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/ -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
