WEBVTT 00:07.090 --> 00:12.700 So, guys, now in this lecture, we will we will going to discuss how to avoid spurious wake up when 00:12.700 --> 00:14.790 we have multiple conditions to deal with. 00:15.790 --> 00:22.480 So women are so familiar with the concept of a spurious wake up and how to handle it by simply replacing 00:22.480 --> 00:24.490 the IFA statement with the wild condition. 00:25.330 --> 00:27.640 All right now, you must be very much familiar with this. 00:28.150 --> 00:31.180 So now we will going to discuss a little bit of extension of it. 00:31.960 --> 00:37.840 So consider that you want to go to the party and let us say that going to the party is some kind of 00:37.840 --> 00:38.800 a critical section. 00:39.490 --> 00:39.850 All right. 00:40.360 --> 00:46.120 And you only go to the party only and only when the following conditions are met. 00:46.720 --> 00:49.240 The condition one is that it must not be raining. 00:49.750 --> 00:55.990 Condition to say is that your best friend must be in the town and condition, three says, is that you 00:55.990 --> 01:03.340 must have a light available so you are willing to go to the party only and only when all these three 01:03.340 --> 01:06.610 conditions combined are satisfied. 01:08.350 --> 01:08.710 All right. 01:09.070 --> 01:18.370 So let us see how this code can be translated to the equivalent piece of code in which a triad is supposed 01:18.370 --> 01:22.180 to get blocked if all these three conditions are not satisfied. 01:23.320 --> 01:29.050 So let's say we have some Bartomeu decks, so we were going to stand with all the conditions to be tested 01:29.050 --> 01:31.390 in between mutex unlock and mutex lock. 01:32.170 --> 01:33.290 Now we very well know. 01:33.310 --> 01:39.250 Why do we sandwich all the conditions to be tested within mutex, lock and mutex some locks? 01:40.210 --> 01:48.250 So now let's say that you test the condition number one and condition number one is that it must not 01:48.250 --> 01:48.790 be raining. 01:49.690 --> 01:52.720 So let's say that this condition comes out to be false. 01:52.840 --> 01:54.580 It means that it is raining. 01:55.270 --> 01:56.260 So what will you do? 01:56.320 --> 01:58.780 You will go and sleep in your bed. 01:59.350 --> 02:04.660 I have intensely choosing that you go and sleep in a bath so that you lie unconscious. 02:05.290 --> 02:11.080 You yourself cannot wake up and look outside the window of your room and see whether it is still raining 02:11.080 --> 02:11.590 or not. 02:12.040 --> 02:13.350 You are sleeping in your bed. 02:13.360 --> 02:14.470 You are unconscious. 02:15.010 --> 02:17.860 Sleeping in the bed represents that. 02:17.860 --> 02:23.230 The threat is in the blog to state which by itself cannot do any kind of inspection. 02:24.870 --> 02:29.400 So let's say after some time, your sovereign comes and he wakes you up. 02:30.330 --> 02:36.600 So coming of the sovereign and waking you up means that some of the threat in the system has sent a 02:36.600 --> 02:38.100 signal to the blocked threat. 02:39.060 --> 02:39.420 All right. 02:39.780 --> 02:47.340 So you wake up, and since you are captured within the wild block, you will go and test the condition 02:47.340 --> 02:47.730 again. 02:48.720 --> 02:55.680 And let's say that it is still raining so you go and sleep in the bed again, and after some time, 02:55.680 --> 02:59.650 your or someone comes again wakes you up and the cycle goes on. 03:00.750 --> 03:07.500 So let's say that at some point of time you whined that when you look outside the window, it does not 03:07.500 --> 03:07.920 raining. 03:08.430 --> 03:14.610 So now you are happy, and now you jump to the condition number two condition number two say, is that 03:14.610 --> 03:17.370 your best friend is in the town or not? 03:18.270 --> 03:21.800 So now let's say that your best friend is not in the town. 03:22.050 --> 03:25.140 Therefore, you are going to decide to go to sleep in the bad. 03:26.270 --> 03:28.220 Your cell phone comes, wakes you up. 03:28.730 --> 03:34.670 You again go and test or check whether your best friend has arrived in the town or not. 03:35.510 --> 03:37.100 So let's say your best friend has now. 03:37.130 --> 03:37.760 All right. 03:38.480 --> 03:45.320 So you are happy and you go ahead and test the condition number three and condition number three. 03:45.350 --> 03:47.780 Let's say that the girl is not available to you. 03:48.320 --> 03:54.170 So you decide to sleep again in the bad, and let's hear someone come and he wakes you up. 03:54.740 --> 03:58.460 So again, you check whether the vehicle is available or not. 03:59.150 --> 04:01.700 And let's say that now the vehicle is available. 04:02.570 --> 04:02.960 All right. 04:03.350 --> 04:07.580 So now you are happy and you decide to go to the party. 04:07.580 --> 04:08.960 But there is one flaw in that. 04:09.800 --> 04:15.440 By the time you check that the vehicle is available, it might be possible that it started raining again. 04:16.800 --> 04:17.190 All right. 04:17.850 --> 04:18.870 So it means that. 04:20.040 --> 04:26.610 As soon as you come to know that the WAKIL is available, you must also check whether it has started 04:26.610 --> 04:27.660 raining again or not. 04:28.290 --> 04:34.080 And you must also check whether your friend is still in the town or not, then only you should go to 04:34.080 --> 04:34.650 the party. 04:36.000 --> 04:42.330 So the thing here is that that every time the tribe wakes up, it must check all the conditions to a 04:42.330 --> 04:43.650 wide, spurious wake up. 04:43.650 --> 04:46.050 All conditions combined should be tested. 04:47.480 --> 04:54.170 So this piece of code is actually a wrong way for testing the conditions to avoid spurious makeup. 04:54.830 --> 05:00.860 In this piece of code, we are checking three different conditions but not combined together, and this 05:00.860 --> 05:02.150 will be the source of error. 05:03.260 --> 05:06.950 So in the next slide, the same code can be rewritten as follows. 05:07.730 --> 05:10.170 And this is the correct code for testing the conditions. 05:10.910 --> 05:17.330 As soon as you wake up while sleeping in the bed at any of these three stages, you will go back to 05:17.330 --> 05:23.690 the first condition and test again from the beginning, and you will end up going to the party only 05:23.690 --> 05:29.660 when you are sure that all the three conditions for going to the party is satisfied. 05:30.710 --> 05:36.860 So every time you sleep from the batch, you go to the beginning and test all the conditions again. 05:38.390 --> 05:38.810 All right. 05:39.260 --> 05:48.110 So the very sunset or precise way of writing such piece of code is that that all the conditions must 05:48.110 --> 05:54.830 be tested along with their negations with the author or logical operator between them. 05:56.090 --> 06:02.420 So to avoid a spurious wake up, you must write, you must test all the conditions combined together 06:02.420 --> 06:04.880 in this way within a single one statement. 06:05.360 --> 06:12.200 And if it is not possible to express checking all of the conditions in a single one statement, because 06:12.200 --> 06:18.290 your conditions may be complicated so you can translate the piece of code on the right hand side into 06:18.590 --> 06:21.050 a little expanded or lengthy piece of code. 06:21.050 --> 06:24.800 On the left hand side, both the codes are logically equivalent. 06:26.180 --> 06:32.030 So this is a common error that most of the people do while writing multithreaded code always check the 06:32.030 --> 06:35.240 conditions combined together within the same wild statement. 06:35.660 --> 06:40.070 And this would ensure that spurious make up do not happen at all.