[RFC][glibc PATCH] fcntl-linux.h: add new definitions and manual updates for open file description locks
Michael Kerrisk (man-pages)
mtk.manpages at gmail.com
Wed Apr 23 06:37:24 MDT 2014
Hi Jeff,
Comments below.
On Wed, Apr 23, 2014 at 1:39 PM, Jeff Layton <jlayton at redhat.com> wrote:
> (I haven't yet sent the patch to rename these to Linus, but here's a
> new draft version of the glibc header/manual updates for open file
> description locks. Please speak up if you see any issues or object
> to the new name.)
>
> 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:
>
> 2014-04-23 Jeff Layton <jlayton at 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 | 75 +++++++++
> manual/llio.texi | 237 ++++++++++++++++++++++++++++-
> sysdeps/unix/sysv/linux/bits/fcntl-linux.h | 17 +++
> 3 files changed, 326 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..544001769272
> --- /dev/null
> +++ b/manual/examples/ofdlocks.c
> @@ -0,0 +1,75 @@
> +/* 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, if not, see <http://www.gnu.org/licenses/>.
s/if not, if not/if not/
> +*/
> +
> +#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;
s/)/) /
> + 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);
s/|/ | /
> +
> + for (i = 0; i < ITERATIONS; i++) {
I am no lover of the GNU coding style, but I suspect this program
should be respecting it with respect to the brace style...
> + 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);
> +
> + usleep(1);
I think the usleep() call needs to be either removed or explained.
> + }
> + pthread_exit(NULL);
> +}
> +
> +int
> +main(int argc, char **argv)
> +{
> + long i;
> + pthread_t threads[NUM_THREADS];
Why the large white space gap?
> +
> + truncate(FILENAME, 0);
> +
> + for (i = 0; i < NUM_THREADS; i++)
> + pthread_create(&threads[i], NULL, thread_start, (void *)i);
> +
> + pthread_exit(NULL);
> + return 0;
The "return 0" isn't needed.
> +}
> diff --git a/manual/llio.texi b/manual/llio.texi
> index 6f8adfc607d7..b5c2d07ad929 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}.
>
> + at item F_OFD_GETLK
> +Test a open file description lock. @xref{Open File Description Locks}.
> +Specific to Linux.
> +
> + at item F_OFD_SETLK
> +Set or clear a file lock. @xref{Open File Description Locks}.
> +Specific to Linux.
> +
> + at item F_OFD_SETLKW
> +Like @code{F_OFD_SETLK}, but wait for completion.
> + at 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 description instead of the process. @xref{Open File Description Locks}.
s/open/open file/
> +
> 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
> + at math{-1}.
> @end table
> @end deftp
>
> @@ -3813,10 +3836,218 @@ 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.
>
> + at node Open File Description Locks
> + at 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
> + at 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
s/file/open file/
> +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 file
I'd s/former/open file description/ for clarity
and
s/file/open file/
> + at emph{description} and not the file @emph{descriptor}.
> +
> +Using @code{dup} (@pxref{Duplicating Descriptors}) to copy a file
> +descriptor does not give you an entirely new file description , but
s/entirely//
(it's redundant, and possibly even confusing.)
> +rather copies a reference to an existing file description and assigns it
s/file/open file/
> +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 file description. Depending on the range and type of lock
s/file/open file/
> +involved, the original lock may be modified by a @code{F_OFD_SETLK} or
> + at 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 cmd values are also declared in the header file
s/cmd/@code{command}/
> + at file{fcntl.h}. To use them, the macro @code{_GNU_SOURCE} must be
> +defined prior to including @file{fcntl.h}.
s/prior to including @file{fcntl.h}/before including any header file/
(See http://man7.org/linux/man-pages/man7/feature_test_macros.7.html )
> +
> +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 a
s/s/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 struct flock is reused as an argument to a
s/struct flock/@code{struct flock}/
> + at code{F_OFD_SETLK} or @code{F_OFD_SETLKW} request after being used for an
> + at code{F_OFD_GETLK} request, it is necessary to inspect and reset the
> + at code{l_pid} field to @math{0}.
> +
> + at pindex fcntl.h.
> +
> + at 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:
> +
> + at smallexample
> +fcntl (@var{filedes}, F_OFD_GETLK, @var{lockp})
> + at 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 overwrites
s/overwrites/is written to/
> + at 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
> + at 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 is set to
> + at code{SEEK_SET} and the @code{l_start} and @code{l_len} fields set to identify
s/set/are set/
> +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:
> +
> + at table @code
> + at item EBADF
> +The @var{filedes} argument is invalid.
> +
> + at 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.
> + at end table
> + at end deftypevr
> +
> + at comment fcntl.h
> + at comment POSIX.1
> + at 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
> + at code{fcntl}, so that the form of the call is:
> +
> + at smallexample
> +fcntl (@var{filedes}, F_OFD_SETLK, @var{lockp})
> + at end smallexample
> +
> +If the opened file already has a lock on any part of the
s/opened/open/
> +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
s/function/command/
> +to release locks. If @code{fcntl} succeeds, it returns @math{0}.
> +
> +The following @code{errno} error conditions are defined for this
> +function:
> +
> + at table @code
> + at item EAGAIN
> +The lock cannot be set because it is blocked by an existing lock on the
> +file.
> +
> + at 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.
> +
> + at 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.
> +
> + at 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.
> + at end table
> + at end deftypevr
> +
> + at comment fcntl.h
> + at comment POSIX.1
> + at 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
> + at 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
> + at code{F_OFD_SETLK} command, but these additional @code{errno} error conditions
> +are defined for this command:
> +
> + at table @code
> + at item EINTR
> +The function was interrupted by a signal while it was waiting.
> + at xref{Interrupted Primitives}.
> +
> + at end table
> + at 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 file description.
s/file/open file/
> +
> +Because they are only released automatically when the last reference to
> +file description is closed, open file description locks avoid the possibility
s/file description/open file description/
But, I'd rewrite the sentence a little:
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 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.
> +
> + at node Open File Description Locks Example
> + at 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 then it would be
s/then then/then/
> +subject to data corruption due since multiple threads are running within
> +the same process.
That last piece is a biut confusing. How about
...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.
> +
> + at smallexample
> + at include ofdlocks.c.texi
> + at end smallexample
> +
> +This example spawns three threads and has each do five iterations of
> +appending to a file.
Better:
This example creates three threads, each of which loops five times,
appending to the file.
> Access to that file is serialized via open file
s/taht/the/
> +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 from
s/from/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
>
Cheers,
Michael
--
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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