Mon Mar 19 11:30:27 2012

Asterisk developer's documentation


func_lock.c

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00001 /*
00002  * Asterisk -- An open source telephony toolkit.
00003  *
00004  * Copyright (C) 2007, Tilghman Lesher
00005  *
00006  * Tilghman Lesher <func_lock_2007@the-tilghman.com>
00007  *
00008  * See http://www.asterisk.org for more information about
00009  * the Asterisk project. Please do not directly contact
00010  * any of the maintainers of this project for assistance;
00011  * the project provides a web site, mailing lists and IRC
00012  * channels for your use.
00013  *
00014  * This program is free software, distributed under the terms of
00015  * the GNU General Public License Version 2. See the LICENSE file
00016  * at the top of the source tree.
00017  */
00018 
00019 /*! \file
00020  *
00021  * \brief Dialplan mutexes
00022  *
00023  * \author Tilghman Lesher <func_lock_2007@the-tilghman.com>
00024  *
00025  * \ingroup functions
00026  * 
00027  */
00028 
00029 /*** MODULEINFO
00030    <support_level>core</support_level>
00031  ***/
00032 
00033 #include "asterisk.h"
00034 
00035 ASTERISK_FILE_VERSION(__FILE__, "$Revision: 350311 $")
00036 
00037 #include <signal.h>
00038 
00039 #include "asterisk/lock.h"
00040 #include "asterisk/file.h"
00041 #include "asterisk/channel.h"
00042 #include "asterisk/pbx.h"
00043 #include "asterisk/module.h"
00044 #include "asterisk/linkedlists.h"
00045 #include "asterisk/astobj2.h"
00046 #include "asterisk/utils.h"
00047 
00048 /*** DOCUMENTATION
00049    <function name="LOCK" language="en_US">
00050       <synopsis>
00051          Attempt to obtain a named mutex.
00052       </synopsis>
00053       <syntax>
00054          <parameter name="lockname" required="true" />
00055       </syntax>
00056       <description>
00057          <para>Attempts to grab a named lock exclusively, and prevents other channels from
00058          obtaining the same lock.  LOCK will wait for the lock to become available.
00059          Returns <literal>1</literal> if the lock was obtained or <literal>0</literal> on error.</para>
00060          <note><para>To avoid the possibility of a deadlock, LOCK will only attempt to
00061          obtain the lock for 3 seconds if the channel already has another lock.</para></note>
00062       </description>
00063    </function>
00064    <function name="TRYLOCK" language="en_US">
00065       <synopsis>
00066          Attempt to obtain a named mutex.
00067       </synopsis>
00068       <syntax>
00069          <parameter name="lockname" required="true" />
00070       </syntax>
00071       <description>
00072          <para>Attempts to grab a named lock exclusively, and prevents other channels
00073          from obtaining the same lock.  Returns <literal>1</literal> if the lock was 
00074          available or <literal>0</literal> otherwise.</para>
00075       </description>
00076    </function>
00077    <function name="UNLOCK" language="en_US">
00078       <synopsis>
00079          Unlocks a named mutex.
00080       </synopsis>
00081       <syntax>
00082          <parameter name="lockname" required="true" />
00083       </syntax>
00084       <description>
00085          <para>Unlocks a previously locked mutex. Returns <literal>1</literal> if the channel 
00086          had a lock or <literal>0</literal> otherwise.</para>
00087          <note><para>It is generally unnecessary to unlock in a hangup routine, as any locks 
00088          held are automatically freed when the channel is destroyed.</para></note>
00089       </description>
00090    </function>
00091  ***/
00092 
00093 
00094 
00095 static AST_LIST_HEAD_STATIC(locklist, lock_frame);
00096 
00097 static void lock_free(void *data);
00098 static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_channel *newchan);
00099 static int unloading = 0;
00100 static pthread_t broker_tid = AST_PTHREADT_NULL;
00101 
00102 static struct ast_datastore_info lock_info = {
00103    .type = "MUTEX",
00104    .destroy = lock_free,
00105    .chan_fixup = lock_fixup,
00106 };
00107 
00108 struct lock_frame {
00109    AST_LIST_ENTRY(lock_frame) entries;
00110    ast_mutex_t mutex;
00111    ast_cond_t cond;
