Communication between processes and thread synchronization is a long-standing and new topic. You should know a little about programming, but you can't explain it in detail.On the one hand, in addition to the work may be used less, on the other hand, these concepts involve more things and are relatively deep.Network programming, server-side programming, concurrent applications and so on are all involved.Its development and debugging process is not intuitive.Since the principles of synchronous communication mechanisms are all conceptual, this paper attempts to materialize abstract concepts by looking at python instances.

Refer to a previous article before reading: python multithreading and multiprocess and their differences To learn about creating threads and processes.

python multithreaded synchronization

Two standard libraries, threading and threading, are provided in python for thread support, which has been discarded in python 3 as a higher-level encapsulated thread library, followed by examples of the latter.

Synchronization and Mutual Exclusion

It is believed that most people who have studied operating systems are confused by these two concepts: what mutually exclusive is a special synchronization, what synchronization is a collaborative relationship between multiple threads or processes on some key nodes in the process of completing a task, and so on.

In fact, both concepts are based on a synergistic relationship, which can be clearly expressed by a specific example:

There are two threads called Thread A and Thread B, where Thread A writes a variable, Thread B reads the variable written by Thread A, and Thread A writes the variable first, then Thread B reads the variable, so there is a synchronization relationship between Thread A and B.

===== Synchronization relationship =====
Thread A:
write(share_data)
V(S)    # Release Resources

Thread B:
P(S)    # Get Resources
read(share_data)

If another thread C writes this variable, then the relationship between thread A and C is mutually exclusive, because only one thread can write the variable at the same time.

===== Mutual exclusion =====
Thread A:
Lock.acquire();       # Acquire locks
write(share_data)
Lock.release()        # Release lock

Thread C:
Lock.acquire();       # Acquire locks
write(share_data)
Lock.release()        # Release lock

Thread Synchronization

The main thread and the thread it creates execute their own code separately until the end.Next, let's look at the synchronization issue between python threads.

Is alternating execution thread safe?

Let's start with this example:

share_data = 0

def tstart(arg):
    time.sleep(0.1)
    global share_data
    for i in xrange(1000):
        share_data += 1

if __name__ == '__main__':
    t1 = threading.Thread(target = tstart, args = ('',))
    t2 = threading.Thread(target = tstart, args = ('',))
    t1.start()
    t2.start()
    t1.join()
    t2.join()
    print 'share_data result:', share_data

The share_data result of this code execution above will be less than 2000 in most cases. As mentioned in the previous article, a global interpreter lock (GIL) has been introduced into the python interpreter CPython, meaning that only one thread is executing at any one time, but there are still problems here. Why?

The root cause is that writing to share_data is not an atomic operation, the thread is interrupted during the writing process, then the thread is switched to execute, and the interrupted write operation continues when it returns, but it may overwrite the result of another thread writing during this time period.

The following is a possible operation:

The actual calculation process may be more complex than described above and can be understood from the perspective of a single thread that, without synchronization, is completely unaware of other threads, reading, calculating, and writing back data.

If after reading the data, the calculation process is interrupted or before writing back the data, and when it is executed again, the process continues from where it was interrupted even though the share_data in memory has changed, overwriting the current share_data value;

This is why only one thread executes at a time, but the result is still wrong.You can imagine that if multiple threads execute in parallel without synchronization, the calculation process becomes more confusing.

If you are interested, you can use a global list of res to record the process of writing share_data for each thread, and you can see the process more intuitively:

share_data = 0
res = []

def tstart(arg):
    time.sleep(0.1)
    global share_data
    for i in xrange(1000):
        res.append((arg, share_data))
        share_data += 1

if __name__ == '__main__':
    t1 = threading.Thread(target = tstart, args = ('1',))
    t2 = threading.Thread(target = tstart, args = ('2',))
    t1.start()
    t2.start()
    t1.join()
    t2.join()
    print res, len(res)
    print 'share_data result:', share_data

The following is a possible result, and you can see that two threads did add 1,000 times to share_data, but the result was not 2,000.

