Re: [Fwd: Re: [Fwd: Re: gdb/725: Crash using debug target and regcaches (in 5.3 branch?)]]

[Andrew Cagney <ac131313@redhat.com>]

>> Er, no I wont :-(
>> 
>> The attached is the refind patch.  I added the comment:
>> 
>> +  /* NOTE: cagney/2002-12-02: This assumes that the target code can
>> +     directly transfer the register values into the register cache.
>> +     This works fine when there is a 1:1 mapping between light weight
>> +     process (LWP) (a.k.a. process on GNU/Linux) and the thread.  On
>> +     an N:1 (user-land threads), or N:M (combination of user-land and
>> +     LWP threading), this does not work.  An LWP can be sitting in the
>> +     thread context switch code and hence, the LWP's registers belong
>> +     to no thread.  */
> 
> 
> First of all, this comment is wrong.

Why?

The code is assuming that the LWP registers belong to the currently 
selected thread's regcache.  That is a pretty scary assumption.

[I'll use that wording]

>  I think we're miscommunicating
> on what the patch does.  At this point the fetch_inferior_registers
> code has an inferior_ptid which looks like this:
>   PID = pid, LWPID = 0, TID = 0
> or
>   PID = pid, LWPID = otherpid, TID = 0

> Don't get confused by the use of TIDGET.  Look at the definition of
> TIDGET; it gets the _LWP_ id.  This's a search and destroy candidate if
> I ever saw one.

I'll add that.

> Some upper layer has already taken the TID, mapped it to an LWP id, and
> is asking for that LWP's registers by the time we get here.  So the LWP
> is known to belong to the thread we are querying.


>> however, with the patch applied, I see (and consistently, well 2 out of 
>> 2, which is pretty amasing for the thread testsuite) the new failure:
>> 
>> 
>> gdb.threads/killed.exp: GDB exits after multi-threaded program exits messily
>> 
>> looking at the log file:
>> 
>> (gdb) run
>> Starting program: /home/cagney/gdb/native/gdb/testsuite/gdb.threads/killed
>> [New Thread 1024 (LWP 6831)]
>> [New Thread 2049 (LWP 6832)]
>> [New Thread 1026 (LWP 6833)]
>> Cannot find user-level thread for LWP 6833: generic error
>> (gdb) PASS: gdb.threads/killed.exp: run program to completion
>> quit
>> The program is running.  Exit anyway? (y or n) y
>> Cannot find thread 2049: generic error
>> (gdb) FAIL: gdb.threads/killed.exp: GDB exits after multi-threaded 
>> program exits
>>  messily (gdb/568)
>> 
>> Which doesn't occure when the patch isn't applied.
> 
> 
> Are you sure about this last bit?  I see this failure even without the
> patch, on an i386 SMP system.  I just checked it moments ago.

Yes.  Not on an SMP machine though.

Andrew