Wed Apr 6 11:29:45 2011

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 #include "asterisk.h"
00030 
00031 ASTERISK_FILE_VERSION(__FILE__, "$Revision: 232012 $")
00032 
00033 #include <signal.h>
00034 
00035 #include "asterisk/lock.h"
00036 #include "asterisk/file.h"
00037 #include "asterisk/channel.h"
00038 #include "asterisk/pbx.h"
00039 #include "asterisk/module.h"
00040 #include "asterisk/linkedlists.h"
00041 #include "asterisk/astobj2.h"
00042 #include "asterisk/utils.h"
00043 
00044 /*** DOCUMENTATION
00045    <function name="LOCK" language="en_US">
00046       <synopsis>
00047          Attempt to obtain a named mutex.
00048       </synopsis>
00049       <syntax>
00050          <parameter name="lockname" required="true" />
00051       </syntax>
00052       <description>
00053          <para>Attempts to grab a named lock exclusively, and prevents other channels from
00054          obtaining the same lock.  LOCK will wait for the lock to become available.
00055          Returns <literal>1</literal> if the lock was obtained or <literal>0</literal> on error.</para>
00056          <note><para>To avoid the possibility of a deadlock, LOCK will only attempt to
00057          obtain the lock for 3 seconds if the channel already has another lock.</para></note>
00058       </description>
00059    </function>
00060    <function name="TRYLOCK" language="en_US">
00061       <synopsis>
00062          Attempt to obtain a named mutex.
00063       </synopsis>
00064       <syntax>
00065          <parameter name="lockname" required="true" />
00066       </syntax>
00067       <description>
00068          <para>Attempts to grab a named lock exclusively, and prevents other channels
00069          from obtaining the same lock.  Returns <literal>1</literal> if the lock was 
00070          available or <literal>0</literal> otherwise.</para>
00071       </description>
00072    </function>
00073    <function name="UNLOCK" language="en_US">
00074       <synopsis>
00075          Unlocks a named mutex.
00076       </synopsis>
00077       <syntax>
00078          <parameter name="lockname" required="true" />
00079       </syntax>
00080       <description>
00081          <para>Unlocks a previously locked mutex. Returns <literal>1</literal> if the channel 
00082          had a lock or <literal>0</literal> otherwise.</para>
00083          <note><para>It is generally unnecessary to unlock in a hangup routine, as any locks 
00084          held are automatically freed when the channel is destroyed.</para></note>
00085       </description>
00086    </function>
00087  ***/
00088 
00089 
00090 
00091 static AST_LIST_HEAD_STATIC(locklist, lock_frame);
00092 
00093 static void lock_free(void *data);
00094 static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_channel *newchan);
00095 static int unloading = 0;
00096 static pthread_t broker_tid = AST_PTHREADT_NULL;
00097 
00098 static struct ast_datastore_info lock_info = {
00099    .type = "MUTEX",
00100    .destroy = lock_free,
00101    .chan_fixup = lock_fixup,
00102 };
00103 
00104 struct lock_frame {
00105    AST_LIST_ENTRY(lock_frame) entries;
00106    ast_mutex_t mutex;
00107    ast_cond_t cond;
00108    /*! count is needed so if a recursive mutex exits early, we know how many times to unlock it. */
00109    unsigned int count;
00110    /*! Container of requesters for the named lock */
00111    struct ao2_container *requesters;
00112    /*! who owns us */
00113    struct ast_channel *owner;
00114    /*! name of the lock */
00115    char name[0];
00116 };
00117 
00118 struct channel_lock_frame {
00119    AST_LIST_ENTRY(channel_lock_frame) list;
00120    /*! Need to save channel pointer here, because during destruction, we won't have it. */
00121    struct ast_channel *channel;
00122    struct lock_frame *lock_frame;
00123 };
00124 
00125 static void lock_free(void *data)
00126 {
