Re: [PATCH, ppc] Fix hw *points for embedded ppc in a threaded environment.

Luis Machado
2012-12-27T19:50:37Z

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Hi,

On 08/06/2012 11:33 AM, Luis Gustavo wrote:
> Hi,
>
> GDB has always assumed that hardware watchpoints and breakpoints should
> be replicated for every new thread in ppc. This worked fine for the old
> DABR-based mechanism since both server and embedded ppc's supported only
> a single hw watchpoint or breakpoint.
>
> With the somewhat recent booke kernel interface, more hw
> watchpoints/breakpoints are available to GDB.
>
> The logic of replicating the existing process' debug state to the new
> thread is still there though, but the new booke interface in the kernel
> already replicates that state. More precisely, the kernel gives the new
> thread the debug state of its parent thread.
>
> When GDB tries to replicate the debug state, it will actually cause the
> kernel to allocate a new hw *point entry, leading to inadequate
> consumption of hw debug resources.
>
> It's still unclear if the kernel is supposed to do this and i'm chasing
> answers with the ppc linux kernel folks
> (https://lists.ozlabs.org/pipermail/linuxppc-dev/2012-August/100083.html).
> Nonetheless, the kernel is out and it has such behavior.
>
> This patch tries to address this problem by clearing any debug state
> prior to replicating *points to the new thread. If the kernel is doing
> something it's not supposed to, then this is a workaround for the broken
> kernels.

This is an updated patch that includes a generic testcase.

The fix is still the same, but the testcase exercises creation of
hardware watchpoints and their replication to all existing threads.

Each thread should cause a watchpoint trigger a certain number of times,
and that's what we expect in the testcase.

Without the fix, the affected ppc targets will throw an error after the
third thread gets created.

Curiously, x86 misses a watchpoint trigger every once in a while. Since
only a single trigger is allowed per thread at a given time, that is, we
try our best not to cause multiple triggers at once, this may be a
different issue, not directly related to the testcase.

It would be nice if someone could try this testcase on different targets
and confirm how it behaves, as well as validate the overall testcase format.

Regards,
Luis

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2012-12-27 Luis Machado <lgustavo@codesourcery.com>

* ppc-linux-nat.c (ppc_linux_new_thread): Clear the new thread's
debug state prior to replicating existing hardware watchpoints or
breakpoints.

* gdb/testsuite/gdb.threads/watchthreads3.c: New file.
* gdb/testsuite/gdb.threads/watchthreads3.exp: New file.

