[RFC] Set process affinity in test to work around ARM ptrace bug

Yao Qi
2016-06-30T13:57:34Z

We recently found a ARM kernel ptrace bug
http://lists.infradead.org/pipermail/linux-arm-kernel/2016-May/431962.html
As a result of this bug, after GDB ptrace set VFP registers, the hardware
registers may not be updated. This bug causes some intermittent fails in
tests, like return.exp, call-rt-st.exp, callfuncs.exp, etc.

The bug is fixed in ARM kernel tree, but it is impractical to upgrade
linux kernel from git tree or most recently release (I don't know when
the fix can be shipped in the mainline kernel release). I am wondering
we can workaround this kernel bug somehow.

My first attempt is to workaround it in GDB, so that GDB still writes
the VFP registers and sync them to hardware. The kernel patch is quite
simple, which moves vfp_flush_hwstate one line below. Probably, we can
call ptrace set vfp registers twice, and then the second vfp set can
flush the state correctly. Unfortunately, it doesn't work, because
every time of ptrace set, kernel loads VFP registers from hardware first,
which might be out of date after the first ptrace set. That is to say,
we can't workaround this kernel bug in GDB.

Then, I am thinking we can workaround this bug in testing, because the
intermittent fails are confusing in comparing test results, by binding
both tracer and tracee on the same core. For example, we can start GDB
or GDBserver with "taskset -c 0 ", but this is a global change, may
have some affects on gdb.threads tests. I also think about doing
"taskset -p PID -c 0" in test harness after the inferior is started,
and do the same to the parent process of inferior (which is either GDB
or GDBserver).

The approach in this patch is to have a small c function which sets
both process affinity and its parent's affinity to core 0. This
function should be called in these tests explicitly, but other tests
are not affected at all. This patch is posted to get comments on the
necessity of workaround this kernel bug, and the proper to workaround
this bug. There are still some test cases affected by this kernel bug,
but this patch doesn't touch them yet.

gdb/testsuite:

2016-06-30 Yao Qi <yao.qi@linaro.org>

* lib/set_process_affinity.c: New file.
* gdb.base/call-rt-st.c: Include lib/set_process_affinity.c.
(main): Call set_process_affinity.
* gdb.base/gnu_vector.c: Likewise.
* gdb.base/return.c: Likewise.
* gdb.base/gnu_vector.exp: Adjust test.
---
gdb/testsuite/gdb.base/call-rt-st.c | 2 ++
gdb/testsuite/gdb.base/gnu_vector.c | 4 +++-
gdb/testsuite/gdb.base/gnu_vector.exp | 3 +++
gdb/testsuite/gdb.base/return.c | 2 ++
gdb/testsuite/lib/set_process_affinity.c | 41 ++++++++++++++++++++++++++++++++
5 files changed, 51 insertions(+), 1 deletion(-)
create mode 100644 gdb/testsuite/lib/set_process_affinity.c

diff --git a/gdb/testsuite/gdb.base/call-rt-st.c b/gdb/testsuite/gdb.base/call-rt-st.c
index 072ea86..ad97e28 100644
--- a/gdb/testsuite/gdb.base/call-rt-st.c
+++ b/gdb/testsuite/gdb.base/call-rt-st.c
@@ -1,3 +1,4 @@
+#include "../lib/set_process_affinity.c"
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
@@ -565,6 +566,7 @@ int main () {
struct two_floats_t *f3;

gdb_unbuffer_output ();
+ set_process_affinity ();

/* Allocate space for large structures
*/
diff --git a/gdb/testsuite/gdb.base/gnu_vector.c b/gdb/testsuite/gdb.base/gnu_vector.c
index ee03ac1..8e0d6a8 100644
--- a/gdb/testsuite/gdb.base/gnu_vector.c
+++ b/gdb/testsuite/gdb.base/gnu_vector.c
@@ -18,6 +18,7 @@
Contributed by Ken Werner <ken.werner@de.ibm.com> */

#include <stdarg.h>
+#include "../lib/set_process_affinity.c"

#define VECTOR(n, type) \
type __attribute__ ((vector_size (n * sizeof(type))))
@@ -137,7 +138,8 @@ main ()
{
int4 res;

- res = add_some_intvecs (i4a, i4a + i4b, i4b);
+ set_process_affinity ();
+ res = add_some_intvecs (i4a, i4a + i4b, i4b); /* breakpoint here */

res = add_some_intvecs (i4a, i4a + i4b, i4b);

diff --git a/gdb/testsuite/gdb.base/gnu_vector.exp b/gdb/testsuite/gdb.base/gnu_vector.exp
index aafaedd..1e57a26 100644
--- a/gdb/testsuite/gdb.base/gnu_vector.exp
+++ b/gdb/testsuite/gdb.base/gnu_vector.exp
@@ -55,6 +55,9 @@ gdb_test_multiple "show endian" "show endian" {
}
}

+gdb_breakpoint [gdb_get_line_number "breakpoint here"]
+gdb_continue_to_breakpoint "breakpoint here"
+
# Test printing of character vector types
gdb_test "print c4" "\\\$$decimal = \\{1, 2, 3, 4\\}"
gdb_test "print c4\[2\]" "\\\$$decimal = 3"
diff --git a/gdb/testsuite/gdb.base/return.c b/gdb/testsuite/gdb.base/return.c
index c365e88..6ff38e6 100644
--- a/gdb/testsuite/gdb.base/return.c
+++ b/gdb/testsuite/gdb.base/return.c
@@ -15,6 +15,7 @@
You should have received a copy of the GNU General Public License
along with this program. If not, see <http://www.gnu.org/licenses/>. */

+#include "../lib/set_process_affinity.c"
#include <stdio.h>
/* Test "return" command. */

@@ -40,6 +41,7 @@ double tmp3;

int main ()
{
+ set_process_affinity ();
func1 ();
printf("in main after func1\n");
tmp2 = func2 ();
diff --git a/gdb/testsuite/lib/set_process_affinity.c b/gdb/testsuite/lib/set_process_affinity.c
new file mode 100644
index 0000000..2615965
--- /dev/null
+++ b/gdb/testsuite/lib/set_process_affinity.c
@@ -0,0 +1,41 @@
+/* Copyright (C) 2016 Free Software Foundation, Inc.
+
+ This file is part of GDB.
+
+ 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/>. */
+
+#if defined(__arm__) && defined(__linux__)
+#define _GNU_SOURCE
+#include <sched.h>
+#include <unistd.h>
+#include <sys/types.h>
+#endif
+
+static int
+set_process_affinity (void)
+{
+#if defined(__arm__) && defined(__linux__)
+ cpu_set_t my_set;
+
+ /* Set both process and parent process (GDB)'s affinity on core 0
+ to workaround ARM linux kernel ptrace bug which doesn't flush the
+ VFP state to hardware after ptrace set VFP registers. */
+
+ CPU_ZERO (&my_set);
+ CPU_SET (0, &my_set);
+
+ sched_setaffinity (0, sizeof(cpu_set_t), &my_set);
+ sched_setaffinity (getppid (), sizeof(cpu_set_t), &my_set);
+#endif
+}
--
1.9.1