Re: [glibc PATCHi v4] fcntl-linux.h: add new definitions and manual updates for open file description locks

Michael Kerrisk (man-pages)
2014-04-24T15:07:40Z

Jeff,

Please let us know what changed from one version to the next. I've no
idea without looking into the patch itself whether this inorporates my
comments on the v2 patch...

Cheers,

Michael

On Thu, Apr 24, 2014 at 4:09 PM, Jeff Layton <jlayton@redhat.com> 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..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 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
> --
> 1.9.0
>

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