00112    /*! count is needed so if a recursive mutex exits early, we know how many times to unlock it. */
00113    unsigned int count;
00114    /*! Container of requesters for the named lock */
00115    struct ao2_container *requesters;
00116    /*! who owns us */
00117    struct ast_channel *owner;
00118    /*! name of the lock */
00119    char name[0];
00120 };
00121 
00122 struct channel_lock_frame {
00123    AST_LIST_ENTRY(channel_lock_frame) list;
00124    /*! Need to save channel pointer here, because during destruction, we won't have it. */
00125    struct ast_channel *channel;
00126    struct lock_frame *lock_frame;
00127 };
00128 
00129 static void lock_free(void *data)
00130 {
00131    AST_LIST_HEAD(, channel_lock_frame) *oldlist = data;
00132    struct channel_lock_frame *clframe;
00133    AST_LIST_LOCK(oldlist);
00134    while ((clframe = AST_LIST_REMOVE_HEAD(oldlist, list))) {
00135       /* Only unlock if we own the lock */
00136       if (clframe->channel == clframe->lock_frame->owner) {
00137          clframe->lock_frame->count = 0;
00138          clframe->lock_frame->owner = NULL;
00139       }
00140       ast_free(clframe);
00141    }
00142    AST_LIST_UNLOCK(oldlist);
00143    AST_LIST_HEAD_DESTROY(oldlist);
00144    ast_free(oldlist);
00145 }
00146 
00147 static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_channel *newchan)
00148 {
00149    struct ast_datastore *lock_store = ast_channel_datastore_find(oldchan, &lock_info, NULL);
00150    AST_LIST_HEAD(, channel_lock_frame) *list;
00151    struct channel_lock_frame *clframe = NULL;
00152 
00153    if (!lock_store) {
00154       return;
00155    }
00156    list = lock_store->data;
00157 
00158    AST_LIST_LOCK(list);
00159    AST_LIST_TRAVERSE(list, clframe, list) {
00160       if (clframe->lock_frame->owner == oldchan) {
00161          clframe->lock_frame->owner = newchan;
00162       }
00163       /* We don't move requesters, because the thread stack is different */
00164       clframe->channel = newchan;
00165    }
00166    AST_LIST_UNLOCK(list);
00167 }
00168 
00169 static void *lock_broker(void *unused)
00170 {
00171    struct lock_frame *frame;
00172    struct timespec forever = { 1000000, 0 };
00173    for (;;) {
00174       int found_requester = 0;
00175 
00176       /* Test for cancel outside of the lock */
00177       pthread_testcancel();
00178       AST_LIST_LOCK(&locklist);
00179 
00180       AST_LIST_TRAVERSE(&locklist, frame, entries) {
00181          if (ao2_container_count(frame->requesters)) {
00182             found_requester++;
00183             ast_mutex_lock(&frame->mutex);
00184             if (!frame->owner) {
00185                ast_cond_signal(&frame->cond);
00186             }
00187             ast_mutex_unlock(&frame->mutex);
00188          }
00189       }
00190 
00191       AST_LIST_UNLOCK(&locklist);
00192       pthread_testcancel();
00193 
00194       /* If there are no requesters, then wait for a signal */
00195       if (!found_requester) {
00196          nanosleep(&forever, NULL);
00197       } else {
00198          sched_yield();
00199       }
00200    }
00201    /* Not reached */
00202    return NULL;
00203 }
00204 
00205 static int ast_channel_hash_cb(const void *obj, const int flags)
00206 {
00207    const struct ast_channel *chan = obj;
00208    return ast_str_case_hash(chan->name);
00209 }
00210 
00211 static int ast_channel_cmp_cb(void *obj, void *arg, int flags)
00212 {
00213    struct ast_channel *chan = obj, *cmp_args = arg;
00214    return strcasecmp(chan->name, cmp_args->name) ? 0 : CMP_MATCH;
00215 }
00216 
00217 static int get_lock(struct ast_channel *chan, char *lockname, int trylock)
00218 {
00219    struct ast_datastore *lock_store = ast_channel_datastore_find(chan, &lock_info, NULL);
00220    struct lock_frame *current;
00221    struct channel_lock_frame *clframe = NULL;
00222    AST_LIST_HEAD(, channel_lock_frame) *list;
00223    int res = 0;
00224    struct timespec timeout = { 0, };
00225    struct timeval now;
00226 
00227    if (!lock_store) {
00228       ast_debug(1, "Channel %s has no lock datastore, so we're allocating one.\n", chan->name);
00229       lock_store = ast_datastore_alloc(&lock_info, NULL);
00230       if (!lock_store) {
00231          ast_log(LOG_ERROR, "Unable to allocate new datastore.  No locks will be obtained.\n");
