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

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

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On 7/28/23 17:36, Pedro Alves wrote:
> On 2023-07-28 11:59, Tom de Vries via Gdb-patches wrote:
>> 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 also initializing on first use.
>>
>> Tested on x86_64-linux.
>> ---
>>   gdb/run-on-main-thread.c | 17 ++++++++++++++++-
>>   1 file changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/gdb/run-on-main-thread.c b/gdb/run-on-main-thread.c
>> index 91d25dae28f..bee5885d9a6 100644
>> --- a/gdb/run-on-main-thread.c
>> +++ b/gdb/run-on-main-thread.c
>> @@ -94,12 +94,27 @@ run_on_main_thread (std::function<void ()> &&func)
>>     serial_event_set (runnable_event);
>>   }
>>   
>> +#if CXX_STD_THREAD
>> +static void
>> +initialize_main_thread_id (void)
>> +{
>> +  static bool initialized = false;
>> +
>> +  if (initialized)
>> +    return;
>> +  initialized = true;
>> +
>> +  main_thread_id = std::this_thread::get_id ();
>> +}
>> +#endif
>> +
>>   /* See run-on-main-thread.h.  */
>>   
>>   bool
>>   is_main_thread ()
>>   {
>>   #if CXX_STD_THREAD
>> +  initialize_main_thread_id ();
> 

Hi Pedro,

thanks for the review.

> This is assuming that is_main_thread() will always be called once by
> the main thread, before any other thread could call it.  Otherwise if
> is_main_thread is called by some other thread first,
> is_main_thread -> initialize_main_thread_id will record the wrong thread
> id in the main_thread_id.  At least a comment somewhere would
> be warranted.  I think putting:
> 
>    gdb_assert (is_main_thread ());
> 
> somewhere in the early main code would be good.  It would prevent such
> situation from ever happening undetected.
> 

Updated accordingly.  Any further comments?

Thanks,
- Tom

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From 3886892551c9e35b6ea36a6df23167fa3690a319 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 | 24 +++++++++++++++++++++++-
 2 files changed, 30 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..5d453ffe612 100644
--- a/gdb/run-on-main-thread.c
+++ b/gdb/run-on-main-thread.c
@@ -94,12 +94,29 @@ run_on_main_thread (std::function<void ()> &&func)
   serial_event_set (runnable_event);
 }
 
+#if CXX_STD_THREAD
+static bool main_thread_id_initialized = false;
+
+static void
+initialize_main_thread_id ()
+{
+  if ( main_thread_id_initialized)
+    return;
+   main_thread_id_initialized = true;
+
+  main_thread_id = std::this_thread::get_id ();
+}
+#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.  */
+  initialize_main_thread_id ();
+
   return std::this_thread::get_id () == main_thread_id;
 #else
   return true;
@@ -111,7 +128,12 @@ 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: b5c37946cce4b41af0436529b139fa3d23e61f73
-- 
2.35.3