Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756225AbaDXMlS (ORCPT ); Thu, 24 Apr 2014 08:41:18 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55454 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753315AbaDXMlQ (ORCPT ); Thu, 24 Apr 2014 08:41:16 -0400 Date: Thu, 24 Apr 2014 08:39:01 -0400 From: Jeff Layton To: "Michael Kerrisk (man-pages)" Cc: libc-alpha@sourceware.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, "Theodore Ts'o" , Rich Felker , samba-technical@lists.samba.org, Ganesha NFS List , "Carlos O'Donell" , "Stefan (metze) Metzmacher" , Christoph Hellwig , Boaz Harrosh , "Alfred M. Szmidt" Subject: Re: [glibc PATCH v2] fcntl-linux.h: add new definitions and manual updates for open file description locks Message-ID: <20140424083901.097bcc50@tlielax.poochiereds.net> In-Reply-To: <5359042F.2050702@gmail.com> References: <1398341744-10249-1-git-send-email-jlayton@redhat.com> <5359042F.2050702@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 24 Apr 2014 14:31:43 +0200 "Michael Kerrisk (man-pages)" wrote: > Jeff, > > Did you receive my mail with comments on the previous patch? > (I got no reply.) It looks like some of those comments that needed to > be addressed were not. > > Cheers, > > Michael > I did get the mail and thought I had addressed them all. The only thing I didn't change based on your comments was the one about the "return 0" in main() being unnecessary in the example program. That's not true since main is an int return function. I suppose we could turn it into a void return, but does it really matter here? > On 04/24/2014 02:15 PM, Jeff Layton 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 > > > > [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 . > > +*/ > > + > > +#define _GNU_SOURCE > > +#include > > +#include > > +#include > > +#include > > +#include > > +#include > > + > > +#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..764f67d469cc 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 a open file description lock. @xref{Open File Description Locks}. > > +Specific to Linux. > > + > > +@item F_OFD_SETLK > > +Set or clear a file lock. @xref{Open File Description Locks}. > > +Specific to Linux. > > + > > +@item F_OFD_SETLKW > > +Like @code{F_OFD_SETLK}, but wait for completion. > > +@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 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 > > +@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 > > +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 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 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{cmd} 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 a 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 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 function 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 inadvertantly 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 > > > > -- Jeff Layton -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/