RE: Displaced stepping not always working as expected

[Marc Khouzam <marc dot khouzam at ericsson dot com>]

> -----Original Message-----
> From: Pedro Alves [mailto:pedro@codesourcery.com] 
> Sent: Wednesday, September 21, 2011 6:20 AM
> To: gdb@sourceware.org
> Cc: Marc Khouzam
> Subject: Re: Displaced stepping not always working as expected
> 
> On Tuesday 20 September 2011 20:54:24, Marc Khouzam wrote:
> > Hi,
> > 
> > I just need a hint on where next to look...
> 
> > I've been asked to look into problems with non-stop on 
> > a user-mode-linux virtual machine
> > (http://user-mode-linux.sourceforge.net/)
> 
> So does this only happen with UML?  UML uses ptrace internally for
> its own business, I wouldn't be surprised if there's something
> wonky going on at that level.

Yes, only on UML.  In fact, only on a particular installation of UML:
I am not able to reproduce the problem on my own installation of UML.

So, I also believe it is because of the UML.  But I'm hoping it might be
something that can be fixed.  Which is why I'm trying to pin-point
the cause.

> > On that AMD 64bit machine, I cannot step or resume past a breakpoint
> > when using non-stop with a multi-threaded program _if_ any of the
> > threads is still running.  If I interrupt all threads, then 
> displaced
> > stepping works.
> 
> I wouldn't be surprised if the UM kernel is reporting a spurious
> SIGTRAP to gdb.  Try "set debug lin-lwp 1" as well, but I don't
> think it'll tell you much.  Maybe peeking at eflags or the siginfo
> of that SIGTRAP reveals something.

Thanks.
No luck with "set debug lin-lwp 1" but I will try to open up the
siginfo.  I've spend the better part of the day getting familiar
with the relevant code.

> Try "set debug lin-lwp 1", and see if the resume was preempted and
> for some bizarre reason the core is getting a cached wait status
> instead of really resuming the thread.
> 
> Otherwise, this smells like a UML problem.

One bizarre thing I noticed when trying "set debug target 1"
is that more threads get started than there should!

Normally, I get (in summary):

set non-stop on
set target-async on
b 8
r&
Breakpoint 1, thread_exec1 (ptr=0x400888) at multithread.c:8
8               i++;
info thr
  Id   Target Id         Frame 
  2    Thread 0x40804940 (LWP 946) "multi3" thread_exec1 (ptr=0x400888) at multithread.c:8
* 1    Thread 0x40003800 (LWP 943) "multi3" (running)

If I add "set debug target 1" before 'r&', I get:

info thr
  Id   Target Id         Frame 
  4    Thread 0x41806940 (LWP 940) "multi3" thread_exec1 (ptr=0x400888) at multithread.c:8
target_core_of_thread (935) = 0
  3    Thread 0x41005940 (LWP 939) "multi3" (running)
target_core_of_thread (935) = 0
  2    Thread 0x40804940 (LWP 938) "multi3" (running)
target_core_of_thread (935) = 0
* 1    Thread 0x40003800 (LWP 935) "multi3" (running)

What the???  The program (copied below), only starts two threads.

I wonder if libthread is causing some problem when using UML?
When using gdbserver, I do often get a gdbserver warning when attaching:
"PID mismatch!  Expected 789, got 791"
where 791 is the LWP of a thread other than the main one. 

Anyway, I'll keep digging.  Thanks for the pointers.

Marc

My test program
===============

#include <pthread.h>
#include <stdio.h>
#include <unistd.h>

void *thread_exec1(void *ptr) {
    int i;
    for (i=0;i<500;i++) {
        i++;
        i--;
        printf("in the second thread %d\n", i);
        sleep(1);
    }
}

int main() {
    pthread_t thread2;
    int iret2 = pthread_create( &thread2, NULL, thread_exec1, (void*) "Thread 2");

    printf("in the first thread\n");
    int i;
    for (i=0;i<30;i++) {
        sleep(2);    // while here, non-stop can't step over breakpoints
    }
    printf("ABOUT TO CALL JOIN\n");

    pthread_join(thread2, NULL);   // but while here, it can!
    return 0;
}