00127    AST_LIST_HEAD(, channel_lock_frame) *oldlist = data;
00128    struct channel_lock_frame *clframe;
00129    AST_LIST_LOCK(oldlist);
00130    while ((clframe = AST_LIST_REMOVE_HEAD(oldlist, list))) {
00131       /* Only unlock if we own the lock */
00132       if (clframe->channel == clframe->lock_frame->owner) {
00133          clframe->lock_frame->count = 0;
00134          clframe->lock_frame->owner = NULL;
00135       }
00136       ast_free(clframe);
00137    }
00138    AST_LIST_UNLOCK(oldlist);
00139    AST_LIST_HEAD_DESTROY(oldlist);
00140    ast_free(oldlist);
00141 }
00142 
00143 static void lock_fixup(void *data, struct ast_channel *oldchan, struct ast_channel *newchan)
00144 {
00145    struct ast_datastore *lock_store = ast_channel_datastore_find(oldchan, &lock_info, NULL);
00146    AST_LIST_HEAD(, channel_lock_frame) *list;
00147    struct channel_lock_frame *clframe = NULL;
00148 
00149    if (!lock_store) {
00150       return;
00151    }
00152    list = lock_store->data;
00153 
00154    AST_LIST_LOCK(list);
00155    AST_LIST_TRAVERSE(list, clframe, list) {
00156       if (clframe->lock_frame->owner == oldchan) {
00157          clframe->lock_frame->owner = newchan;
00158       }
00159       /* We don't move requesters, because the thread stack is different */
00160       clframe->channel = newchan;
00161    }
00162    AST_LIST_UNLOCK(list);
00163 }
00164 
00165 static void *lock_broker(void *unused)
00166 {
00167    struct lock_frame *frame;
00168    struct timespec forever = { 1000000, 0 };
00169    for (;;) {
00170       int found_requester = 0;
00171 
00172       /* Test for cancel outside of the lock */
00173       pthread_testcancel();
00174       AST_LIST_LOCK(&locklist);
00175 
00176       AST_LIST_TRAVERSE(&locklist, frame, entries) {
00177          if (ao2_container_count(frame->requesters)) {
00178             found_requester++;
00179             ast_mutex_lock(&frame->mutex);
00180             if (!frame->owner) {
00181                ast_cond_signal(&frame->cond);
00182             }
00183             ast_mutex_unlock(&frame->mutex);
00184          }
00185       }
00186 
00187       AST_LIST_UNLOCK(&locklist);
00188       pthread_testcancel();
00189 
00190       /* If there are no requesters, then wait for a signal */
00191       if (!found_requester) {
00192          nanosleep(&forever, NULL);
00193       } else {
00194          sched_yield();
00195       }
00196    }
00197    /* Not reached */
00198    return NULL;
00199 }
00200 
00201 static int ast_channel_hash_cb(const void *obj, const int flags)
00202 {
00203    const struct ast_channel *chan = obj;
00204    return ast_str_case_hash(chan->name);
00205 }
00206 
00207 static int ast_channel_cmp_cb(void *obj, void *arg, int flags)
00208 {
00209    struct ast_channel *chan = obj, *cmp_args = arg;
00210    return strcasecmp(chan->name, cmp_args->name) ? 0 : CMP_MATCH;
00211 }
00212 
00213 static int get_lock(struct ast_channel *chan, char *lockname, int try)
00214 {
00215    struct ast_datastore *lock_store = ast_channel_datastore_find(chan, &lock_info, NULL);
00216    struct lock_frame *current;
00217    struct channel_lock_frame *clframe = NULL;
00218    AST_LIST_HEAD(, channel_lock_frame) *list;
00219    int res = 0;
00220    struct timespec three_seconds = { .tv_sec = 3 };
00221 
00222    if (!lock_store) {
00223       ast_debug(1, "Channel %s has no lock datastore, so we're allocating one.\n", chan->name);
00224       lock_store = ast_datastore_alloc(&lock_info, NULL);
00225       if (!lock_store) {
00226          ast_log(LOG_ERROR, "Unable to allocate new datastore.  No locks will be obtained.\n");
00227          return -1;
00228       }
00229 
00230       list = ast_calloc(1, sizeof(*list));
00231       if (!list) {
00232          ast_log(LOG_ERROR, "Unable to allocate datastore list head.  %sLOCK will fail.\n", try ? "TRY" : "");