00232          return -1;
00233       }
00234 
00235       list = ast_calloc(1, sizeof(*list));
00236       if (!list) {
00237          ast_log(LOG_ERROR,
00238             "Unable to allocate datastore list head.  %sLOCK will fail.\n",
00239             trylock ? "TRY" : "");
00240          ast_datastore_free(lock_store);
00241          return -1;
00242       }
00243 
00244       lock_store->data = list;
00245       AST_LIST_HEAD_INIT(list);
00246       ast_channel_datastore_add(chan, lock_store);
00247    } else
00248       list = lock_store->data;
00249 
00250    /* Lock already exists? */
00251    AST_LIST_LOCK(&locklist);
00252    AST_LIST_TRAVERSE(&locklist, current, entries) {
00253       if (strcmp(current->name, lockname) == 0) {
00254          break;
00255       }
00256    }
00257 
00258    if (!current) {
00259       if (unloading) {
00260          /* Don't bother */
00261          AST_LIST_UNLOCK(&locklist);
00262          return -1;
00263       }
00264 
00265       /* Create new lock entry */
00266       current = ast_calloc(1, sizeof(*current) + strlen(lockname) + 1);
00267       if (!current) {
00268          AST_LIST_UNLOCK(&locklist);
00269          return -1;
00270       }
00271 
00272       strcpy(current->name, lockname); /* SAFE */
00273       if ((res = ast_mutex_init(&current->mutex))) {
00274          ast_log(LOG_ERROR, "Unable to initialize mutex: %s\n", strerror(res));
00275          ast_free(current);
00276          AST_LIST_UNLOCK(&locklist);
00277          return -1;
00278       }
00279       if ((res = ast_cond_init(&current->cond, NULL))) {
00280          ast_log(LOG_ERROR, "Unable to initialize condition variable: %s\n", strerror(res));
00281          ast_mutex_destroy(&current->mutex);
00282          ast_free(current);
00283          AST_LIST_UNLOCK(&locklist);
00284          return -1;
00285       }
00286       if (!(current->requesters = ao2_container_alloc(1, ast_channel_hash_cb, ast_channel_cmp_cb))) {
00287          ast_mutex_destroy(&current->mutex);
00288          ast_cond_destroy(&current->cond);
00289          ast_free(current);
00290          AST_LIST_UNLOCK(&locklist);
00291          return -1;
00292       }
00293       AST_LIST_INSERT_TAIL(&locklist, current, entries);
00294    }
00295    AST_LIST_UNLOCK(&locklist);
00296 
00297    /* Found lock or created one - now find or create the corresponding link in the channel */
00298    AST_LIST_LOCK(list);
00299    AST_LIST_TRAVERSE(list, clframe, list) {
00300       if (clframe->lock_frame == current) {
00301          break;
00302       }
00303    }
00304 
00305    if (!clframe) {
00306       if (unloading) {
00307          /* Don't bother */
00308          AST_LIST_UNLOCK(list);
00309          return -1;
00310       }
00311 
00312       if (!(clframe = ast_calloc(1, sizeof(*clframe)))) {
00313          ast_log(LOG_ERROR,
00314             "Unable to allocate channel lock frame.  %sLOCK will fail.\n",
00315             trylock ? "TRY" : "");
00316          AST_LIST_UNLOCK(list);
00317          return -1;
00318       }
00319 
00320       clframe->lock_frame = current;
00321       clframe->channel = chan;
00322       AST_LIST_INSERT_TAIL(list, clframe, list);
00323    }
00324    AST_LIST_UNLOCK(list);
00325 
00326    /* If we already own the lock, then we're being called recursively.
00327     * Keep track of how many times that is, because we need to unlock
00328     * the same amount, before we'll release this one.
00329     */
00330    if (current->owner == chan) {
00331       current->count++;
00332       return 0;
00333    }
00334 
00335    /* Okay, we have both frames, so now we need to try to lock.
00336     *
00337     * Locking order: always lock locklist first.  We need the
00338     * locklist lock because the broker thread counts whether
00339     * there are requesters with the locklist lock held, and we
00340     * need to hold it, so that when we send our signal, below,
00341     * to wake up the broker thread, it definitely will see that
00342     * a requester exists at that point in time.  Otherwise, we
00343     * could add to the requesters after it has already seen that
00344     * that lock is unoccupied and wait forever for another signal.
