Re: [PATCH v2 18/24] Multi-target support

Simon Marchi
2020-01-17T03:47:33Z

On 2019-10-17 6:50 p.m., Pedro Alves wrote:
> This commit adds multi-target support to GDB. What this means is that
> with this commit, GDB can now be connected to different targets at the
> same time. E.g., you can debug a live native process and a core dump
> at the same time, connect to multiple gdbservers, etc.

Hi Pedro,

I think I found something odd happening starting with this commit. When
the test gdb.threads/tid-reuse.exp runs, we are adding a thread_info to the
inferior's thread list when a thread_info object with the same ptid is
already present. I don't think this is ever supposed to happen, right?

This can be observed by applying the following diff and running
gdb.threads/tid-reuse.exp. Note that some distributions are starting to bump
pid_max to a really large value, so make sure it's not that on your system.

diff --git a/gdb/thread.c b/gdb/thread.c
index 4959f938c7f3..8909e371e2ce 100644
--- a/gdb/thread.c
+++ b/gdb/thread.c
@@ -254,7 +254,10 @@ new_thread (struct inferior *inf, ptid_t ptid)
struct thread_info *last;

for (last = inf->thread_list; last->next != NULL; last = last->next)
- ;
+ gdb_assert (ptid != last->ptid);
+
+ gdb_assert (ptid != last->ptid);
+
last->next = tp;
}

What I have found so far, by breaking at thread.c:274 and stepping into
the "delete_thread" call, is that `inferior_ptid == ptid` is false
(inferior_ptid is (0,0,0)). In delete_thread, we are actually not
deleting the thread because ->deletable () returns false, because
the refcount is 2.

Before this patch, we would enter the `if (inferior_ptid == ptid)`.

If it can help, here is the command line I'm using to debug this, which is
mimicking the test:

./gdb -nx --data-directory=data-directory testsuite/outputs/gdb.threads/tid-reuse/tid-reuse -iex "set confirm off" -iex "set pagination off" -ex start -ex "set print thread-events off" -ex "b after_count" -ex c -ex "b do_nothing_thread_func" -ex c -ex d -ex "b after_reuse_time" -ex c

Simon