lib/lockfile.c

   1  /* Copyright (c) 2008, 2009, 2010, 2011, 2012, 2013 Nicira, Inc.
   2  *
   3  * Licensed under the Apache License, Version 2.0 (the "License");
   4  * you may not use this file except in compliance with the License.
   5  * You may obtain a copy of the License at:
   6  *
   7  *     http://www.apache.org/licenses/LICENSE-2.0
   8  *
   9  * Unless required by applicable law or agreed to in writing, software
  10  * distributed under the License is distributed on an "AS IS" BASIS,
  11  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  12  * See the License for the specific language governing permissions and
  13  * limitations under the License.
  14  */
  15
  16 #include <config.h>
  17
  18 #include "lockfile.h"
  19
  20 #include <errno.h>
  21 #include <fcntl.h>
  22 #include <stdlib.h>
  23 #include <string.h>
  24 #include <sys/stat.h>
  25 #include <unistd.h>
  26
  27 #include "coverage.h"
  28 #include "hash.h"
  29 #include "hmap.h"
  30 #include "ovs-thread.h"
  31 #include "timeval.h"
  32 #include "util.h"
  33 #include "vlog.h"
  34
  35 VLOG_DEFINE_THIS_MODULE(lockfile);
  36
  37 COVERAGE_DEFINE(lockfile_lock);
  38 COVERAGE_DEFINE(lockfile_error);
  39 COVERAGE_DEFINE(lockfile_unlock);
  40
  41 struct lockfile {
  42     struct hmap_node hmap_node;
  43     char *name;
  44     dev_t device;
  45     ino_t inode;
  46     int fd;
  47 };
  48
  49 /* Lock table.
  50  *
  51  * We have to do this stupid dance because POSIX says that closing *any* file
  52  * descriptor for a file on which a process holds a lock drops *all* locks on
  53  * that file.  That means that we can't afford to open a lockfile more than
  54  * once. */
  55 static struct ovs_mutex lock_table_mutex = OVS_MUTEX_INITIALIZER;
  56 static struct hmap lock_table__ = HMAP_INITIALIZER(&lock_table__);
  57 static struct hmap *const lock_table OVS_GUARDED_BY(lock_table_mutex)
  58     = &lock_table__;
  59
  60 static void lockfile_unhash(struct lockfile *);
  61 static int lockfile_try_lock(const char *name, pid_t *pidp,
  62                              struct lockfile **lockfilep);
  63
  64 /* Returns the name of the lockfile that would be created for locking a file
  65  * named 'filename_'.  The caller is responsible for freeing the returned name,
  66  * with free(), when it is no longer needed. */
  67 char *
  68 lockfile_name(const char *filename_)
  69 {
  70     char *filename;
  71     const char *slash;
  72     char *lockname;
  73
  74     /* If 'filename_' is a symlink, base the name of the lockfile on the
  75      * symlink's target rather than the name of the symlink.  That way, if a
  76      * file is symlinked, but there is no symlink for its lockfile, then there
  77      * is only a single lockfile for both the source and the target of the
  78      * symlink, not one for each. */
  79     filename = follow_symlinks(filename_);
  80     slash = strrchr(filename, '/');
  81     lockname = (slash
  82                 ? xasprintf("%.*s/.%s.~lock~",
  83                             (int) (slash - filename), filename, slash + 1)
  84                 : xasprintf(".%s.~lock~", filename));
  85     free(filename);
  86
  87     return lockname;
  88 }
  89
  90 /* Locks the configuration file against modification by other processes and
  91  * re-reads it from disk.
  92  *
  93  * Returns 0 on success, otherwise a positive errno value.  On success,
  94  * '*lockfilep' is set to point to a new "struct lockfile *" that may be
  95  * unlocked with lockfile_unlock().  On failure, '*lockfilep' is set to
  96  * NULL.  Will not block if the lock cannot be immediately acquired. */
  97 int
  98 lockfile_lock(const char *file, struct lockfile **lockfilep)
  99 {
 100     /* Only exclusive ("write") locks are supported.  This is not a problem
 101      * because the Open vSwitch code that currently uses lock files does so in
 102      * stylized ways such that any number of readers may access a file while it
 103      * is being written. */
 104     char *lock_name;
 105     pid_t pid;
 106     int error;
 107
 108     COVERAGE_INC(lockfile_lock);
 109
 110     lock_name = lockfile_name(file);
 111
 112     ovs_mutex_lock(&lock_table_mutex);
 113     error = lockfile_try_lock(lock_name, &pid, lockfilep);
 114     ovs_mutex_unlock(&lock_table_mutex);
 115
 116     if (error) {
 117         COVERAGE_INC(lockfile_error);
 118         if (error == EACCES) {
 119             error = EAGAIN;
 120         }
 121         if (pid) {
 122             VLOG_WARN("%s: cannot lock file because it is already locked by "
 123                       "pid %ld", lock_name, (long int) pid);
 124         } else {
 125             VLOG_WARN("%s: failed to lock file: %s",