Index: gdb/gdb/testsuite/gdb.threads/watchthreads3.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ gdb/gdb/testsuite/gdb.threads/watchthreads3.c 2012-12-27 17:05:20.529201750 -0200
@@ -0,0 +1,144 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+ Copyright 2012 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/>.
+
+ Check that hardware watchpoints get propagated to all existing
+ threads when the hardware watchpoint is created.
+*/
+
+#include <stdio.h>
+#include <unistd.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <pthread.h>
+
+#ifndef NR_THREADS
+#define NR_THREADS 4
+#endif
+
+#ifndef X_INCR_COUNT
+#define X_INCR_COUNT 10
+#endif
+
+void *thread_function (void *arg); /* Function executed by each thread. */
+
+/* Used to hold threads back until watchthreads3.exp is ready. */
+int test_ready = 0;
+
+/* Used to push the program out of the waiting loop after the
+ testcase is done counting the number of hardware watchpoints
+ available for our target. */
+int watch_count_done = 0;
+
+/* Array with elements we can create watchpoints for. */
+static int watched_data[NR_BYTES];
+pthread_mutex_t data_mutex;
+
+/* Wait function to keep threads busy while the testcase does
+ what it needs to do. */
+void empty_cycle (void)
+{
+ usleep (1);
+}
+
+int
+main ()
+{
+ int res;
+ pthread_t threads[NR_THREADS];
+ int i;
+
+ while (watch_count_done == 0)
+ {
+ /* GDB will modify the value of "i" at runtime and we will
+ get past this point. */
+ empty_cycle ();
+ }
+
+ pthread_mutex_init (&data_mutex, NULL);
+
+ for (i = 0; i < NR_THREADS; ++i)
+ {
+ res = pthread_create (&threads[i],
+ NULL,
+ thread_function,
+ (void *) (intptr_t) i);
+ if (res != 0)
+ {
+ fprintf (stderr, "error in thread %d create\n", i);
+ abort ();
+ }
+ }
+
+ for (i = 0; i < NR_THREADS; ++i)
+ {
+ res = pthread_join (threads[i], NULL);
+ if (res != 0)
+ {
+ fprintf (stderr, "error in thread %d join\n", i);
+ abort ();
+ }
+ }
+
+ exit (EXIT_SUCCESS);
+}
+
+/* Easy place for a breakpoint.
+ watchthreads3.exp uses this to track when all threads are running
+ instead of, for example, the program keeping track
+ because we don't need the program to know when all threads are running,
+ instead we need gdb to know when all threads are running.
+ There is a delay between when a thread has started and when the thread
+ has been registered with gdb. */
+
+void
+thread_started ()
+{
+}
+
+void *
+thread_function (void *arg)
+{
+ int i, j;
+ long thread_number = (long) arg;
+
+ thread_started ();
+
+ /* Don't start incrementing X until watchthreads3.exp is ready. */
+ while (!test_ready)
+ usleep (1);
+
+ for (i = 0; i < NR_TRIGGERS_PER_THREAD; ++i)
+ {
+ for (j = 0; j < NR_THREADS; j++)
+ {
+ pthread_mutex_lock (&data_mutex);
+ /* For debugging. */
+ printf ("Thread %ld changing watch_thread[%d] data"
+ " from %d -> %d\n", thread_number, j,
+ watched_data[j], watched_data[j] + 1);
+ /* The call to usleep is so that when the watchpoint triggers,
+ the pc is still on the same line. */
+ /* Increment the watched data field. */
+ ++watched_data[j];
+ usleep (1);
+
+ pthread_mutex_unlock (&data_mutex);
+ }
+ }
+
+ pthread_exit (NULL);
+}
Index: gdb/gdb/testsuite/gdb.threads/watchthreads3.exp
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ gdb/gdb/testsuite/gdb.threads/watchthreads3.exp 2012-12-27 17:09:19.817211552 -0200
@@ -0,0 +1,143 @@
+# This testcase is part of GDB, the GNU debugger.
+
+# Copyright 2012 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/>.
+
+# Check that hardware watchpoints get propagated to all existing
+# threads when the hardware watchpoint is created.
+
+
+set NR_THREADS 4
+set NR_TRIGGERS_PER_THREAD 10
+set NR_BYTES 100
+
+# This test verifies that a hardware watchpoint gets replicated to
+# every existing thread and is detected properly. This test is
+# only meaningful on a target with hardware watchpoints.
+if {[skip_hw_watchpoint_tests]} {
+ return 0
+}
+
+standard_testfile
+if {[gdb_compile_pthreads "${srcdir}/${subdir}/${srcfile}" "${binfile}" executable [list debug "additional_flags=-DNR_THREADS=$NR_THREADS -DNR_TRIGGERS_PER_THREAD=$NR_TRIGGERS_PER_THREAD -DNR_BYTES=$NR_BYTES"]] != "" } {
+ return -1
+}
+
+clean_restart ${binfile}
+
+# Force hardware watchpoints to be used.
