Re: [RFA] Fix internal error with 'set debug infrun 1' under high load

[Kevin Buettner <kevinb at redhat dot com>]

Hi Philippe,

There is definitely a bug in this section of code from infrun.c:

	  else if (ws.kind == TARGET_WAITKIND_THREAD_EXITED
		   || ws.kind == TARGET_WAITKIND_EXITED
		   || ws.kind == TARGET_WAITKIND_SIGNALLED)
	    {
	      if (debug_infrun)
		{
		  ptid_t ptid = ptid_t (ws.value.integer);

		  fprintf_unfiltered (gdb_stdlog,
				      "infrun: %s exited while "
				      "stopping threads\n",
				      target_pid_to_str (ptid).c_str ());
		}
	    }

This line...

		  ptid_t ptid = ptid_t (ws.value.integer);

...doesn't make sense to me since ws.value.integer is supposed to
be the exit status for TARGET_WAITKIND_THREAD_EXITED and
TARGET_WAITKIND_EXITED.

However, for TARGET_WAITKIND_SIGNALLED, the signal number is in
ws.value.sig (which, due to being part of a union occupies some
of the same bytes as ws.value.integer).

So trying to find the ptid in that manner makes no sense at all.

I'm guessing that the ptid values are bogus when it does work.

Does it work when you use 

		  ptid_t ptid = ptid_t (event_pid);

instead?

Kevin


On Sun, 24 Mar 2019 15:25:05 +0100
Philippe Waroquiers <philippe.waroquiers@skynet.be> wrote:

