Re: [9fans] a question on APE

[Kernel Panic <cinap_lenrek@gmx.de>]

Kernel Panic wrote:

> ron minnich wrote:
>
>> we're doing some work here with Andrey's port of ssh2. It *almost*
>> works. But I'm seeing a stack trace I don't understand.
>>
>> I can't give you all the details -- it's ssh, therefore it is pretty
>> awful -- but here is the short form: There is a proc called fromnet()
>> which has this inner loop:
>>     for(;;){
>>         if((n = libssh2_channel_read(c, buf, Bufsize)) > 0)
>>             write(1, buf, n);
>>         else
>>             goto Donenet;
>>     }
>>
>> When this proc is entered, ape has forked off two procs to handle the
>> fd 'c'. From the fromnet function, we see the libssh2_channel_read
>> does a select. here is where I get confused. The stk() for the two
>> procs looks like this:
>> pread()+0x7 /sys/src/libc/9syscall/pread.s:5
>> read(fd=0x5,buf=0x110414,n=0x1000)+0x2f /sys/src/libc/9sys/read.c:7
>> recv(flags=0x0,fd=0x5,a=0x110414,n=0x1000)+0x3e 
>> /sys/src/ape/lib/bsd/send.c:30
>> libssh2_packet_read(session=0x1102f8)+0x176
>> /usr/bootes/libssh2/libssh2-0.18/src/transport.c:326
>> libssh2_channel_read_ex(channel=0x114460,buflen=0x1000,stream_id=0x0,buf=0xdfffdee8)+0x2a7 
>>
>> /usr/bootes/libssh2/libssh2-0.18/src/channel.c:1442
>> fromnet(c=0x114460,s=0x1102f8)+0x2e
>> /usr/bootes/libssh2/libssh2-0.18/clients/ssh2.c:75
>> main(argc=0x2,argv=0xdfffef94)+0x47c
>> /usr/bootes/libssh2/libssh2-0.18/clients/ssh2.c:253
>> _main+0x31 /sys/src/libc/386/main9.s:16
>>
>> The the read on fd 5. That's the socket. Here is the other proc.
>>
>> _PREAD()+0x7 /sys/src/ape/lib/ap/syscall/_PREAD.s:5
>> _READ(fd=0x5,buf=0x600003c,n=0x2000)+0x2f 
>> /sys/src/ape/lib/ap/plan9/9read.c:10
>> _copyproc(b=0x6000028,fd=0x5)+0x86 /sys/src/ape/lib/ap/plan9/_buf.c:166
>> _startbuf(fd=0x5)+0x1dd /sys/src/ape/lib/ap/plan9/_buf.c:107
>> select(timeout=0xdfffde90,rfds=0xdfffde80,wfds=0x0,efds=0x0,nfds=0x6)+0xe9 
>>
>> /sys/src/ape/lib/ap/plan9/_buf.c:292
>> libssh2_waitsocket(session=0x1102f8,seconds=0x0)+0x7b
>> /usr/bootes/libssh2/libssh2-0.18/src/packet.c:1054
>> libssh2_channel_read_ex(channel=0x114460,buflen=0x1000,stream_id=0x0,buf=0xdfffdee8)+0x69 
>>
>> /usr/bootes/libssh2/libssh2-0.18/src/channel.c:1408
>> fromnet(c=0x114460,s=0x1102f8)+0x2e
>> /usr/bootes/libssh2/libssh2-0.18/clients/ssh2.c:75
>> main(argc=0x2,argv=0xdfffef94)+0x47c
>> /usr/bootes/libssh2/libssh2-0.18/clients/ssh2.c:253
>> _main+0x31 /sys/src/libc/386/main9.s:16
>>
>> ok, I think this stack is a bit messed up, since I don't see how we
>> can have the coyproc in the call chain from select(), but ... is it?
>

Ahh... just looked at the code...
Ok, as i expected... recv() calls a different read() from 
/sys/src/libc/9sys/read.c. It will all work if
recv() would call the thing from this one: 
/sys/src/ape/lib/ap/plan9/read.c.

I guess you could work arround it by using read() instead of recv() in 
ssh-code, but the right
thing is to fix ape and have recv() call the read() from ap/plan9/read.c.

> Plan9 has no select functionality. Select is emulated in APE by 
> forking a childproc that reads an fd and
> fills a buffer (on a shared memory area). Read() should then pick up 
> the data from the buffer and
> wakeup the reader proc if it sleeps (because the buffer got filled 
> up). Select() will startup such a
> reader proc (startbuf()) if it is not already "bufferd" and then check 
> if the buffer has data available,
> so the stacktrace looks valid to me.
>
> Maybe the bufferd filedescriptors doesnt work with the recv() call and 
> are only implemented for read()?
> I think you should find some kind of switch in read() that checks if 
> the fd is bufferd and then calls
> some _buf.c function that copies the data from the buffer.
> Maybe this is missing for recv()?
>
>> I realize there is very little information here, sorry ... here's what
>> is bothering me. It seems we have two procs hanging on a read on fd 5.
>> I think the copyproc and some other proc are in conflict but ... I am
>> unsure. The problems we are seeing might be explained by the wrong
>> proc grabbing output at the wrong time -- it feels like a race
>> condition. And acid trips we can take to hammer this one down?
>>
>> Anyone ever done a select on a socket in ape?
>>