00345     */
00346    AST_LIST_LOCK(&locklist);
00347    ast_mutex_lock(&current->mutex);
00348    /* Add to requester list */
00349    ao2_link(current->requesters, chan);
00350    pthread_kill(broker_tid, SIGURG);
00351    AST_LIST_UNLOCK(&locklist);
00352 
00353    /* Wait up to three seconds from now for LOCK. */
00354    now = ast_tvnow();
00355    timeout.tv_sec = now.tv_sec + 3;
00356    timeout.tv_nsec = now.tv_usec * 1000;
00357 
00358    if (!current->owner
00359       || (!trylock
00360          && !(res = ast_cond_timedwait(&current->cond, &current->mutex, &timeout)))) {
00361       res = 0;
00362       current->owner = chan;
00363       current->count++;
00364    } else {
00365       res = -1;
00366    }
00367    /* Remove from requester list */
00368    ao2_unlink(current->requesters, chan);
00369    ast_mutex_unlock(&current->mutex);
00370 
00371    return res;
00372 }
00373 
00374 static int unlock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
00375 {
00376    struct ast_datastore *lock_store = ast_channel_datastore_find(chan, &lock_info, NULL);
00377    struct channel_lock_frame *clframe;
00378    AST_LIST_HEAD(, channel_lock_frame) *list;
00379 
00380    if (!lock_store) {
00381       ast_log(LOG_WARNING, "No datastore for dialplan locks.  Nothing was ever locked!\n");
00382       ast_copy_string(buf, "0", len);
00383       return 0;
00384    }
00385 
00386    if (!(list = lock_store->data)) {
00387       ast_debug(1, "This should NEVER happen\n");
00388       ast_copy_string(buf, "0", len);
00389       return 0;
00390    }
00391 
00392    /* Find item in the channel list */
00393    AST_LIST_LOCK(list);
00394    AST_LIST_TRAVERSE(list, clframe, list) {
00395       if (clframe->lock_frame && clframe->lock_frame->owner == chan && strcmp(clframe->lock_frame->name, data) == 0) {
00396          break;
00397       }
00398    }
00399    /* We never destroy anything until channel destruction, which will never
00400     * happen while this routine is executing, so we don't need to hold the
00401     * lock beyond this point. */
00402    AST_LIST_UNLOCK(list);
00403 
00404    if (!clframe) {
00405       /* We didn't have this lock in the first place */
00406       ast_copy_string(buf, "0", len);
00407       return 0;
00408    }
00409 
00410    if (--clframe->lock_frame->count == 0) {
00411       clframe->lock_frame->owner = NULL;
00412    }
00413 
00414    ast_copy_string(buf, "1", len);
00415    return 0;
00416 }
00417 
00418 static int lock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
00419 {
00420    if (chan)
00421       ast_autoservice_start(chan);
00422 
00423    ast_copy_string(buf, get_lock(chan, data, 0) ? "0" : "1", len);
00424 
00425    if (chan)
00426       ast_autoservice_stop(chan);
00427 
00428    return 0;
00429 }
00430 
00431 static int trylock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
00432 {
00433    if (chan)
00434       ast_autoservice_start(chan);
00435 
00436    ast_copy_string(buf, get_lock(chan, data, 1) ? "0" : "1", len);
00437 
00438    if (chan)
00439       ast_autoservice_stop(chan);
00440 
00441    return 0;
00442 }
00443 
00444 static struct ast_custom_function lock_function = {
00445    .name = "LOCK",
00446    .read = lock_read,
00447    .read_max = 2,
00448 };
00449 
00450 static struct ast_custom_function trylock_function = {
00451    .name = "TRYLOCK",
00452    .read = trylock_read,
00453    .read_max = 2,
00454 };
00455 
00456 static struct ast_custom_function unlock_function = {
00457    .name = "UNLOCK",
00458    .read = unlock_read,
00459    .read_max = 2,
00460 };
00461 
00462 static int unload_module(void)
00463 {
00464    struct lock_frame *current;
00465 
00466    /* Module flag */
00467    unloading = 1;
00468 
00469    AST_LIST_LOCK(&locklist);
00470    while ((current = AST_LIST_REMOVE_HEAD(&locklist, entries))) {
00471       /* If any locks are currently in use, then we cannot unload this module */
00472       if (current->owner || ao2_container_count(current->requesters)) {
00473          /* Put it back */
00474          AST_LIST_INSERT_HEAD(&locklist, current, entries);
00475          AST_LIST_UNLOCK(&locklist);
00476          unloading = 0;
00477          return -1;
00478       }
00479       ast_mutex_destroy(&current->mutex);
00480       ao2_ref(current->requesters, -1);
00481       ast_free(current);
00482    }
00483 
00484    /* No locks left, unregister functions */
00485    ast_custom_function_unregister(&lock_function);
00486    ast_custom_function_unregister(&trylock_function);
00487    ast_custom_function_unregister(&unlock_function);
00488 
00489    pthread_cancel(broker_tid);
00490    pthread_kill(broker_tid, SIGURG);
00491    pthread_join(broker_tid, NULL);
00492 
00493    AST_LIST_UNLOCK(&locklist);
00494 
00495    return 0;
00496 }
00497 
00498 static int load_module(void)
00499 {
00500    int res = ast_custom_function_register(&lock_function);
00501    res |= ast_custom_function_register(&trylock_function);
00502    res |= ast_custom_function_register(&unlock_function);
00503    ast_pthread_create_background(&broker_tid, NULL, lock_broker, NULL);
00504    return res;
00505 }
00506 
00507 AST_MODULE_INFO_STANDARD(ASTERISK_GPL_KEY, "Dialplan mutexes");

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