Re: [PATCH resend #7] fcntl-linux.h: add new definitions and manual updates for open file description locks

From: Jeff Layton
Date: Wed Jul 30 2014 - 07:02:01 EST

On Tue, 29 Jul 2014 23:30:36 -0400
"Carlos O'Donell" <carlos@xxxxxxxxxx> wrote:

> On 07/23/2014 02:21 PM, Jeff Layton wrote:
> > From: Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
>
> Thanks for resending. Sorry for the delay.
>
> Your use of 3 different emails caused me to miss the recent
> resends. That's my fault and tied to the way I'm tracking
> everything from patchwork using the first email you used.
>

Sorry about that. I was hoping to get this wrapped up before changing
jobs, but that obviously didn't happen...

> I am assuming that this work is under Red Hat's copyright
> status since you submitted it originally from @redhat.com.
>

Yeah, that should be fine. This work was all done under the aegis of
Red Hat. The small tweaks you mention below are also OK. Thanks for
getting this merged!


> I've committed this to trunk with small tweaks and the following
> NEWS entry:
>
> * Support for file description locks is added to systems running the
> Linux kernel. The standard file locking interfaces are extended to
> operate on file descriptions, not file descriptors, via the use of
> F_OFD_GETLK, F_OFD_SETLK, and F_OFD_SETLKW. File description locks
> are associated with an open file instead of a process.
>
> This will be in 2.20 when we cut the branch.
>
> > 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@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.
>
> I simplified it a bit.
>
> A space in the ChangeLog indicates a distinct commit, which this is
> not, so I lump them together.
>
> This is what went in:
>
> 2014-07-29 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: Define
> F_OFD_GETLK, F_OFD_SETLK, and F_OFD_SETLKW.
>
> As the committer I added #16839 to the fixed bug list following:
>
> https://sourceware.org/glibc/wiki/Committer%20checklist
>
> > ---
> > 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
>
> This was changed back to `#ifdef' given Joseph's request to avoid
> partial conversion of macro usages.
>
> > +# 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
> >
>
> Cheers,
> Carlos.


--
Jeff Layton <jlayton@xxxxxxxxxxxxxxx>
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