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From: "simark at simark dot ca" <sourceware-bugzilla@sourceware.org>
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Subject: [Bug gdb/24694] FAIL: gdb.multi/multi-arch-exec.exp: first_arch=1:
 selected_thread=1: follow_exec_mode=same: continue across exec that changes
 architecture (Couldn't get registers: No such process)
Date: Tue, 01 Dec 2020 02:57:25 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D24694

Simon Marchi <simark at simark dot ca> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |simark at simark dot ca

--- Comment #8 from Simon Marchi <simark at simark dot ca> ---
This is due to an exec race.  We have two threads

- leader thread, doing the exec
- other thread, minding its own business

Things to remember are:

- GDB resumes threads one by one, one after the other
- When a multi-threaded program execs, regardless of which thread did the e=
xec,
all non-leader threads disappear and it looks like the leader is now execut=
ing
the new executable.

When we do a "continue" after stopping at the all_started function, GDB tri=
es
to resume leader first, then other.  In the failing case, leader has the ti=
me
to run its exec before GDB tries to resume the other thread.  So the ptrace
resume on the other thread fails with

  Couldn't get registers: No such process.

That aborts the resumption command and causes unexpected output, making the
test fail.

Since this particular test is not about testing this particular corner case=
, I
think we can avoid it by adding a synchronization point after the continue,=
 so
the exec only happens after both threads were resumed.

But I also think we should handle the situation better.  When resuming mult=
iple
threads and one of them fails with "no such process", we should probably pr=
int
a warning but carry on, not abort the continue.  There would be a separate =
test
for that (or whatever behavior we decide to have).

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