1 00:00:00,003 --> 00:00:05,005 (upbeat music) 2 00:00:05,005 --> 00:00:09,000 - [Instructor] Let's look at those pretty good challenges. 3 00:00:09,000 --> 00:00:10,001 So the first thing was 4 00:00:10,001 --> 00:00:13,009 to go get some Linux kernel source code. 5 00:00:13,009 --> 00:00:15,009 So I did a little grep from my history here. 6 00:00:15,009 --> 00:00:19,000 You can see I did a few things, 7 00:00:19,000 --> 00:00:22,006 but it's this one right here 8 00:00:22,006 --> 00:00:25,009 with noble in lowercase that was correct. 9 00:00:25,009 --> 00:00:28,003 So maybe you did something like that. 10 00:00:28,003 --> 00:00:33,009 Then after that we wanted you to do a make cscope. 11 00:00:33,009 --> 00:00:35,003 And we'll do that again. 12 00:00:35,003 --> 00:00:38,007 And then a make tags. 13 00:00:38,007 --> 00:00:40,008 All right, and if those worked, 14 00:00:40,008 --> 00:00:45,002 then you got some cscope.out files, 15 00:00:45,002 --> 00:00:48,001 and you got a tags file. 16 00:00:48,001 --> 00:00:49,003 Those are text files. 17 00:00:49,003 --> 00:00:50,001 You can look at those. 18 00:00:50,001 --> 00:00:52,007 They're essentially indexes of where stuff is. 19 00:00:52,007 --> 00:00:57,003 So now that you got those, you can use cscope and tags. 20 00:00:57,003 --> 00:01:00,002 So let's go cscope -d, 21 00:01:00,002 --> 00:01:03,003 and we want to look for the write zero function. 22 00:01:03,003 --> 00:01:05,008 We want the definition of that. 23 00:01:05,008 --> 00:01:11,002 Write_zero. 24 00:01:11,002 --> 00:01:13,003 And it's a macro. 25 00:01:13,003 --> 00:01:17,006 Write_Zero is actually the same as write_null. 26 00:01:17,006 --> 00:01:18,008 They didn't make a new function, 27 00:01:18,008 --> 00:01:22,000 so they're just reusing the write_null, 28 00:01:22,000 --> 00:01:24,001 and we looked at that earlier. 29 00:01:24,001 --> 00:01:27,006 And write_null just says, 30 00:01:27,006 --> 00:01:32,001 yeah, I wrote how many ever you wanted me to write. 31 00:01:32,001 --> 00:01:36,000 But we also want to look at read_zero. 32 00:01:36,000 --> 00:01:38,006 So we'll go back to Cscope. 33 00:01:38,006 --> 00:01:40,008 Back here where we started by getting out of VI, 34 00:01:40,008 --> 00:01:42,003 just a colon Q. 35 00:01:42,003 --> 00:01:45,007 Then I do a tab to get to the bottom, 36 00:01:45,007 --> 00:01:49,002 and then I'm going to do read_zero 37 00:01:49,002 --> 00:01:53,003 looking for the definition of that, and I hit enter, 38 00:01:53,003 --> 00:01:54,002 and there's only one. 39 00:01:54,002 --> 00:01:56,004 So it brings me right to it. 40 00:01:56,004 --> 00:01:59,003 So here's read_zero, 41 00:01:59,003 --> 00:02:03,008 and let me get the loop in the window here. 42 00:02:03,008 --> 00:02:07,001 So count is the number of bites we want to read. 43 00:02:07,001 --> 00:02:11,005 So we're going to loop while that's not zero, 44 00:02:11,005 --> 00:02:15,008 and we chunk it up a little bit, and we use some function. 45 00:02:15,008 --> 00:02:17,008 That's probably what zeroes it out. 46 00:02:17,008 --> 00:02:23,002 Clear user, and we check to make sure stuff worked. 47 00:02:23,002 --> 00:02:26,001 But down here we got a couple interesting things. 48 00:02:26,001 --> 00:02:29,008 Here we check to see if our current process, 49 00:02:29,008 --> 00:02:33,002 that's the process essentially that's called read, 50 00:02:33,002 --> 00:02:34,005 has a signal pending. 51 00:02:34,005 --> 00:02:36,002 So did a signal happen? 52 00:02:36,002 --> 00:02:40,008 So in the kernel, signals don't interrupt us 53 00:02:40,008 --> 00:02:41,008 at arbitrary points. 54 00:02:41,008 --> 00:02:46,005 The kernel has to check to see if a signal was sent, 55 00:02:46,005 --> 00:02:48,003 and if a signal was sent, 56 00:02:48,003 --> 00:02:51,007 then we break out of the while loop, right? 57 00:02:51,007 --> 00:02:56,007 Then the cond_resched checks 58 00:02:56,007 --> 00:02:59,004 to see essentially if some other process 59 00:02:59,004 --> 00:03:01,007 has its turn to run 60 00:03:01,007 --> 00:03:04,009 like it has a higher priority or we used up our time slice. 61 00:03:04,009 --> 00:03:07,004 So the cond_resched checks to see 62 00:03:07,004 --> 00:03:09,003 if the scheduler has set a flag 63 00:03:09,003 --> 00:03:12,005 saying I need to reschedule you. 64 00:03:12,005 --> 00:03:14,009 And if that flag's been set, 65 00:03:14,009 --> 00:03:19,002 then cond_resched will call the scheduler function 66 00:03:19,002 --> 00:03:21,003 which will then schedule another function. 67 00:03:21,003 --> 00:03:22,005 So we do that in the loop. 68 00:03:22,005 --> 00:03:27,005 Every time through, we do some chunk 69 00:03:27,005 --> 00:03:30,002 and then we check for signals, 70 00:03:30,002 --> 00:03:32,002 and we check to see if we lost our turn. 71 00:03:32,002 --> 00:03:33,009 Then we do another chunk. 72 00:03:33,009 --> 00:03:38,001 So if you're reading a large amount, 73 00:03:38,001 --> 00:03:40,008 you can't just keep the kernel busy 74 00:03:40,008 --> 00:03:42,009 for that whole time. 75 00:03:42,009 --> 00:03:46,000 All right, that should have been pretty interesting.