[('2', 0), ('2', 1), ('1', 2), ('1', 3), ('1', 4), ('1', 5), ('1', 6), ('1', 7), ('1', 8), ('1', 9), ('1', 10), ('1', 11), ('1', 12), ('2', 12), ('2', 13), ('2', 14), ('2', 15), ('2', 16), ('2', 17), ('2', 18), ('2', 19), ('2', 20), ('2', 21), ('2', 22), ('1', 13), ('1', 14), ('1', 15), ('1', 16), ('1', 17), ('1', 18), ('1', 19), ('1', 20), ('1', 21), ('1', 22), ('1', 23), ('2', 24), ('2', 25), ('2', 26), ('2', 27), ('2', 28), ('2', 29), ('2', 30), ('2', 31), ('2', 32), ('2', 33), ('2', 34), ('1', 35), ('1', 36), ('1', 37), ('1', 38), ('1', 39), ('1', 40), ('1', 41), ('1', 42), ('1', 43), ('1', 44), ('1', 45), ('2', 35), ('2', 47), ('2', 48), ('2', 49), ('2', 50), ('2', 51), ('2', 52), ('2', 53), ('2', 54), ('2', 55), ('2', 56), ('2', 57), ('1', 57), ('1', 58), ('1', 59), ('1', 60), ('1', 61), ('1', 62), ('1', 63), ('1', 64), ('1', 65), ('1', 66), ('1', 67), ('2', 58), ('2', 59), ('2', 60), ('2', 61), ('2', 62), ('2', 63), ('2', 64), ('2', 65), ('2', 66), ('2', 67), ('2', 68), ('1', 68), ('1', 70), ('1', 71), ('1', 72), ('1', 73), ('1', 74), ('1', 75), ('1', 76), ('1', 77), ('1', 78), ('1', 79), ('1', 80), ('2', 80), ('2', 81), ('2', 82), ('2', 83), ('2', 84), ('2', 85), ('2', 86), ('2', 87), ('2', 88), ('2', 89), ('2', 90), ('1', 81), ('1', 82), ('1', 83), ('1', 84), ('1', 85), ('1', 86), ('1', 87), ('1', 88), ('1', 89), ('1', 90), ('1', 91), ('2', 92), ('2', 93), ('2', 94), ('2', 95), ('2', 96), ('2', 97), ('2', 98), ('2', 99), ('2', 100), ('2', 101), ('2', 102), ('1', 103), ('1', 104), ('1', 105), ('1', 106), ('1', 107), ('1', 108), ('1', 109), ('1', 110), ('1', 111), ('1', 112), ('1', 113), ('2', 103), ('2', 115), ('2', 116), ('2', 117), ('2', 118), ('2', 119), ('2', 120), ('2', 121), ('2', 122), ('2', 123), ('2', 124), ('2', 125), ('1', 125), ('1', 126), ('1', 127), ('1', 128), ('1', 129), ('1', 130), ('1', 131), ('1', 132), ('1', 133), ('1', 134), ('1', 135), ('2', 137), ('2', 138), ('2', 139), ('2', 140), ('2', 141), ('2', 142), ('2', 143), ('2', 144), ('2', 145), ('2', 146), ('2', 147), ('1', 136), ('1', 148), ('1', 149), ('1', 150), ('1', 151), ('1', 152), ('1', 153), ('1', 154), ('1', 155), ('1', 156), ('1', 157), ('1', 158), ('2', 148), ('2', 149), ('2', 150), ('2', 151), ('2', 152), ('2', 153), ('2', 154), ('2', 155), ('2', 156), ('2', 157), ('2', 158), ('1', 160), ('1', 161), ('1', 162), ('1', 163), ('1', 164), ('1', 165), ('1', 166), ('1', 167), ('1', 168), ('1', 169), ('1', 170), ('2', 170), ('2', 171), ('2', 172), ('2', 173), ('2', 174), ('2', 175), ('2', 176), ('2', 177), ('2', 178), ('2', 179), ('2', 180), ('1', 171), ('1', 172), ('1', 173), ('1', 174), ('1', 175), ('1', 176), ('1', 177), ('1', 178), ('1', 179), ('1', 180), ('1', 181), ('2', 182), ('2', 183), ('2', 184), ('2', 185), ('2', 186), ('2', 187), ('2', 188), ('2', 189), ('2', 190), ('2', 191), ('2', 192), ('1', 193), ('1', 194), ('1', 195), ('1', 196), ('1', 197), ('1', 198), ('1', 199), ('1', 200), ('1', 201), ('1', 202), ('1', 203), ('2', 193), ('2', 205), ('2', 206), ('2', 207), ('2', 208), ('2', 209), ('2', 210), ('2', 211), ('2', 212), ('2', 213), ('2', 214), ('2', 215), ('1', 215), ('1', 216), ('1', 