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

From: Michael Kerrisk (man-pages)
Date: Thu Apr 24 2014 - 11:35:44 EST

On Thu, Apr 24, 2014 at 5:11 PM, Jeff Layton <jlayton@xxxxxxxxxx> wrote:
> On Thu, 24 Apr 2014 17:07:11 +0200
> "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx> wrote:
>
>> 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
>>
>>
>
> It's a little hard to do that specifically since much of what changed
> were trivial s/a/an/ type changes. But basically I just incorporated
> your changes and fixed a few other places.

I think you missed my point. At the moment, there is no feedback loop.
Either reply to my mail or just add a note such as:

* Incorporated changes after Michael Kerrisk's comments on v2 patch

In the next patch revision mail.

> Since this is already so close, what might be best is to just ask the
> glibc folks to merge what we have, and then if there are other changes
> that are needed later, we can base patches on top of it?

I don't think I'll have more comments. The text looks pretty close to
done to me. Good work!

Cheers,

Michael

>> On Thu, Apr 24, 2014 at 4:09 PM, Jeff Layton <jlayton@xxxxxxxxxx> 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@xxxxxxxxxx>
>> >
>> > [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
>> >
>>
>>
>>
>
>
> --
> Jeff Layton <jlayton@xxxxxxxxxx>



--
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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