Re: [PATCH 2/2] [gdb] Initialize main_thread_id earlier

[Tom de Vries <tdevries@suse.de>]

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On 8/2/23 18:55, Tom Tromey wrote:
>>>>>> "Tom" == Tom de Vries via Gdb-patches <gdb-patches@sourceware.org> writes:
> 
>>> I wonder if it's enough to have:
>>> static std::thread::id main_thread_id = std::this_thread::get_id ();
> 
> Tom> In principle it's enough but it does expose us to the "Static
> Tom> Initialization Order Fiasco" [1], so I prefer this solution.
> 
> If we think it's safe enough to initialize it on first use, just moving
> the definition into is_main_thread is enough to avoid this problem.
> 

Done.

> Now, in theory it is possible for gdb to start a thread very early and
> then have that thread call is_main_thread, setting the global
> incorrectly.  If we care about that then we need an explicit call to set
> it early during startup.

Yes, Pedro had the same remark, that was already fixed in the previous 
version.

Any further comments?

Thanks,
- Tom


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From dc19a86c9fd189bb2e7394199e5fda0179d8e6cb Mon Sep 17 00:00:00 2001
From: Tom de Vries <tdevries@suse.de>
Date: Fri, 28 Jul 2023 11:41:07 +0200
Subject: [PATCH] [gdb] Initialize main_thread_id earlier

I wrote a patch using is_main_thread (), and found it returning false in the
main thread due to main_thread_id not being initialized yet.

Initialization currently takes place in _initialize_run_on_main_thread, but
that's too late for earlier uses.

Fix this by initializing, either:
- when entering main, or
- on an earlier first use.

Tested on x86_64-linux.
---
 gdb/gdb.c                |  7 +++++++
 gdb/run-on-main-thread.c | 19 ++++++++++++++++++-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/gdb/gdb.c b/gdb/gdb.c
index 71a3fd12ba1..6e3ff0755ab 100644
--- a/gdb/gdb.c
+++ b/gdb/gdb.c
@@ -19,10 +19,17 @@
 #include "defs.h"
 #include "main.h"
 #include "interps.h"
+#include "run-on-main-thread.h"
 
 int
 main (int argc, char **argv)
 {
+  /* The first call to is_main_thread () should be from the main thread.
+     If this is the first call, then that requirement is fulfilled here.
+     If this is not the first call, then this verifies that the first call
+     fulfilled that requirement.  */
+  gdb_assert (is_main_thread ());
+
   struct captured_main_args args;
 
   memset (&args, 0, sizeof args);
diff --git a/gdb/run-on-main-thread.c b/gdb/run-on-main-thread.c
index 91d25dae28f..e5c9a775b13 100644
--- a/gdb/run-on-main-thread.c
+++ b/gdb/run-on-main-thread.c
@@ -94,12 +94,24 @@ run_on_main_thread (std::function<void ()> &&func)
   serial_event_set (runnable_event);
 }
 
+#if CXX_STD_THREAD
+static bool main_thread_id_initialized = false;
+#endif
+
 /* See run-on-main-thread.h.  */
 
 bool
 is_main_thread ()
 {
 #if CXX_STD_THREAD
+  /* Initialize main_thread_id on first use of is_main_thread.  */
+  if (!main_thread_id_initialized)
+    {
+      main_thread_id_initialized = true;
+
+      main_thread_id = std::this_thread::get_id ();
+    }
+
   return std::this_thread::get_id () == main_thread_id;
 #else
   return true;
@@ -111,7 +123,12 @@ void
 _initialize_run_on_main_thread ()
 {
 #if CXX_STD_THREAD
-  main_thread_id = std::this_thread::get_id ();
+  /* The variable main_thread_id should be initialized when entering main, or
+     at an earlier use, so it should already be initialized here.  */
+  gdb_assert (main_thread_id_initialized);
+
+  /* Assume that we execute this in the main thread.  */
+  gdb_assert (is_main_thread ());
 #endif
   runnable_event = make_serial_event ();
   add_file_handler (serial_event_fd (runnable_event), run_events, nullptr,

base-commit: 86bb2d76c901a558e5b2dd29df95874ce6e78dae
-- 
2.35.3