217), ('1', 218), ('1', 219), ('1', 220), ('1', 221), ('1', 222), ('1', 223), ('1', 224), ('1', 225), ('2', 216), ('2', 217), ('2', 218), ('2', 219), ('2', 220), ('2', 221), ('2', 222), ('2', 223), ('2', 224), ('2', 225), ('2', 226), ('1', 226), ('1', 228), ('1', 229), ('1', 230), ('1', 231), ('1', 232), ('1', 233), ('1', 234), ('1', 235), ('1', 236), ('1', 237), ('1', 238), ('2', 238), ('2', 239), ('2', 240), ('2', 241), ('2', 242), ('2', 243), ('2', 244), ('2', 245), ('2', 246), ('2', 247), ('2', 248), ('1', 239), ('1', 240), ('1', 241), ('1', 242), ('1', 243), ('1', 244), ('1', 245), ('1', 246), ('1', 247), ('1', 248), ('1', 249), ('2', 250), ('2', 251), ('2', 252), ('2', 253), ('2', 254), ('2', 255), ('2', 256), ('2', 257), ('2', 258), ('2', 259), ('2', 260), ('1', 261), ('1', 262), ('1', 263), ('1', 264), ('1', 265), ('1', 266), 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('2', 849), ('2', 850), ('2', 851), ('2', 852), ('2', 853), ('2', 854), ('2', 855), ('2', 856), ('2', 857), ('2', 858), ('1', 858), ('1', 860), ('1', 861), ('1', 862), ('1', 863), ('1', 864), ('1', 865), ('1', 866), ('1', 867), ('1', 868), ('1', 869), ('1', 870), ('2', 870), ('2', 871), ('2', 872), ('2', 873), ('2', 874), ('2', 875), ('2', 876), ('2', 877), ('2', 878), ('2', 879), ('2', 880), ('1', 871), ('1', 872), ('1', 873), ('1', 874), ('1', 875), ('1', 876), ('1', 877), ('1', 878), ('1', 879), ('1', 880), ('1', 881), ('2', 882), ('2', 883), ('2', 884), ('2', 885), ('2', 886), ('2', 887), ('2', 888), ('2', 889), ('2', 890), ('2', 891), ('2', 892), ('1', 893), ('1', 894), ('1', 895), ('1', 896), ('1', 897), ('1', 898), ('1', 899), ('1', 900), ('1', 901), ('1', 902), ('1', 903), ('2', 893), ('2', 905), ('2', 906), ('2', 907), ('2', 908), ('2', 909), ('2', 910), ('2', 911), ('2', 912), ('2', 913), ('2', 914), ('2', 915), ('1', 915), ('1', 916), ('1', 917), ('1', 918), ('1', 919), ('1', 920), ('1', 921), ('1', 922), ('1', 923), ('1', 924), ('1', 925), ('2', 927), ('2', 928), ('2', 929), ('2', 930), ('2', 931), ('2', 932), ('2', 933), ('2', 934), ('2', 935), ('2', 936), ('2', 937), ('1', 926), ('1', 938), ('1', 939), ('1', 940), ('1', 941), ('1', 942), ('1', 943), ('1', 944), ('1', 945), ('1', 946), ('1', 947), ('1', 948), ('2', 938), ('2', 939), ('2', 940), ('2', 941), ('2', 942), ('2', 943), ('2', 944), ('2', 945), ('2', 946), ('2', 947), ('2', 948), ('1', 950), ('1', 951), ('1', 952), ('1', 953), ('1', 954), ('1', 955), ('1', 956), ('1', 957), ('1', 958), ('1', 959), ('1', 960), ('2', 960), ('2', 961), ('2', 962), ('2', 963), ('2', 964), ('2', 965), ('2', 966), ('2', 967), ('2', 968), ('2', 969), ('2', 970), ('1', 961), ('1', 962), ('1', 963), ('1', 964), ('1', 965), ('1', 966), ('1', 967), ('1', 968), ('1', 969), ('1', 970), ('1', 971), ('2', 972), ('2', 973), ('2', 974), ('2', 975), ('2', 976), ('2', 977), ('2', 978), ('2', 979), ('2', 980), ('2', 981), ('2', 982), ('1', 983), ('1', 984), ('1', 985), ('1', 986), ('1', 987), ('1', 988), ('1', 989), ('1', 990), ('1', 991), ('1', 992), ('1', 993), ('2', 983), ('2', 995), ('2', 996), ('2', 997), ('2', 998), ('2', 999), ('2', 1000), ('2', 1001), ('2', 1002), ('2', 1003), ('2', 1004), ('2', 1005), ('1', 1005), ('1', 1006), ('1', 1007), ('1', 1008), ('1', 1009), ('1', 1010), ('1', 1011), ('1', 1012), ('1', 1013), ('1', 1014), ('1', 1015), ('2', 1006), ('2', 1007), ('2', 1008), ('2', 1009), ('2', 1010), ('2', 1011), ('2', 1012), ('2', 1013), ('2', 1014), ('2', 1015), ('2', 1016), ('1', 1016), ('1', 1018), ('1', 1019), ('1', 1020), ('1', 1021), ('1', 1022), ('1', 1023), ('1', 1024), ('1', 1025), ('1', 1026), ('1', 1027), ('1', 1028), ('2', 1028), ('2', 1029), ('2', 1030), ('2', 1031), ('2', 1032), ('2', 1033), ('2', 1034), ('2', 1035), ('2', 1036), ('2', 1037), ('2', 1038), ('1', 1029), ('1', 1030), ('1', 1031), ('1', 1032), ('1', 1033), ('1', 1034), ('1', 1035), ('1', 1036), ('1', 1037), ('1', 1038), ('1', 1039), ('2', 1040), ('2', 1041), ('2', 1042), ('2', 1043), ('2', 1044), ('2', 1045), ('2', 1046), ('2', 1047), ('2', 1048), ('2', 1049), ('2', 1050), ('1', 1051), ('1', 1052), ('1', 1053), ('1', 1054), ('1', 1055), ('1', 1056), ('1', 1057), ('1', 1058), ('1', 1059), ('1', 1060), ('1', 1061), ('2', 1051), ('2', 1063), ('2', 1064), ('2', 1065), ('2', 1066), ('2', 1067), ('2', 1068), ('2', 1069), ('2', 1070), ('2', 1071), ('2', 1072), ('2', 1073), ('1', 1073), ('1', 1074), ('1', 1075), ('1', 1076), ('1', 1077), ('1', 1078), ('1', 1079), ('1', 1080), ('1', 1081), ('1', 1082), ('1', 1083), ('2', 1085), ('2', 1086), ('2', 1087), ('2', 1088), ('2', 1089), ('2', 1090), ('2', 1091), ('2', 1092), ('2', 1093), ('2', 1094), ('2', 1095), ('1', 1084), ('1', 1096), ('1', 1097), ('1', 1098), ('1', 1099), ('1', 1100), ('1', 1101), ('1', 1102), ('1', 1103), ('1', 1104), ('1', 1105), ('1', 1106), ('2', 1096), ('2', 1097), ('2', 1098), ('2', 1099), ('2', 1100), ('2', 1101), ('2', 1102), ('2', 1103), ('2', 1104), ('2', 1105), ('2', 1106), ('1', 1108), ('1', 1109), ('1', 1110), ('1', 1111), ('1', 1112), ('1', 1113), ('1', 1114), ('1', 1115), ('1', 1116), ('1', 1117), ('1', 1118), ('2', 1118), ('2', 1119), ('2', 1120), ('2', 1121), ('2', 1122), ('2', 1123), ('2', 1124), ('2', 1125), ('2', 1126), ('2', 1127), ('2', 1128), ('1', 1119), ('1', 1120), ('1', 1121), ('1', 1122), ('1', 1123), ('1', 1124), ('1', 1125), ('1', 1126), ('1', 1127), ('1', 1128), ('1', 1129), ('2', 1130), ('2', 1131), ('2', 1132), ('2', 1133), ('2', 1134), ('2', 1135), ('2', 1136), ('2', 1137), ('2', 1138), ('2', 1139), ('2', 1140), ('1', 1141), ('1', 1142), ('1', 1143), ('1', 1144), ('1', 1145), ('1', 1146), ('1', 1147), ('1', 1148), ('1', 1149), ('1', 1150), ('1', 1151), ('2', 1141), ('2', 1153), ('2', 1154), ('2', 1155), ('2', 1156), ('2', 1157), ('2', 1158), ('2', 1159), ('2', 1160), ('2', 1161), ('2', 1162), ('2', 1163), ('1', 1163), ('1', 1164), ('1', 1165), ('1', 1166), ('1', 1167), ('1', 1168), ('1', 1169), ('1', 1170), ('1', 1171), ('1', 1172), ('1', 1173), ('2', 1164), ('2', 1165), ('2', 1166), ('2', 1167), ('2', 1168), ('2', 1169), ('2', 1170), ('2', 1171), ('2', 1172), ('2', 1173), ('2', 1174), ('1', 1174), ('1', 1176), ('1', 1177), ('1', 1178), ('1', 1179), ('1', 1180), ('1', 1181), ('1', 1182), ('1', 1183), ('1', 1184), ('1', 1185), ('1', 1186), ('2', 1186), ('2', 1187), ('2', 