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* [Bug gdb/12702] New: gdb can hang waiting for thread group leader
@ 2011-04-26  0:39 dje at google dot com
  2011-04-26 17:58 ` [Bug gdb/12702] " jan.kratochvil at redhat dot com
                   ` (10 more replies)
  0 siblings, 11 replies; 12+ messages in thread
From: dje at google dot com @ 2011-04-26  0:39 UTC (permalink / raw)
  To: gdb-prs

http://sourceware.org/bugzilla/show_bug.cgi?id=12702

           Summary: gdb can hang waiting for thread group leader
           Product: gdb
           Version: HEAD
            Status: NEW
          Severity: normal
          Priority: P2
         Component: gdb
        AssignedTo: unassigned@sourceware.org
        ReportedBy: dje@google.com


Created attachment 5685
  --> http://sourceware.org/bugzilla/attachment.cgi?id=5685
Patch to workaround the issue.

When ptracing, waitpid with options == 0 will hang if the thread group leader
exits while there are still other threads around.
[Blech!  IWBN if it could at least return an error instead of hanging.]

When gdb detects a thread has stopped, it will stop other threads before
returning control to the user (in "all-stop mode").
A race occurs when the main thread exits between gdb detecting a 2nd thread has
stopped and gdb waits for the main thread to stop.
If the main thread has exited and there are other threads then waitpid
(main_thread_pid, &status, 0) will hang.

This patch to gdb and the accompanying testcase illustrates the issue.
Apply to cvs head as of 11apr25.

diff -u -p -r1.199 linux-nat.c
--- linux-nat.c 9 Mar 2011 12:48:55 -0000       1.199
+++ linux-nat.c 25 Apr 2011 23:48:13 -0000
@@ -4047,6 +4108,21 @@ linux_thread_alive (ptid_t ptid)
                        target_pid_to_str (ptid),
                        err ? safe_strerror (tmp_errno) : "OK");

+  if (debug_linux_nat && GET_PID (ptid) != GET_LWP (ptid))
+    {
+      char buf[200];
+      sprintf (buf, "cat /proc/%ld/task/%ld/status",
+              (long) GET_PID (ptid), (long) GET_LWP (ptid));
+      system (buf);
+      sleep (3);
+      err = kill_lwp (GET_LWP (ptid), 0);
+      fprintf_unfiltered (gdb_stdlog,
+                         "LLTA: KILL(SIG0) %s (%s)\n",
+                         target_pid_to_str (ptid),
+                         err ? safe_strerror (err) : "OK");
+      system (buf);
+    }
+
   if (err != 0)
     return 0;


bash$ cat testcase.c

#include <stdio.h>
#include <pthread.h>
#include <syscall.h>
#include <stdarg.h>
#include <stdlib.h>

int
gettid ()
{
 return syscall (__NR_gettid);
}

void
printf_flushed (const char *msg, ...)
{
 va_list args;
 va_start (args, msg);
 vprintf (msg, args);
 va_end (args);
 fflush (stdout);
}

void*
thread_function (void* dummy_ptr)
{
 printf_flushed ("Thread self 0x%x, pid %d, lwp %d\n",
                 pthread_self (), getpid (), gettid ());
 asm volatile ("int3");
 pthread_exit ((void *) 0);
 abort ();
}

int
main ()
{
 pthread_t thread_id;
 pthread_create (&thread_id, NULL, thread_function, NULL);
 sleep (1);
 return 0;
}
bash$ gcc -g testcase.c -o testcase.x64 -lpthread
bash$ ./gdb --batch -nx -ex "set debug infrun 1" -ex "set debug lin-lwp 1" -ex
run -ex quit ./testcase.x64
[...]
LLW: waitpid 15050 received Trace/breakpoint trap (stopped)
LLTA: KILL(SIG0) Thread 0x7ffff783c700 (LWP 15050) (OK)
Name:   testcase.x64
State:  T (tracing stop)
Tgid:   15047
Pid:    15050
PPid:   15045
TracerPid:      15045
[...]
State:  Z (zombie)
Tgid:   15047
Pid:    15050
PPid:   15045
TracerPid:      15045
[...]
LLW: Candidate event Trace/breakpoint trap (stopped) in Thread 0x7ffff783c700
(LWP 15050).
SC:  kill Thread 0x7ffff7fd7700 (LWP 15047) **<SIGSTOP>**
SC:  lwp kill 0 ERRNO-OK
hang

The attached patch works around the bug^wissue.

I suspect all calls to waitpid with options == 0 need to be audited and either
fixed or documented that they can't trip over this issue.

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^ permalink raw reply	[flat|nested] 12+ messages in thread
end of thread, other threads:[~2014-02-27 15:39 UTC | newest]

Thread overview: 12+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2011-04-26  0:39 [Bug gdb/12702] New: gdb can hang waiting for thread group leader dje at google dot com
2011-04-26 17:58 ` [Bug gdb/12702] " jan.kratochvil at redhat dot com
2011-04-27  1:10 ` dje at google dot com
2011-05-02 17:02 ` dje at google dot com
2011-05-17 15:57 ` dje at google dot com
2011-05-17 16:06 ` jan.kratochvil at redhat dot com
2011-05-27 16:57 ` cvs-commit at gcc dot gnu.org
2011-05-27 17:03 ` jan.kratochvil at redhat dot com
2013-07-20 10:01 ` teawater at gmail dot com
2014-02-27 14:31 ` cvs-commit at gcc dot gnu.org
2014-02-27 15:02 ` palves at redhat dot com
2014-02-27 15:39 ` palves at redhat dot com

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