[RFC,gdb/doc] Document non-stop attach behaviour

Message ID 20210607132127.GA10588@delia.home
State New
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  • [RFC,gdb/doc] Document non-stop attach behaviour
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Commit Message

Tom de Vries June 7, 2021, 1:21 p.m.
Hi,

While investigating PR27908, I got confused about what the proper behaviour
is when attaching to a multi-threaded program in non-stop mode.

In particular, when running a script that issues an "info threads" after an
attach in non-stop mode, it can show a non-current thread as still running:
...
$ ./a.out & pid=$!; \
  gdb -q \
    -iex "set trace-commands on" \
    -iex "set pagination off" \
    -iex "set non-stop on" \
    -p $pid \
    -ex "info threads"
[2] 10038
+set pagination off
+set non-stop on
Attaching to process 10038
[New LWP 10040]
main () at test.c:37
37        while (1)
+info threads
  Id   Target Id                                 Frame
* 1    Thread 0x7f63547a8740 (LWP 10038) "a.out" main () at t.c:37
  2    Thread 0x7f6353fd7700 (LWP 10040) "a.out" (running)
(gdb)
Thread 2 "a.out" stopped.
thread_func (p=0x0) at test.c:13
13        while (1)
...

An "info threads" after the 'Thread 2 "a.out" stopped' message will show it as
stopped though:
...
info threads
+info threads
  Id   Target Id                                 Frame
* 1    Thread 0x7f63547a8740 (LWP 10038) "a.out" main () at t.c:37
  2    Thread 0x7f6353fd7700 (LWP 10040) "a.out" thread_func (p=0x0) at t.c:13
(gdb)
...

In conclusion, it seems that attaching in non-stop mode stops all threads,
just like in all-stop mode.  But the stopping of non-current threads is
visible to the user.

Update the non-stop documentation to describe the attach behaviour in some
detail.

Any comments?

Thanks,
- Tom

[gdb/doc] Document non-stop attach behaviour

gdb/doc/ChangeLog:

2021-06-07  Tom de Vries  <tdevries@suse.de>

	* gdb.texinfo (Non-Stop Mode): Describe non-stop attach behaviour.

---
 gdb/doc/gdb.texinfo | 7 +++++++
 1 file changed, 7 insertions(+)

Patch

diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index d09b86cda95..0fd7ab16150 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -6957,6 +6957,13 @@  one thread while allowing others to run freely, stepping
 one thread while holding all others stopped, or stepping several threads
 independently and simultaneously.
 
+Note that attaching in non-stop mode stops all threads, just as in
+all-stop mode.  However, while in all-stop mode the attach command is
+finished only once all threads are stopped, in non-stop mode it's
+finished once the current thread is stopped.  Consequently, it's
+briefly possible to observe non-current threads still running after an
+attach.
+
 To enter non-stop mode, use this sequence of commands before you run
 or attach to your program: