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From: Jacob Moody <moody@posixcafe.org>
To: 9fans@9fans.net
Subject: Re: [9fans] syscall silently kill processes
Date: Sun, 19 Jun 2022 00:13:50 -0600	[thread overview]
Message-ID: <48e83724-8ea9-79b2-536a-b6d3c952c152@posixcafe.org> (raw)
In-Reply-To: <f75f7af1-7d33-5422-17b-3cccc92ce20@SDF.ORG>

On 6/18/22 23:54, adr wrote:
> On Sat, 18 Jun 2022, Jacob Moody wrote:
>> I've attempted to reproduce it, trying to remove the libthread/notify
>> factors. I've come up with this:
>>
>> #include <u.h>
>> #include <libc.h>
>>
>> static void
>> proc_udp(void*)
>> {
>>        char resp[512];
>>        char req[] = "request";
>>        int fd;
>>        int n;
>>        int pid;
>>
>>        fd = dial("udp!185.157.221.201!5678", nil, nil, nil);
>>        if(fd < 0)
>>                exits("can't dial");
>>
>>        if(write(fd, req, strlen(req)) != strlen(req))
>>                exits("can't write");
>>
>>        pid = getpid();
>>        fprint(1, "start %d\n", pid);
>>        n = read(fd, resp, sizeof(resp)-1);
>>        fprint(1, "end %d %d\n", pid, n);
>>        exits(nil);
>> }
>>
>> void
>> main(int, char**)
>> {
>>        int i;
>>        Waitmsg *wm;
>>
>>        for(i = 0; i < 10; i++){
>>                switch(fork()){
>>                case -1:
>>                        sysfatal("fork %r");
>>                case 0:
>>                        proc_udp(nil);
>>                        sysfatal("ret");
>>                default:
>>                        break;
>>                }
>>        }
>>        for(i = 0; i < 10; i++){
>>                wm = wait();
>>                print("proc %d died with message %s\n", wm->pid, wm->msg);
>>        }
>>        exits(nil);
>> }
>>
>> This code makes it pretty obvious that we are losing some children;
>> on my machine this program never exits. I see some portion of the
>> readers correctly returning -1, and the parent is able to get their
>> Waitmsg but not all of them.
> 
> Moody I think this old thread will interest you:
> 
> https://marc.info/?t=112730920400001&r=1&w=2
> 
> Russ Cox explained there:
>   It appears that your program, at its core, it is doing this:
> 
>   void
>   readproc(void *v)
>   {
>       int fd;
>       char buf[100];
>       fd = (int)v;
>       read(fd, buf, sizeof buf);
>   }
> 
>   void
>   threadmain(int argc, char **argv)
>   {
>       int p[2];
>       pipe(p);
>       proccreate(readproc, (void*)p[0], 8192);
>       proccreate(readproc, (void*)p[1], 8192);
>       close(p[0]);
>       /* and here you expect the first readproc to be done */
>       close(p[1]);
>       /* and here the second */
>   }
> 
>   Each read call is holding up a reference to its channel
>   inside the kernel, so that even though you've closed the fd
>   and removed the ref from the fd table, there is still a reference
>   to each side of the pipe in the form of the process blocked
>   on the read.
> 
>   I've never been sure whether the implicit ref held during
>   the system call is good behavior, but it's hard to change.
> 
>   In your case, writing 0 (or anything) makes the read
>   finish, releasing the last ref to the underlying pipe when
>   the system call finishes, and then everything cleans up
>   as expected.  So you've found your workaround, and now
>   we understand why it works.
> 

I was just making the wrong observation here.
I thought I had observed the child procs getting
murdered mid read, and the parent never getting
the Waitmsg. Testing again I see as Andrej had observed,
they are just blocking. I thought I was seeing a bug
related to just udp, nothing to do with notes/threads.

I apologize for the confusion, interesting thread
you linked regardless.

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  reply	other threads:[~2022-06-19  6:14 UTC|newest]

Thread overview: 50+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-06-17  9:37 andrey100100100
2022-06-17 13:46 ` Thaddeus Woskowiak
2022-06-17 14:11   ` Jacob Moody
2022-06-17 14:39     ` Thaddeus Woskowiak
2022-06-17 15:06     ` andrey100100100
2022-06-17 16:08       ` Skip Tavakkolian
2022-06-17 16:11         ` Skip Tavakkolian
2022-06-17 16:16           ` Skip Tavakkolian
2022-06-17 17:42             ` adr
2022-06-17 16:11       ` Jacob Moody
2022-06-17 18:48         ` andrey100100100
2022-06-17 19:28           ` Jacob Moody
2022-06-17 21:15           ` adr
2022-06-18  6:40             ` andrey100100100
2022-06-18  8:37               ` adr
2022-06-18  9:22                 ` adr
2022-06-18 12:53                   ` Jacob Moody
2022-06-18 22:03                     ` andrey100100100
2022-06-19  5:54                     ` adr
2022-06-19  6:13                       ` Jacob Moody [this message]
2022-06-18 22:22                   ` andrey100100100
2022-06-18 16:57                 ` andrey100100100
2022-06-19  2:40                   ` adr
2022-06-19  5:01                     ` adr
2022-06-19  8:52                       ` andrey100100100
2022-06-19 10:32                         ` adr
2022-06-19 11:40                           ` andrey100100100
2022-06-19 12:01                             ` andrey100100100
2022-06-19 15:10                           ` andrey100100100
2022-06-19 16:41                             ` adr
2022-06-19 21:22                               ` andrey100100100
2022-06-19 21:26                                 ` andrey100100100
2022-06-20  4:41                                 ` adr
2022-06-20  5:39                                   ` andrey100100100
2022-06-20  5:59                                   ` adr
2022-06-20 15:56                                     ` andrey100100100
2022-06-20 22:29                                       ` Skip Tavakkolian
2022-06-21  7:07                                         ` andrey100100100
2022-06-21 11:26                                           ` adr
2022-06-21 13:03                                             ` andrey100100100
2022-06-21 13:22                                               ` adr
2022-06-28 15:28                                                 ` adr
2022-06-28 16:43                                                   ` ori
2022-06-28 18:19                                                   ` adr
2022-06-28 18:28                                                     ` adr
2022-06-28 19:09                                                   ` andrey100100100
2022-06-28 19:42                                                     ` adr
2022-06-29 13:14                                                       ` adr
2022-06-21 13:47                                             ` andrey100100100
2022-06-21  7:22                                         ` adr

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