 126                       lock_name, ovs_strerror(error));
 127         }
 128     }
 129
 130     free(lock_name);
 131     return error;
 132 }
 133
 134 /* Unlocks 'lockfile', which must have been created by a call to
 135  * lockfile_lock(), and frees 'lockfile'. */
 136 void
 137 lockfile_unlock(struct lockfile *lockfile)
 138 {
 139     if (lockfile) {
 140         ovs_mutex_lock(&lock_table_mutex);
 141         lockfile_unhash(lockfile);
 142         ovs_mutex_unlock(&lock_table_mutex);
 143
 144         COVERAGE_INC(lockfile_unlock);
 145         free(lockfile->name);
 146         free(lockfile);
 147     }
 148 }
 149
 150 /* Marks all the currently locked lockfiles as no longer locked.  It makes
 151  * sense to call this function after fork(), because a child created by fork()
 152  * does not hold its parents' locks. */
 153 void
 154 lockfile_postfork(void)
 155 {
 156     struct lockfile *lockfile;
 157
 158     ovs_mutex_lock(&lock_table_mutex);
 159     HMAP_FOR_EACH (lockfile, hmap_node, lock_table) {
 160         if (lockfile->fd >= 0) {
 161             VLOG_WARN("%s: child does not inherit lock", lockfile->name);
 162             lockfile_unhash(lockfile);
 163         }
 164     }
 165     ovs_mutex_unlock(&lock_table_mutex);
 166 }
 167 \f
 168 static uint32_t
 169 lockfile_hash(dev_t device, ino_t inode)
 170 {
 171     return hash_bytes(&device, sizeof device,
 172                       hash_bytes(&inode, sizeof inode, 0));
 173 }
 174
 175 static struct lockfile *
 176 lockfile_find(dev_t device, ino_t inode) OVS_REQUIRES(&lock_table_mutex)
 177 {
 178     struct lockfile *lockfile;
 179
 180     HMAP_FOR_EACH_WITH_HASH (lockfile, hmap_node,
 181                              lockfile_hash(device, inode), lock_table) {
 182         if (lockfile->device == device && lockfile->inode == inode) {
 183             return lockfile;
 184         }
 185     }
 186     return NULL;
 187 }
 188
 189 static void
 190 lockfile_unhash(struct lockfile *lockfile) OVS_REQUIRES(&lock_table_mutex)
 191 {
 192     if (lockfile->fd >= 0) {
 193         close(lockfile->fd);
 194         lockfile->fd = -1;
 195         hmap_remove(lock_table, &lockfile->hmap_node);
 196     }
 197 }
 198
 199 static struct lockfile *
 200 lockfile_register(const char *name, dev_t device, ino_t inode, int fd)
 201     OVS_REQUIRES(&lock_table_mutex)
 202 {
 203     struct lockfile *lockfile;
 204
 205     lockfile = lockfile_find(device, inode);
 206     if (lockfile) {
 207         VLOG_ERR("%s: lock file disappeared and reappeared!", name);
 208         lockfile_unhash(lockfile);
 209     }
 210
 211     lockfile = xmalloc(sizeof *lockfile);
 212     lockfile->name = xstrdup(name);
 213     lockfile->device = device;
 214     lockfile->inode = inode;
 215     lockfile->fd = fd;
 216     hmap_insert(lock_table, &lockfile->hmap_node,
 217                 lockfile_hash(device, inode));
 218     return lockfile;
 219 }
 220
 221 static int
 222 lockfile_try_lock(const char *name, pid_t *pidp, struct lockfile **lockfilep)
 223     OVS_REQUIRES(&lock_table_mutex)
 224 {
 225     struct flock l;
 226     struct stat s;
 227     int error;
 228     int fd;
 229
 230     *lockfilep = NULL;
 231     *pidp = 0;
 232
 233     /* Check whether we've already got a lock on that file. */
 234     if (!stat(name, &s)) {
 235         if (lockfile_find(s.st_dev, s.st_ino)) {
 236             return EDEADLK;
 237         }
 238     } else if (errno != ENOENT) {
 239         VLOG_WARN("%s: failed to stat lock file: %s",
 240                   name, ovs_strerror(errno));
 241         return errno;
 242     }
 243
 244     /* Open the lock file. */
 245     fd = open(name, O_RDWR | O_CREAT, 0600);
 246     if (fd < 0) {
 247         VLOG_WARN("%s: failed to open lock file: %s",
 248                   name, ovs_strerror(errno));
 249         return errno;
 250     }
 251
 252     /* Get the inode and device number for the lock table. */
 253     if (fstat(fd, &s)) {
 254         VLOG_ERR("%s: failed to fstat lock file: %s",
 255                  name, ovs_strerror(errno));
 256         close(fd);
 257         return errno;
 258     }
 259
 260     /* Try to lock the file. */
 261     memset(&l, 0, sizeof l);
 262     l.l_type = F_WRLCK;
 263     l.l_whence = SEEK_SET;
 264     l.l_start = 0;
 265     l.l_len = 0;
 266
 267     error = fcntl(fd, F_SETLK, &l) == -1 ? errno : 0;
 268
 269     if (!error) {
 270         *lockfilep = lockfile_register(name, s.st_dev, s.st_ino, fd);
 271     } else {
 272         if (!fcntl(fd, F_GETLK, &l) && l.l_type != F_UNLCK) {
 273             *pidp = l.l_pid;
 274         }
 275         close(fd);
 276     }
 277     return error;
 278 }
 279