From 0961f7e1e300ef633b0c1ad95d0999fb5c169f4e Mon Sep 17 00:00:00 2001 From: Jeff Layton Date: Wed, 23 Jul 2014 14:21:05 -0400 Subject: [PATCH] fcntl-linux.h: add new definitions and manual updates for open file description locks 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 | 9 ++ NEWS | 34 ++-- manual/examples/ofdlocks.c | 77 +++++++++ manual/llio.texi | 241 ++++++++++++++++++++++++++++- sysdeps/unix/sysv/linux/bits/fcntl-linux.h | 17 ++ 5 files changed, 361 insertions(+), 17 deletions(-) create mode 100644 manual/examples/ofdlocks.c diff --git a/ChangeLog b/ChangeLog index e60dfcc..1c263b3 100644 --- a/ChangeLog +++ b/ChangeLog @@ -1,3 +1,12 @@ +2014-07-29 Jeff Layton + + [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. + 2014-07-23 Allan McRae * po/es.po: Update Spanish translation from translation project. diff --git a/NEWS b/NEWS index a88d3e0..d195913 100644 --- a/NEWS +++ b/NEWS @@ -9,20 +9,26 @@ Version 2.20 * The following bugs are resolved with this release: - 6804, 9894, 12994, 13347, 13651, 14308, 14770, 15119, 15132, 15347, 15514, - 15698, 15804, 15894, 15946, 16002, 16064, 16095, 16198, 16284, 16287, - 16315, 16348, 16349, 16354, 16357, 16362, 16447, 16516, 16532, 16539, - 16545, 16561, 16562, 16564, 16574, 16599, 16600, 16609, 16610, 16611, - 16613, 16619, 16623, 16629, 16632, 16634, 16639, 16642, 16648, 16649, - 16670, 16674, 16677, 16680, 16681, 16683, 16689, 16695, 16701, 16706, - 16707, 16712, 16713, 16714, 16724, 16731, 16739, 16740, 16743, 16754, - 16758, 16759, 16760, 16770, 16786, 16789, 16791, 16796, 16799, 16800, - 16815, 16823, 16824, 16831, 16838, 16849, 16854, 16876, 16877, 16878, - 16882, 16885, 16888, 16890, 16912, 16915, 16916, 16917, 16918, 16922, - 16927, 16928, 16932, 16943, 16958, 16965, 16966, 16967, 16977, 16978, - 16984, 16990, 16996, 17009, 17022, 17031, 17042, 17048, 17050, 17058, - 17061, 17062, 17069, 17075, 17078, 17079, 17084, 17086, 17088, 17092, - 17097, 17125, 17135, 17137, 17150, 17153. + 6804, 9894, 12994, 13347, 13651, 14308, 14770, 15119, 15132, 15347, + 15514, 15698, 15804, 15894, 15946, 16002, 16064, 16095, 16198, 16284, + 16287, 16315, 16348, 16349, 16354, 16357, 16362, 16447, 16516, 16532, + 16539, 16545, 16561, 16562, 16564, 16574, 16599, 16600, 16609, 16610, + 16611, 16613, 16619, 16623, 16629, 16632, 16634, 16639, 16642, 16648, + 16649, 16670, 16674, 16677, 16680, 16681, 16683, 16689, 16695, 16701, + 16706, 16707, 16712, 16713, 16714, 16724, 16731, 16739, 16740, 16743, + 16754, 16758, 16759, 16760, 16770, 16786, 16789, 16791, 16796, 16799, + 16800, 16815, 16823, 16824, 16831, 16838, 16839, 16849, 16854, 16876, + 16877, 16878, 16882, 16885, 16888, 16890, 16912, 16915, 16916, 16917, + 16918, 16922, 16927, 16928, 16932, 16943, 16958, 16965, 16966, 16967, + 16977, 16978, 16984, 16990, 16996, 17009, 17022, 17031, 17042, 17048, + 17050, 17058, 17061, 17062, 17069, 17075, 17078, 17079, 17084, 17086, + 17088, 17092, 17097, 17125, 17135, 17137, 17150, 17153. + +* 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. * Optimized strchr implementation for AArch64. Contributed by ARM Ltd. diff --git a/manual/examples/ofdlocks.c b/manual/examples/ofdlocks.c new file mode 100644 index 0000000..85e193c --- /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 6f8adfc..864060d 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 527eb5c..e224dab 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. */ +#ifdef __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 -- 2.7.4