> The test  gdb.threads/watchthreads-reorder.exp verifies that the
> 'set debug infrun 1' debug output does not crash GDB.
> 
> Under high load, the test can still cause a GDB internal error (see details
> below).
> 
> This patch checks that ptid differs from null_ptid to call
> pid_to_str so that it does not crash.
> The change also outputs event_ptid.
> 
>   (top-gdb) bt
>   #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
>   #1  0x00007f3d54a0642a in __GI_abort () at abort.c:89
>   #2  0x0000555c24c60e66 in dump_core () at ../../fixleaks/gdb/utils.c:201
>   #3  0x0000555c24c63d49 in internal_vproblem(internal_problem *, const char *, int, const char *, typedef __va_list_tag __va_list_tag *) (problem=problem@entry=0x555c25338d40 <internal_error_problem>, file=<optimized out>, line=287,
>       fmt=<optimized out>, ap=<optimized out>) at ../../fixleaks/gdb/utils.c:411
>   #4  0x0000555c24c63eab in internal_verror (file=<optimized out>, line=<optimized out>, fmt=<optimized out>,
>       ap=<optimized out>) at ../../fixleaks/gdb/utils.c:436
>   #5  0x0000555c249e8c22 in internal_error (file=file@entry=0x555c24e0f2ad "../../fixleaks/gdb/inferior.c",
>       line=line@entry=287, fmt=<optimized out>) at ../../fixleaks/gdb/common/errors.c:55
>   #6  0x0000555c247d3f5c in find_inferior_pid (pid=<optimized out>) at ../../fixleaks/gdb/inferior.c:287
>   #7  0x0000555c24ad2248 in find_inferior_pid (pid=<optimized out>) at ../../fixleaks/gdb/inferior.c:302
>   #8  find_inferior_ptid (ptid=...) at ../../fixleaks/gdb/inferior.c:301
>   #9  0x0000555c24c35f25 in find_thread_ptid (ptid=...) at ../../fixleaks/gdb/thread.c:522
>   #10 0x0000555c24b0ab4d in thread_db_target::pid_to_str[abi:cxx11](ptid_t) (
>       this=0x555c2532e3e0 <the_thread_db_target>, ptid=...) at ../../fixleaks/gdb/linux-thread-db.c:1637
>   #11 0x0000555c24c2f420 in target_pid_to_str[abi:cxx11](ptid_t) (ptid=...) at ../../fixleaks/gdb/target.c:2083
>   #12 0x0000555c24ad9cab in stop_all_threads () at ../../fixleaks/gdb/infrun.c:4373
>   #13 0x0000555c24ada00f in stop_waiting (ecs=<optimized out>) at ../../fixleaks/gdb/infrun.c:7464
>   #14 0x0000555c24adc401 in process_event_stop_test (ecs=ecs@entry=0x7ffc9402d9d0) at ../../fixleaks/gdb/infrun.c:6181
>   ...
>   (top-gdb) fr 12
>   #12 0x0000555c24ad9cab in stop_all_threads () at ../../fixleaks/gdb/infrun.c:4373
>   (top-gdb) p event_ptid
>   $5 = {m_pid = 25419, m_lwp = 25427, m_tid = 0}
>   (top-gdb) p ptid
>   $6 = {m_pid = 0, m_lwp = 0, m_tid = 0}
>   (top-gdb) p ws
>   $7 = {kind = TARGET_WAITKIND_THREAD_EXITED, value = {integer = 0, sig = GDB_SIGNAL_0, related_pid = {m_pid = 0,
>         m_lwp = 0, m_tid = 0}, execd_pathname = 0x0, syscall_number = 0}}
>   (top-gdb)
> 
> The gdb.log corresponding to the above crash is:
>   (gdb) PASS: gdb.threads/watchthreads-reorder.exp: reorder1: set debug infrun 1
>   continue
>   Continuing.
>   infrun: clear_proceed_status_thread (Thread 0x7ffff7fcfb40 (LWP 25419))
>   infrun: clear_proceed_status_thread (Thread 0x7ffff7310700 (LWP 25427))
>   infrun: clear_proceed_status_thread (Thread 0x7ffff6b0f700 (LWP 25428))
>   infrun: proceed (addr=0xffffffffffffffff, signal=GDB_SIGNAL_DEFAULT)
>   infrun: proceed: resuming Thread 0x7ffff7fcfb40 (LWP 25419)
>   infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 0x7ffff7fcfb40 (LWP 25419)] at 0x7ffff7344317
>   infrun: infrun_async(1)
>   infrun: prepare_to_wait
>   infrun: proceed: resuming Thread 0x7ffff7310700 (LWP 25427)
>   infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 0x7ffff7310700 (LWP 25427)] at 0x5555555553d7
>   infrun: prepare_to_wait
>   infrun: proceed: resuming Thread 0x7ffff6b0f700 (LWP 25428)
>   infrun: resume (step=0, signal=GDB_SIGNAL_0), trap_expected=0, current thread [Thread 0x7ffff6b0f700 (LWP 25428)] at 0x5555555554c8
>   infrun: prepare_to_wait
>   infrun: target_wait (-1.0.0, status) =
>   infrun:   -1.0.0 [process -1],
>   infrun:   status->kind = ignore
>   infrun: TARGET_WAITKIND_IGNORE
>   infrun: prepare_to_wait
>   Joining the threads.
>   [Thread 0x7ffff6b0f700 (LWP 25428) exited]
>   infrun: target_wait (-1.0.0, status) =
>   infrun:   -1.0.0 [process -1],
>   infrun:   status->kind = ignore
>   infrun: TARGET_WAITKIND_IGNORE
>   infrun: prepare_to_wait
>   infrun: target_wait (-1.0.0, status) =
>   infrun:   25419.25419.0 [Thread 0x7ffff7fcfb40 (LWP 25419)],
>   infrun:   status->kind = stopped, signal = GDB_SIGNAL_TRAP
>   infrun: TARGET_WAITKIND_STOPPED
>   infrun: stop_pc = 0x555555555e50
>   infrun: context switch
>   infrun: Switching context from Thread 0x7ffff6b0f700 (LWP 25428) to Thread 0x7ffff7fcfb40 (LWP 25419)
>   infrun: BPSTAT_WHAT_STOP_NOISY
>   infrun: stop_waiting
>   infrun: stop_all_threads
>   infrun: stop_all_threads, pass=0, iterations=0
>   infrun:   Thread 0x7ffff7fcfb40 (LWP 25419) not executing
>   infrun:   Thread 0x7ffff7310700 (LWP 25427) executing, need stop
>   [Thread 0x7ffff7310700 (LWP 25427) exited]
>   infrun: target_wait (-1.0.0, status) =
>   infrun:   25419.25427.0 [LWP 25427],
>   infrun:   status->kind = thread exited, status = 0
>   infrun: infrun_async(0)
>   ../../fixleaks/gdb/inferior.c:287: internal-error: inferior* find_inferior_pid(int): Assertion `pid != 0' failed.
>   A problem internal to GDB has been detected,
>   further debugging may prove unreliable.
>   Quit this debugging session? (y or n) FAIL: gdb.threads/watchthreads-reorder.exp: reorder1: continue to breakpoint: break-at-exit (GDB internal error)
>   Resyncing due to internal error.
>   n
>   infrun: infrun_async(1)
> 
>   This is a bug, please report it.  For instructions, see:
>   <http://www.gnu.org/software/gdb/bugs/>.
> 
>   infrun: infrun_async(0)
>   ../../fixleaks/gdb/inferior.c:287: internal-error: inferior* find_inferior_pid(int): Assertion `pid != 0' failed.
>   A problem internal to GDB has been detected,
>   further debugging may prove unreliable.
>   Create a core file of GDB? (y or n) y
> ---
>  gdb/infrun.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index ad7892105a..111879c318 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -4368,9 +4368,13 @@ stop_all_threads (void)
>  		  ptid_t ptid = ptid_t (ws.value.integer);
>  
>  		  fprintf_unfiltered (gdb_stdlog,
> -				      "infrun: %s exited while "
> +				      "infrun: %s "
> +				      "(event_ptid %s) exited while "
>  				      "stopping threads\n",
> -				      target_pid_to_str (ptid).c_str ());
> +				      ptid == null_ptid
> +				      ? "<null thread>"
> +				      : target_pid_to_str (ptid).c_str (),
> +				      target_pid_to_str (event_ptid).c_str ());
>  		}
>  	    }
>  	  else
> -- 
> 2.20.1
>