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* [PATCH] gdb: don't pass TARGET_WNOHANG to targets that can't async (PR 26642)
@ 2020-10-01  2:56 Simon Marchi
  2020-10-12 14:48 ` Pedro Alves
  2020-10-13 15:27 ` Simon Marchi
  0 siblings, 2 replies; 8+ messages in thread
From: Simon Marchi @ 2020-10-01  2:56 UTC (permalink / raw)
  To: gdb-patches

Debugging with "maintenance set target-async off" on Linux has been
broken since 5b6d1e4fa4f ("Multi-target support").

The issue is easy to reproduce:

    $ ./gdb -q --data-directory=data-directory -nx ./test
    Reading symbols from ./test...
    (gdb) maintenance set target-async off
    (gdb) start
    Temporary breakpoint 1 at 0x1151: file test.c, line 5.
    Starting program: /home/simark/build/binutils-gdb/gdb/test

... and it hangs there.

The difference between pre-5b6d1e4fa4f and 5b6d1e4fa4f is that
fetch_inferior_event now calls target_wait with TARGET_WNOHANG for
non-async-capable targets, whereas it didn't before.

For non-async-capable targets, this is how it's expected to work when
resuming execution:

1. we call resume
2. the infrun async handler is marked in prepare_to_wait, to immediatly
   wake up the event loop
3. fetch_inferior_event calls the target's wait method without
   TARGET_WNOHANG, effectively blocking until the target has something
   to report

However, since we call the target's wait method with TARGET_WNOHANG,
this happens:

1. we call resume
2. the infrun async handler is marked in prepare_to_wait, to immediatly
   wake up the event loop
3. fetch_inferior_event calls the target's wait method with
   TARGET_WNOHANG, the target has nothing to report yet
4. we go back to blocking on the event loop
5. SIGCHLD finally arrives, but the event loop is not woken up, because
   we are not in async mode.  Normally, we should have been stuck in
   waitpid the SIGCHLD would have unblocked us.

We end up in this situation because these two necessary conditions are
met:

1. GDB uses the TARGET_WNOHANG option with a target that can't do async.
   I don't think this makes sense.  I mean, it's technically possible,
   the doc for TARGET_WNOHANG is:

  /* Return immediately if there's no event already queued.  If this
     options is not requested, target_wait blocks waiting for an
     event.  */
  TARGET_WNOHANG = 1,

   ... which isn't in itself necessarily incompatible with synchronous
   targets.  But I don't see when it would be useful to ask a sync
   target to do a non-blocking wait.

   Pass TARGET_WNOHANG to sync targets also poses the risk of passing
   TARGET_WNOHANG to a target that doesn't implement it.  The caller of
   target_wait would expect the call not to block, but the call may
   indeed block.

   Since supporting TARGET_WNOHANG is a requirement for targets that can
   do async (I believe), I propose (as implemented in this patch) that
   we add an assertion in target_wait to make sure we don't ask a target
   that can't do async to handle TARGET_WNOHANG.

   <off-topic>
   A question in my mind is: could we bind the blocking or non-blocking
   behavior of wait with can_async_p?  In other words:

     - Do we ever need to do a blocking wait on a target that supports
       async?
     - Do we ever need to do a non-blocking wait on a target that does
       not support async?

   So, could we make it so that if target's can_async_p method returns
   true, its wait method is necessarily non-blocking?  And if
   can_async_p returns false, its wait method is necessarily blocking?

   It sounds to me like it would simplify the semantic of
   target_ops::wait a little bit.
   </off-topic>

2. The linux-nat target, even in the mode where it emulates a
   synchronous target (with "maintenance set target-async off") respects
   TARGET_WNOHANG.

   Non-async targets, such as windows_nat_target, simply don't check /
   support TARGET_WNOHANG.  Their wait method is always blocking.  So,
   to properly emulate a non-async target, I believe that linux-nat
   should also ignore it when in "maintenance set target-async off"
   mode.  Its behavior would be closer to a "true" non-async target.

The problem disappears if we simply fix either of these issues, but I
think it wouldn't hurt to fix both.

The new test gdb.base/maint-target-async-off.exp is a simple test that
just tries running to main and then to the end of the program, with
"maintenance set target-async off".

gdb/ChangeLog:

	PR gdb/26642
	* infrun.c (do_target_wait_1): Clear TARGET_WNOHANG if the
	target can't do async.
	* linux-nat.c (linux_nat_wait_1): Ignore TARGET_WNOHANG if
	target_async_permitted is false.
	* target.c (target_wait): Assert that we don't pass
	TARGET_WNOHANG to a target that can't async.

gdb/testsuite/ChangeLog:

	PR gdb/26642
	* gdb.base/maint-target-async-off.c: New test.
	* gdb.base/maint-target-async-off.exp: New test.

