Programming with POSIX® Threads

116 }

117 }

118 /*

119 * Yield processor, if needed to be sure locks get

120 * interleaved on a uniprocessor.

121 */

122 if (yield_flag) {

123 if (yield_flag > 0)

124 sched_yield ();

125 else

126 sleep (1);

127 }

128 }

129 /*

130 * Report that we got 'em, and unlock to try again.

131 */

132 printf (

133 "lock backward got all locks, %d backoffs\n", backoffs);

134 pthread_mutex_unlock (&mutex[0]);

135 pthread_mutex_unlock (&mutex[l]);

136 pthread_mutex_unlock (&mutex[2]);

137 sched_yield ();

138 }

139 return NULL;

140 } 141

142 int main (int argc, char *argv[])

143 {

144 pthread_t forward, backward;

145 int status;

146

147 #ifdef sun

148 /*

149 * On Solaris 2.5, threads are not timesliced. To ensure

150 * that our threads can run concurrently, we need to

151 * increase the concurrency level.

152 */

153 DPRINTF (("Setting concurrency level to 2\n"));

154 thr_setconcurrency (2);

155 #endif

156

157 /*

158 * If the first argument is absent, or nonzero, a backoff

159 * algorithm will be used to avoid deadlock. If the first

160 * argument is zero, the program will deadlock on a lock

161 * "collision."

162 */

163 if (argc > 1)

164 backoff = atoi (argv[l]);

165

166 /*

167 * If the second argument is absent, or zero, the two threads

168 * run "at speed." On some systems, especially uniprocessors,

169 * one thread may complete before the other has a chance to run,

170 * and you won't see a deadlock or backoffs. In that case, try

171 * running with the argument set to a positive number to cause

172 * the threads to call sched_yield() at each lock; or, to make

173 * it even more obvious, set to a negative number to cause the

174 * threads to call sleep(l) instead.

175 */

176 if (argc > 2)

177 yield_flag = atoi (argv[2]);

178 status = pthread_create (

179 &forward, NULL, lock_forward, NULL);

180 if (status != 0)

181 err_abort (status, "Create forward");

182 status = pthread_create (

183 &backward, NULL, lock_backward, NULL);

184 if (status != 0)

185 err_abort (status, "Create backward");

186 pthread_exit (NULL);

187 }

Whatever type of hierarchy you choose, document it, carefully, completely, and often. Document it in each function that uses any of the mutexes. Document it where the mutexes are defined. Document it where they are declared in a project

header file. Document it in the project design notes. Write it on your whiteboard. And then tie a string around your finger to be sure that you do not forget.