1188), ('2', 1189), ('2', 1190), ('2', 1191), ('2', 1192), ('2', 1193), ('2', 1194), ('2', 1195), ('2', 1196), ('1', 1187), ('1', 1188), ('1', 1189), ('1', 1190), ('1', 1191), ('1', 1192), ('1', 1193), ('1', 1194), ('1', 1195), ('1', 1196), ('1', 1197), ('2', 1198), ('2', 1199), ('2', 1200), ('2', 1201), ('2', 1202), ('2', 1203), ('2', 1204), ('2', 1205), ('2', 1206), ('2', 1207), ('2', 1208), ('1', 1209), ('1', 1210), ('1', 1211), ('1', 1212), ('1', 1213), ('1', 1214), ('1', 1215), ('1', 1216), ('1', 1217), ('1', 1218), ('1', 1219), ('2', 1209), ('2', 1221), ('2', 1222), ('2', 1223), ('2', 1224), ('2', 1225), ('2', 1226), ('2', 1227), ('2', 1228), ('2', 1229), ('2', 1230), ('2', 1231), ('1', 1231), ('1', 1232), ('1', 1233), ('1', 1234), ('1', 1235), ('1', 1236), ('1', 1237), ('1', 1238), ('1', 1239), ('1', 1240), ('1', 1241), ('2', 1243), ('2', 1244), ('2', 1245), ('2', 1246), ('2', 1247), ('2', 1248), ('2', 1249), ('2', 1250), ('2', 1251), ('2', 1252), ('2', 1253), ('1', 1242), ('1', 1254), ('1', 1255), ('1', 1256), ('1', 1257), ('1', 1258), ('1', 1259), ('1', 1260), ('1', 1261), ('1', 1262), ('1', 1263), ('1', 1264), ('2', 1254), ('2', 1255), ('2', 1256), ('2', 1257), ('2', 1258), ('2', 1259), ('2', 1260), ('2', 1261), ('2', 1262), ('2', 1263), ('2', 1264), ('1', 1266), ('1', 1267), ('1', 1268), ('1', 1269), ('1', 1270), ('1', 1271), ('1', 1272), ('1', 1273), ('1', 1274), ('1', 1275), ('1', 1276), ('2', 1276), ('2', 1277), ('2', 1278), ('2', 1279), ('2', 1280), ('2', 1281), ('2', 1282), ('2', 1283), ('2', 1284), ('2', 1285), ('2', 1286), ('1', 1277), ('1', 1278), ('1', 1279), ('1', 1280), ('1', 1281), ('1', 1282), ('1', 1283), ('1', 1284), ('1', 1285), ('1', 1286), ('1', 1287), ('2', 1288), ('2', 1289), ('2', 1290), ('2', 1291), ('2', 1292), ('2', 1293), ('2', 1294), ('2', 1295), ('2', 1296), ('2', 1297), ('2', 1298), ('1', 1299), ('1', 1300), ('1', 1301), ('1', 1302), ('1', 1303), ('1', 1304), ('1', 1305), ('1', 1306), ('1', 1307), ('1', 1308), ('1', 1309), ('2', 1299), ('2', 1311), ('2', 1312), ('2', 1313), ('2', 1314), ('2', 1315), ('2', 1316), ('2', 1317), ('2', 1318), ('2', 1319), ('2', 1320), ('2', 1321), ('1', 1321), ('1', 1322), ('1', 1323), ('1', 1324), ('1', 1325), ('1', 1326), ('1', 1327), ('1', 1328), ('1', 1329), ('1', 1330), ('1', 1331), ('2', 1322), ('2', 1323), ('2', 1324), ('2', 1325), ('2', 1326), ('2', 1327), ('2', 1328), ('2', 1329), ('2', 1330), ('2', 1331), ('2', 1332), ('1', 1332), ('1', 1334), ('1', 1335), ('1', 1336), ('1', 1337), ('1', 1338), ('1', 1339), ('1', 1340), ('1', 1341), ('1', 1342), ('1', 1343), ('1', 1344), ('2', 1344), ('2', 1345), ('2', 1346), ('2', 1347), ('2', 1348), ('2', 1349), ('2', 1350), ('2', 1351), ('2', 1352), ('2', 1353), ('2', 1354), ('1', 1345), ('1', 1346), ('1', 1347), ('1', 1348), ('1', 1349), ('1', 1350), ('1', 1351), ('1', 1352), ('1', 1353), ('1', 1354), ('1', 1355), ('2', 1356), ('2', 1357), ('2', 1358), ('2', 1359), ('2', 1360), ('2', 1361), ('2', 1362), ('2', 1363), ('2', 1364), ('2', 1365), ('2', 1366), ('1', 1367), ('1', 1368), ('1', 1369), ('1', 1370), ('1', 1371), ('1', 1372), ('1', 1373), ('1', 1374), ('1', 