Change-Id: I69ad3a14598863d21338a8c4e78700a58ce7ad86
---
 gdb/infrun.c                                  |  5 +++
 gdb/linux-nat.c                               |  6 +++-
 gdb/target.c                                  |  5 +++
 .../gdb.base/maint-target-async-off.c         | 22 +++++++++++++
 .../gdb.base/maint-target-async-off.exp       | 32 +++++++++++++++++++
 5 files changed, 69 insertions(+), 1 deletion(-)
 create mode 100644 gdb/testsuite/gdb.base/maint-target-async-off.c
 create mode 100644 gdb/testsuite/gdb.base/maint-target-async-off.exp

diff --git a/gdb/infrun.c b/gdb/infrun.c
index daf10417842f..1b88514c9cc7 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -3533,6 +3533,11 @@ do_target_wait_1 (inferior *inf, ptid_t ptid,
 
   /* But if we don't find one, we'll have to wait.  */
 
+  /* We can't ask a non-async target to do a non-blocking wait, so this will be
+     a blocking wait.  */
+  if (!target_can_async_p ())
+    options &= ~TARGET_WNOHANG;
+
   if (deprecated_target_wait_hook)
     event_ptid = deprecated_target_wait_hook (ptid, status, options);
   else
diff --git a/gdb/linux-nat.c b/gdb/linux-nat.c
index fbb538859429..1064fc4f8c72 100644
--- a/gdb/linux-nat.c
+++ b/gdb/linux-nat.c
@@ -3238,7 +3238,11 @@ linux_nat_wait_1 (ptid_t ptid, struct target_waitstatus *ourstatus,
 
       /* No interesting event to report to the core.  */
 
-      if (target_options & TARGET_WNOHANG)
+      /* If target_async_permitted is false (maintenance set target-async off
+	 is used), pretend that we don't know about TARGET_WNOHANG and go block
+	 in wait_for_signal.  */
+      if (target_options & TARGET_WNOHANG
+	  && target_async_permitted)
 	{
 	  linux_nat_debug_printf ("exit (ignore)");
 
diff --git a/gdb/target.c b/gdb/target.c
index dd78a848caec..6f340678b7ca 100644
--- a/gdb/target.c
+++ b/gdb/target.c
@@ -1997,6 +1997,11 @@ ptid_t
 target_wait (ptid_t ptid, struct target_waitstatus *status,
 	     target_wait_flags options)
 {
+  target_ops *target = current_top_target ();
+
+  if (!target->can_async_p ())
+    gdb_assert ((options & TARGET_WNOHANG) == 0);
+
   return current_top_target ()->wait (ptid, status, options);
 }
 
diff --git a/gdb/testsuite/gdb.base/maint-target-async-off.c b/gdb/testsuite/gdb.base/maint-target-async-off.c
new file mode 100644
index 000000000000..9d7b2f1a4c28
--- /dev/null
+++ b/gdb/testsuite/gdb.base/maint-target-async-off.c
@@ -0,0 +1,22 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2020 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+int
+main (void)
+{
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.base/maint-target-async-off.exp b/gdb/testsuite/gdb.base/maint-target-async-off.exp
new file mode 100644
index 000000000000..e77bc79a21e1
--- /dev/null
+++ b/gdb/testsuite/gdb.base/maint-target-async-off.exp
@@ -0,0 +1,32 @@
+# Copyright 2020 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Verify that debugging with "maintenance target-async off" works somewhat.  At
+# least running to main and to the end of the program.
+
+standard_testfile
+
+if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
+    return
+}
+
+gdb_test_no_output "maintenance set target-async off"
+
+if { ![runto_main] } {
+    fail "can't run to main"
+    return
+}
+
+gdb_continue_to_end
-- 
2.28.0


^ permalink raw reply	[flat|nested] 8+ messages in thread
end of thread, other threads:[~2020-10-13 16:01 UTC | newest]

Thread overview: 8+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2020-10-01  2:56 [PATCH] gdb: don't pass TARGET_WNOHANG to targets that can't async (PR 26642) Simon Marchi
2020-10-12 14:48 ` Pedro Alves
2020-10-13 15:06   ` Simon Marchi
2020-10-13 15:21     ` Pedro Alves
2020-10-13 15:27 ` Simon Marchi
2020-10-13 15:31   ` [PATCH v2] " Simon Marchi
2020-10-13 15:44     ` Pedro Alves
2020-10-13 16:00       ` Simon Marchi

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