00233          ast_datastore_free(lock_store);
00234          return -1;
00235       }
00236 
00237       lock_store->data = list;
00238       AST_LIST_HEAD_INIT(list);
00239       ast_channel_datastore_add(chan, lock_store);
00240    } else
00241       list = lock_store->data;
00242 
00243    /* Lock already exists? */
00244    AST_LIST_LOCK(&locklist);
00245    AST_LIST_TRAVERSE(&locklist, current, entries) {
00246       if (strcmp(current->name, lockname) == 0) {
00247          break;
00248       }
00249    }
00250 
00251    if (!current) {
00252       if (unloading) {
00253          /* Don't bother */
00254          AST_LIST_UNLOCK(&locklist);
00255          return -1;
00256       }
00257 
00258       /* Create new lock entry */
00259       current = ast_calloc(1, sizeof(*current) + strlen(lockname) + 1);
00260       if (!current) {
00261          AST_LIST_UNLOCK(&locklist);
00262          return -1;
00263       }
00264 
00265       strcpy(current->name, lockname); /* SAFE */
00266       if ((res = ast_mutex_init(&current->mutex))) {
00267          ast_log(LOG_ERROR, "Unable to initialize mutex: %s\n", strerror(res));
00268          ast_free(current);
00269          AST_LIST_UNLOCK(&locklist);
00270          return -1;
00271       }
00272       if ((res = ast_cond_init(&current->cond, NULL))) {
00273          ast_log(LOG_ERROR, "Unable to initialize condition variable: %s\n", strerror(res));
00274          ast_mutex_destroy(&current->mutex);
00275          ast_free(current);
00276          AST_LIST_UNLOCK(&locklist);
00277          return -1;
00278       }
00279       if (!(current->requesters = ao2_container_alloc(1, ast_channel_hash_cb, ast_channel_cmp_cb))) {
00280          ast_mutex_destroy(&current->mutex);
00281          ast_cond_destroy(&current->cond);
00282          ast_free(current);
00283          AST_LIST_UNLOCK(&locklist);
00284          return -1;
00285       }
00286       AST_LIST_INSERT_TAIL(&locklist, current, entries);
00287    }
00288    AST_LIST_UNLOCK(&locklist);
00289 
00290    /* Found lock or created one - now find or create the corresponding link in the channel */
00291    AST_LIST_LOCK(list);
00292    AST_LIST_TRAVERSE(list, clframe, list) {
00293       if (clframe->lock_frame == current) {
00294          break;
00295       }
00296    }
00297 
00298    if (!clframe) {
00299       if (unloading) {
00300          /* Don't bother */
00301          AST_LIST_UNLOCK(list);
00302          return -1;
00303       }
00304 
00305       if (!(clframe = ast_calloc(1, sizeof(*clframe)))) {
00306          ast_log(LOG_ERROR, "Unable to allocate channel lock frame.  %sLOCK will fail.\n", try ? "TRY" : "");
00307          AST_LIST_UNLOCK(list);
00308          return -1;
00309       }
00310 
00311       clframe->lock_frame = current;
00312       clframe->channel = chan;
00313       AST_LIST_INSERT_TAIL(list, clframe, list);
00314    }
00315    AST_LIST_UNLOCK(list);
00316 
00317    /* If we already own the lock, then we're being called recursively.
00318     * Keep track of how many times that is, because we need to unlock
00319     * the same amount, before we'll release this one.
00320     */
00321    if (current->owner == chan) {
00322       current->count++;
00323       return 0;
00324    }
00325 
00326    /* Okay, we have both frames, so now we need to try to lock.
00327     *
00328     * Locking order: always lock locklist first.  We need the
00329     * locklist lock because the broker thread counts whether
00330     * there are requesters with the locklist lock held, and we
00331     * need to hold it, so that when we send our signal, below,
00332     * to wake up the broker thread, it definitely will see that
00333     * a requester exists at that point in time.  Otherwise, we
00334     * could add to the requesters after it has already seen that
00335     * that lock is unoccupied and wait forever for another signal.