+gdb_test_no_output "set can-use-hw-watchpoints 1" ""
+
+# Run to `main' where we begin our tests.
+if ![runto_main] then {
+ gdb_suppress_tests
+}
+
+# First, break at empty_cycle.
+gdb_test "break empty_cycle" \
+ "Breakpoint 2 at .*: file .*${srcfile}, line .*" \
+ "breakpoint on empty_cycle"
+
+# Set some default values.
+set hwatch_count 0
+set done 0
+
+# Count the number of hardware watchpoints available on
+# this target.
+while { $done == 0 } {
+
+ gdb_test "continue" \
+ ".*Breakpoint 2, empty_cycle \\(\\) at .*${srcfile}.*" \
+ "continue to empty_cycle"
+
+ # Some targets do resource counting as we insert watchpoints.
+ # Such targets won't cause a watchpoint insertion failure, but
+ # will switch to software watchpoints silently. We check for
+ # both cases here.
+ gdb_test_multiple "watch watched_data\[$hwatch_count\]" \
+ "watch watched_data\[$hwatch_count\]" {
+ -re "Hardware watchpoint .*" {
+ continue
+ }
+ -re "Watchpoint .*" {
+ set done 1
+ break
+ }
+ }
+
+ gdb_test_multiple "continue" "watchpoint created successfully" {
+ -re ".*Breakpoint 2, empty_cycle \\(\\).*$gdb_prompt $" {
+ incr hwatch_count
+ continue
+ }
+ -re ".*Could not insert hardware watchpoint.*" {
+ set done 1
+ break
+ }
+ }
+}
+
+if { $hwatch_count == 0} {
+ gdb_exit;
+}
+
+# At this point, we know how many hardware watchpoints
+# the target supports. Use that to do further testing.
+delete_breakpoints
+
+# Break out of the empty_cycle loop by changing the
+# controlling variable.
+gdb_test_no_output "set var watch_count_done=1" \
+ "set var watch_count_done=1"
+
+# Prepare to create all the threads.
+gdb_test "break thread_started" \
+ "Breakpoint \[0-9\]+ at .*: file .*${srcfile}, line .*" \
+ "Breakpoint on thread_started"
+
+# Move all threads to where they're supposed to be for testing.
+for { set i 0 } { $i < $NR_THREADS } { incr i} {
+
+ # We want to set the maximum number of hardware watchpoints
+ # and make sure the target can handle that without an error.
+ # That will show us the watchpoints got replicated to all the
+ # threads correctly, and that no new watchpoints got created
+ # in the background for a specific thread.
+ if {$i < $hwatch_count} {
+ gdb_test "watch watched_data\[$i\]" \
+ "Hardware watchpoint .*" \
+ "watch watched_data\[$i\]"
+ }
+
+ gdb_test continue "Continuing\\..*Breakpoint \[0-9\]+, thread_started \\(\\) at .*$srcfile.*" \
+ "Break at thread_started"
+}
+
+# Let the threads run and change the watched data, leading
+# to watchpoint triggers.
+gdb_test_no_output "set var test_ready=1" \
+ "set var test_ready=1"
+
+# Set the number of expected watchpoint triggers.
+set TRIGGERS [expr "$NR_THREADS * $NR_THREADS * $NR_TRIGGERS_PER_THREAD"]
+
+# Move the threads and hit the watchpoints
+# TRIGGERS times.
+for { set i 0 } { $i < $TRIGGERS } { incr i} {
+ gdb_test continue "Continuing\\..*Hardware watchpoint \[0-9\]+: watched_data\[\[0-9\]+\].*Old value = \[0-9\]+.*New value = \[0-9\]+.*thread_function \\(arg=$hex\\) at .*$srcfile.*" \
+ "Continue to watchpoint trigger on watched_data"
+}
+
+gdb_exit
Index: gdb/gdb/ppc-linux-nat.c
===================================================================
--- gdb.orig/gdb/ppc-linux-nat.c 2012-12-27 14:22:58.844802770 -0200
+++ gdb/gdb/ppc-linux-nat.c 2012-12-27 14:23:14.568803412 -0200
@@ -2179,7 +2179,21 @@ ppc_linux_new_thread (struct lwp_info *l
/* Copy that thread's breakpoints and watchpoints to the new thread. */
for (i = 0; i < max_slots_number; i++)
if (hw_breaks[i].hw_break)
- booke_insert_point (hw_breaks[i].hw_break, tid);
+ {
+ /* The ppc Linux kernel causes a thread to inherit its parent
+ thread's debug state, and that includes any hardware
+ watchpoints or breakpoints that the parent thread may have set.
+
+ For this reason, the debug state of the new thread is cleared
+ before trying to replicate any hardware watchpoints or
+ breakpoints contained in other threads. */
+
+ /* The ppc debug resource accounting is done through "slots".
+ Ask the kernel the deallocate this specific *point's slot. */
+ ptrace (PPC_PTRACE_DELHWDEBUG, tid, 0, hw_breaks[i].slot);
+
+ booke_insert_point (hw_breaks[i].hw_break, tid);
+ }
}
else
ptrace (PTRACE_SET_DEBUGREG, tid, 0, saved_dabr_value);