1375), ('1', 1376), ('1', 1377), ('2', 1367), ('2', 1379), ('2', 1380), ('2', 1381), ('2', 1382), ('2', 1383), ('2', 1384), ('2', 1385), ('2', 1386), ('2', 1387), ('2', 1388), ('2', 1389), ('1', 1389), ('1', 1390), ('1', 1391), ('1', 1392), ('1', 1393), ('1', 1394), ('1', 1395), ('1', 1396), ('1', 1397), ('1', 1398), ('1', 1399), ('2', 1401), ('2', 1402), ('2', 1403), ('2', 1404), ('2', 1405), ('2', 1406), ('2', 1407), ('2', 1408), ('2', 1409), ('2', 1410), ('2', 1411), ('1', 1400), ('1', 1412), ('1', 1413), ('1', 1414), ('1', 1415), ('1', 1416), ('1', 1417), ('1', 1418), ('1', 1419), ('1', 1420), ('1', 1421), ('1', 1422), ('2', 1412), ('2', 1413), ('2', 1414), ('2', 1415), ('2', 1416), ('2', 1417), ('2', 1418), ('2', 1419), ('2', 1420), ('2', 1421), ('2', 1422), ('1', 1424), ('1', 1425), ('1', 1426), ('1', 1427), ('1', 1428), ('1', 1429), ('1', 1430), ('1', 1431), ('1', 1432), ('1', 1433), ('1', 1434), ('2', 1434), ('2', 1435), ('2', 1436), ('2', 1437), ('2', 1438), ('2', 1439), ('2', 1440), ('2', 1441), ('2', 1442), ('2', 1443), ('2', 1444), ('1', 1435), ('1', 1436), ('1', 1437), ('1', 1438), ('1', 1439), ('1', 1440), ('1', 1441), ('1', 1442), ('1', 1443), ('1', 1444), ('1', 1445), ('2', 1446), ('2', 1447), ('2', 1448), ('2', 1449), ('2', 1450), ('2', 1451), ('2', 1452), ('2', 1453), ('2', 1454), ('2', 1455), ('2', 1456), ('1', 1457), ('1', 1458), ('1', 1459), ('1', 1460), ('1', 1461), ('1', 1462), ('1', 1463), ('1', 1464), ('1', 1465), ('1', 1466), ('1', 1467), ('2', 1457), ('2', 1469), ('2', 1470), ('2', 1471), ('2', 1472), ('2', 1473), ('2', 1474), ('2', 1475), ('2', 1476), ('2', 1477), ('2', 1478), ('2', 1479), ('1', 1479), ('1', 1480), ('1', 1481), ('1', 1482), ('1', 1483), ('1', 1484), ('1', 1485), ('1', 1486), ('1', 1487), ('1', 1488), ('1', 1489), ('2', 1480), ('2', 1481), ('2', 1482), ('2', 1483), ('2', 1484), ('2', 1485), ('2', 1486), ('2', 1487), ('2', 1488), ('2', 1489), ('2', 1490), ('1', 1490), ('1', 1492), ('1', 1493), ('1', 1494), ('1', 1495), ('1', 1496), ('1', 1497), ('1', 1498), ('1', 1499), ('1', 1500), ('1', 1501), ('1', 1502), ('2', 1502), ('2', 1503), ('2', 1504), ('2', 1505), ('2', 1506), ('2', 1507), ('2', 1508), ('2', 1509), ('2', 1510), ('2', 1511), ('2', 1512), ('1', 1503), ('1', 1504), ('1', 1505), ('1', 1506), ('1', 1507), ('1', 1508), ('1', 1509), ('1', 1510), ('1', 1511), ('1', 1512), ('1', 1513), ('2', 1514), ('2', 1515), ('2', 1516), ('2', 1517), ('2', 1518), ('2', 1519), ('2', 1520), ('2', 1521), ('2', 1522), ('2', 1523), ('2', 1524), ('1', 1525), ('1', 1526), ('1', 1527), ('1', 1528), ('1', 1529), ('1', 1530), ('1', 1531), ('1', 1532), ('1', 1533), ('1', 1534), ('1', 1535), ('2', 1525), ('2', 1537), ('2', 1538), ('2', 1539), ('2', 1540), ('2', 1541), ('2', 1542), ('2', 1543), ('2', 1544), ('2', 1545), ('2', 1546), ('2', 1547), ('1', 1547), ('1', 1548), ('1', 1549), ('1', 1550), ('1', 1551), ('1', 1552), ('1', 1553), ('1', 1554), ('1', 1555), ('1', 1556), ('1', 1557), ('2', 1559), ('2', 1560), ('2', 1561), ('2', 1562), ('2', 1563), ('2', 1564), ('2', 1565), ('2', 1566), ('2', 1567), ('2', 1568), ('1', 1558), ('1', 1570)] 2000
share_data result: 1571

