 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 inadvertently 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 527eb5c8b5d9..b5b5d0fbfa4c 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
> 


-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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