Re: [PATCH] Limit the switch_to_thread() calls in startup_inferior()

[Samuel Thibault <samuel.thibault@ens-lyon.org>]

Hello,

(Note that I have never really hacked very much into gdb's inferior
management, so I don't know much how things are supposed to be working.)

Simon Marchi, le sam. 24 oct. 2020 22:58:05 -0400, a ecrit:
> I tried to wrap my head around this and understand the need for the
> switch_to_thread, what could be the impacts of your change, and I just
> can't give anything close to a definitive answer.
> 
[...]
> Samuel, as the last person who touched gnu-nat.c, do you have a bit of
> time to test Kamil's patch on Hurd, see if it breaks starting inferiors?

I am not really sure what I am supposed to test. I tried a program that
starts a second thread which just pause()s. With Kamil's patch I can
switch & backtrace between the two threads.

(gdb) r
Starting program: /home/samy/test
^C[New Thread 13012.6]
[New Thread 13012.7]

Thread 5 received signal SIGINT, Interrupt.
0x0107951c in ?? () from /lib/i386-gnu/libc.so.0.3
(gdb) info thread
  Id   Target Id         Frame
* 5    Thread 13012.5    0x0107951c in ?? () from /lib/i386-gnu/libc.so.0.3
  6    Thread 13012.6    0x0109dc74 in _hurd_intr_rpc_mach_msg ()
   from /lib/i386-gnu/libc.so.0.3
  7    Thread 13012.7    0x0107951c in ?? () from /lib/i386-gnu/libc.so.0.3
(gdb) bt
#0  0x0107951c in ?? () from /lib/i386-gnu/libc.so.0.3
#1  0x01079cc6 in mach_msg () from /lib/i386-gnu/libc.so.0.3
[...]
#5  0x0104cdca in pthread_join () from /lib/i386-gnu/libpthread.so.0.3
#6  0x0803254b in main () at test.c:13
(gdb) thread 7
[Switching to thread 7 (Thread 13012.7)]
#0  0x0107951c in ?? () from /lib/i386-gnu/libc.so.0.3
(gdb) bt
#0  0x0107951c in ?? () from /lib/i386-gnu/libc.so.0.3
[...]
#3  0x0114e4ce in pause () from /lib/i386-gnu/libc.so.0.3
#4  0x080324fa in f (foo=0x0) at test.c:6
#5  0x0104c3ff in entry_point () from /lib/i386-gnu/libpthread.so.0.3
#6  0x00000000 in ?? ()

> And what if you remove all the switch_to_thread from that function, does
> it still work?

If I remove all calls from that fonction I am getting

(gdb) r
Starting program: /home/samy/test
Can't fetch registers from thread bogus thread id 1: No such thread
(gdb) info thread
  Id   Target Id         Frame
  5    Thread 18821.5    0x000021e0 in ?? ()

The current thread <Thread ID 1> has terminated.  See `help thread'.

Since I now have a built tree I can easily experiment more.

Samuel