This is the thread security issue of multithreaded write operations.Specifically, this thread synchronization is mutually exclusive.Next, take a look at the multithreaded synchronization measures provided by python.

The threading module provides some synchronization mechanisms for python threads, which can be used with reference to documentation on the official website.

Mutex Lock

Its basic usage is very simple:

1. Create a lock: Lock()
2. Acquire locks: acquire([blocking])
3. Release lock: release()

import threading
import time
lock = threading.Lock()     # step 1: Create Mutex
share_data = 0

def tstart(arg):
    time.sleep(0.1)
    global share_data
    if lock.acquire():       # step 2: Get mutex, otherwise block current thread
        share_data += 1
    lock.release()          # step 3: Release Mutex

if __name__ == '__main__':
    tlst = list()
    for i in xrange(10):
        t = threading.Thread(target=tstart, args=('',))
        tlst.append(t)
    for t in tlst:
        t.start()
    tlst[2].join()
    print("This is main function at: %s" % time.time())
    print 'share_data result:', share_data

Result:

This is main function at: 1564909315.86
share_data result: 7

The share_data results above are somewhat random because we simply wait for the second thread to finish executing, read the results directly, and end the main thread.

However, from the above results, we can infer that at least seven threads completed the add-1 operation on share_data when the third thread ended and the main thread executed to output the result of share_data.

Reentry Lock RLock

Since the current thread acquires the lock, it is possible to acquire the lock again leading to a deadlock before releasing it.python introduces reentrant locks.

import threading
import time
rlock = threading.RLock()     # step 1: Create Reentry Lock
share_data = 0

def check_data():
    global share_data
    if rlock.acquire():
        if share_data > 10:
            share_data = 0
    rlock.release()

def tstart(arg):
    time.sleep(0.1)
    global share_data
    if rlock.acquire():       # step 2: Obtain reentrant locks or block the current thread
        check_data()
        share_data += 1
    rlock.release()          # step 3: Release Re-entry Lock

if __name__ == '__main__':
    t1 = threading.Thread(target = tstart, args = ('',))
    t1.start()
    t1.join()
    print("This is main function at: %s" % time.time())
    print 'share_data result:', share_data

This example uses a mutex, which can cause the current thread to block.

Semaphore semaphore

A semaphore has an initial value that represents the number of resources currently available, dynamic semaphores are added or subtracted during multithreaded execution, and the value at a certain time of the semaphore represents the number of threads that can currently be increased for execution.

There are two operations for semaphores:

1. Acquire (that is, P operation)
2. Release (that is, V action)

Threads that perform V operations are unrestricted, and threads that perform P operations are blocked when resources are low;

def get_wait_time():
    return random.random()/5.0

# Number of resources 0
S = threading.Semaphore(0)
def consumer(name):
    S.acquire()
    time.sleep(get_wait_time())
    print name

def producer(name):
    # time.sleep(0.1)
    time.sleep(get_wait_time())
    print name
    S.release()

if __name__ == "__main__":
    for i in xrange(5, 10):
        c = threading.Thread(target=consumer, args=("consumer:%s"%i, ))
        c.start()
    for i in xrange(5):
        p = threading.Thread(target=producer, args=("producer:%s"%i, ))
        p.start()
    time.sleep(2)

The following is a possible execution result:

producer:3
producer:0
consumer:6
producer:4
producer:1
consumer:5
producer:2
consumer:7
consumer:8
consumer:9

Conditional Condition

Next, let's look at another synchronization mechanism, Condition, which is not very intuitive. To better see how it works, first define some functions:

def get_time():
    return time.strftime("%Y-%m-%d %H:%M:%S")

def show_start_info(tname):
    print '%s start at: %s' %(tname, get_time())

def show_acquire_info(tname):
    print '%s acquire at: %s' % (tname, time.time())

def show_add_once_res(tname):
    print '%s add: %s at: %s' % (tname, share_data, time.time())

def show_end_info(tname):
    print 'End %s with: %s at: %s' % (tname, share_data, time.time())

def show_wait_info(tname):
    print '%s wait at: %s' % (tname, time.time())

Conditional variables allow threads that have acquired locks to voluntarily discard locks when conditions are not met, notifying them to wake up other blocked threads; the basic process is as follows:

• Create a global conditional variable object;
• Each thread acquires a conditional variable before execution, acquires executes, otherwise blocks.
• The current execution thread will judge some conditions during its progression. If the conditions are not satisfied, wait and actively release the lock. Calling wait will block the current thread.
• The judgement condition is satisfied. After some processing changes the condition, the current thread notifies and wakes up other threads through the notify method. The other threads in the wait state will re-judge the condition after being notified and execute if their execution condition is met.Note that calling notify does not release the lock;
• Repeat this process until the task is completed.

It's a bit hard to understand here. After the current thread modifies the condition, notify other threads to check if their respective execution conditions are met, but the only lock held by the condition variable is owned by the current thread and not released. How do other threads execute?

The Python documentation gives the following instructions:

Note: an awakened thread does not actually return from its wait() call until it can reacquire the lock. Since notify() does not release the lock, its caller should.

That is, the notify notify wake-up thread does not return from its wait function and continues execution until it acquires a lock in the conditional variable.The thread calling notify should actively release the lock because the notify function will not release it.