00336     */
00337    AST_LIST_LOCK(&locklist);
00338    ast_mutex_lock(&current->mutex);
00339    /* Add to requester list */
00340    ao2_link(current->requesters, chan);
00341    pthread_kill(broker_tid, SIGURG);
00342    AST_LIST_UNLOCK(&locklist);
00343 
00344    if ((!current->owner) ||
00345       (!try && !(res = ast_cond_timedwait(&current->cond, &current->mutex, &three_seconds)))) {
00346       res = 0;
00347       current->owner = chan;
00348       current->count++;
00349    } else {
00350       res = -1;
00351    }
00352    /* Remove from requester list */
00353    ao2_unlink(current->requesters, chan);
00354    ast_mutex_unlock(&current->mutex);
00355 
00356    return res;
00357 }
00358 
00359 static int unlock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
00360 {
00361    struct ast_datastore *lock_store = ast_channel_datastore_find(chan, &lock_info, NULL);
00362    struct channel_lock_frame *clframe;
00363    AST_LIST_HEAD(, channel_lock_frame) *list;
00364 
00365    if (!lock_store) {
00366       ast_log(LOG_WARNING, "No datastore for dialplan locks.  Nothing was ever locked!\n");
00367       ast_copy_string(buf, "0", len);
00368       return 0;
00369    }
00370 
00371    if (!(list = lock_store->data)) {
00372       ast_debug(1, "This should NEVER happen\n");
00373       ast_copy_string(buf, "0", len);
00374       return 0;
00375    }
00376 
00377    /* Find item in the channel list */
00378    AST_LIST_LOCK(list);
00379    AST_LIST_TRAVERSE(list, clframe, list) {
00380       if (clframe->lock_frame && clframe->lock_frame->owner == chan && strcmp(clframe->lock_frame->name, data) == 0) {
00381          break;
00382       }
00383    }
00384    /* We never destroy anything until channel destruction, which will never
00385     * happen while this routine is executing, so we don't need to hold the
00386     * lock beyond this point. */
00387    AST_LIST_UNLOCK(list);
00388 
00389    if (!clframe) {
00390       /* We didn't have this lock in the first place */
00391       ast_copy_string(buf, "0", len);
00392       return 0;
00393    }
00394 
00395    if (--clframe->lock_frame->count == 0) {
00396       clframe->lock_frame->owner = NULL;
00397    }
00398 
00399    ast_copy_string(buf, "1", len);
00400    return 0;
00401 }
00402 
00403 static int lock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
00404 {
00405    if (chan)
00406       ast_autoservice_start(chan);
00407 
00408    ast_copy_string(buf, get_lock(chan, data, 0) ? "0" : "1", len);
00409 
00410    if (chan)
00411       ast_autoservice_stop(chan);
00412 
00413    return 0;
00414 }
00415 
00416 static int trylock_read(struct ast_channel *chan, const char *cmd, char *data, char *buf, size_t len)
00417 {
00418    if (chan)
00419       ast_autoservice_start(chan);
00420 
00421    ast_copy_string(buf, get_lock(chan, data, 1) ? "0" : "1", len);
00422 
00423    if (chan)
00424       ast_autoservice_stop(chan);
00425 
00426    return 0;
00427 }
00428 
00429 static struct ast_custom_function lock_function = {
00430    .name = "LOCK",
00431    .read = lock_read,
00432    .read_max = 2,
00433 };
00434 
00435 static struct ast_custom_function trylock_function = {
00436    .name = "TRYLOCK",
00437    .read = trylock_read,
00438    .read_max = 2,
00439 };
00440 
00441 static struct ast_custom_function unlock_function = {
00442    .name = "UNLOCK",
00443    .read = unlock_read,
00444    .read_max = 2,
00445 };
00446 
00447 static int unload_module(void)
00448 {
00449    struct lock_frame *current;
00450 
00451    /* Module flag */
00452    unloading = 1;
00453 
00454    AST_LIST_LOCK(&locklist);
00455    while ((current = AST_LIST_REMOVE_HEAD(&locklist, entries))) {
00456       /* If any locks are currently in use, then we cannot unload this module */
00457       if (current->owner || ao2_container_count(current->requesters)) {
00458          /* Put it back */
00459          AST_LIST_INSERT_HEAD(&locklist, current, entries);
00460          AST_LIST_UNLOCK(&locklist);
00461          unloading = 0;
00462          return -1;
00463       }
00464       ast_mutex_destroy(&current->mutex);
00465       ao2_ref(current->requesters, -1);
00466       ast_free(current);
00467    }
00468 
00469    /* No locks left, unregister functions */
00470    ast_custom_function_unregister(&lock_function);
00471    ast_custom_function_unregister(&trylock_function);
00472    ast_custom_function_unregister(&unlock_function);
00473 
00474    pthread_cancel(broker_tid);
00475    pthread_kill(broker_tid, SIGURG);
00476    pthread_join(broker_tid, NULL);
00477 
00478    AST_LIST_UNLOCK(&locklist);
00479 
00480    return 0;
00481 }
00482 
00483 static int load_module(void)
00484 {
00485    int res = ast_custom_function_register(&lock_function);
00486    res |= ast_custom_function_register(&trylock_function);
00487    res |= ast_custom_function_register(&unlock_function);
00488    ast_pthread_create_background(&broker_tid, NULL, lock_broker, NULL);
00489    return res;
00490 }
00491 
00492 AST_MODULE_INFO_STANDARD(ASTERISK_GPL_KEY, "Dialplan mutexes");

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