There will be a problem here. The current thread modifies the execution conditions of other threads, notifies other threads, and actively calls wait to release the lock to suspend itself. If the execution conditions of other threads are not met, all threads will block.

The following illustrates the use of conditional variables by alternating the printing characters A and B on two threads:

share_data, max_len = '#', 6
cond = threading.Condition()

def addA(tname):
    show_start_info(tname)
    cond.acquire()
    time.sleep(1)
    show_acquire_info(tname)
    global share_data
    while len(share_data) <= max_len:
        if share_data[-1] != 'A':
            share_data += 'A'
            time.sleep(1)
            cond.notify()
            show_add_once_res(tname)
        else:
            # show_wait_info(tname)
            cond.wait()
    cond.release()
    show_end_info(tname)

def addB(tname):
    show_start_info(tname)
    cond.acquire()
    time.sleep(1)
    show_acquire_info(tname)
    global share_data
    while len(share_data) <= max_len:
        if share_data[-1] != 'B':
            share_data += 'B'
            time.sleep(1)
            cond.notify()
            show_add_once_res(tname)
        else:
            # show_wait_info(tname)
            cond.wait()
    cond.release()
    show_end_info(tname)

if __name__ == "__main__":
    t1 = threading.Thread(target=addA, args=("Thread 1", ))
    t2 = threading.Thread(target=addB, args=("Thread 2", ))
    t1.start()
    t2.start()
    t1.join()
    t2.join()
    print "share_data:", share_data

Result:

Thread 1 start at: 2019-08-10 17:47:54
Thread 2 start at: 2019-08-10 17:47:54
Thread 1 acquire at: 1565430475.68
Thread 1 add: #A at: 1565430476.68
Thread 2 acquire at: 1565430477.68
Thread 2 add: #AB at: 1565430478.68
Thread 1 add: #ABA at: 1565430479.68
Thread 2 add: #ABAB at: 1565430480.68
Thread 1 add: #ABABA at: 1565430481.68
Thread 2 add: #ABABAB at: 1565430482.68
End Thread 2 with: #ABABAB at: 1565430482.68
End Thread 1 with: #ABABAB at: 1565430482.68
share_data: #ABABAB

The result shows the process performed by two threads and the time node.

Event Event

Finally, let's look at a simple and rough way to synchronize threads: Event.

The core of this approach is to use events to control the state of a global variable: True or False.Thread executes by first determining the value of the variable and executes if it is true, otherwise call wait to block itself;

When the state of the global variable is set to True, all threads calling wait are awakened to a blocked state; when all threads need to be paused, use clear to set the global variable to False;

The following demonstrates the use of event events using a three-round game in which two players roll dices and a referee decides the winner or loser.

E = threading.Event()
E.clear()
res1, res2, cnt, lst = 0, 0, 3, ("player2", 'both', 'player1')

def show_round_res():
    print ("Stop! judging... %s win!" % lst[cmp(res1, res2) + 1])

def judge():
    global cnt 
    while cnt > 0:
        print 'start game!'
        E.set()
        time.sleep(1)
        E.clear()
        show_round_res()
        time.sleep(1)
        cnt -= 1
    print "game over by judge!"

def player1():
    global res1
    while cnt > 0:
        if E.is_set():
            res1 = random.randint(1, 6)
            print "player1 get %d" % res1
            time.sleep(1.5)
            E.wait()
    print "player1 quit!"

def player2():
    global res2
    while cnt > 0:
        if E.is_set():
            res2 = random.randint(1, 6)
            print "player2 get %d" % res2
            time.sleep(1.5)
            E.wait()
    print "player2 quit!"


if __name__ == "__main__":
    t1 = threading.Thread(target=judge, args=( ))
    t2 = threading.Thread(target=player1, args=( ))
    t3 = threading.Thread(target=player2, args=( ))
    t1.start()
    t2.start()
    t3.start()
    t1.join()
    E.set()

It is important to note that when a thread calls wait, the current thread is blocked only if the variable value is False, and if the global variable is True, it is returned immediately.

Here is one possible result:

start game!
player1 get 2
player2 get 2
Stop! judging... both win!
start game!
player2 get 3
player1 get 4
Stop! judging... player1 win!
start game!
player1 get 1
player2 get 3
Stop! judging... player2 win!
game over by judge!
player2 quit!
player1 quit!

summary

The above are some synchronization measures to address thread security issues.Python provides thread-like synchronization for process synchronization, such as locks, semaphores, and so on.

Unlike sharing process resources between threads, processes have separate address spaces and different process memory spaces are isolated.

So for processes we usually focus on how they communicate with each other, and there will be subsequent articles on synchronization between processes.