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Re: [PATCH 1/3] PR remote/19496, internal err forking-threads-plus-bkpt
- From: Pedro Alves <palves at redhat dot com>
- To: Don Breazeal <donb at codesourcery dot com>, gdb-patches at sourceware dot org
- Date: Mon, 01 Feb 2016 20:38:02 +0000
- Subject: Re: [PATCH 1/3] PR remote/19496, internal err forking-threads-plus-bkpt
- Authentication-results: sourceware.org; auth=none
- References: <1453942111-1215-1-git-send-email-donb at codesourcery dot com> <1453942111-1215-2-git-send-email-donb at codesourcery dot com>
On 01/28/2016 12:48 AM, Don Breazeal wrote:
> This patch fixes an internal error that occurs in
> gdb.threads/forking-threads-plus-breakpoint.exp:
>
> /blah/binutils-gdb/gdb/target.c:2723: internal-error: Can't determine the
> current address space of thread Thread 3170.3170
>
> In default_thread_address_space, find_inferior_ptid couldn't find 3170.3170
> because it had been overwritten in inferior_appeared, called as follows:
>
> inferior_appeared
> remote_add_inferior
> remote_notice_new_inferior
> remote_update_thread_list
>
> The cause of the problem was the following sequence of events:
>
> * GDB knows only about the main thread
>
> * the first fork event is reported to GDB, saved as pending_event
>
> * qXfer:threads_read gets the threads from the remote.
> remove_new_fork_children id's the fork child from the pending event
> and removes it from the list reported to GDB. All the rest of the
> threads, including the fork parent, are added to the GDB thread list.
>
> * GDB stops all the threads. All the stop events are pushed onto the
> stop reply queue behind the pending fork event.
>
> * remote_wait_ns calls queued_stop_reply and process_stop_reply to
> remove the fork event from the front of the stop reply queue and save
> event information in the thread_info structure for the fork parent
> thread. Unfortunately, none of the information saved in this way is
> the fork-specific information, so the actual fork event info is lost.
>
> * A subsequent qXfer:threads:read packet gets the thread list including
> the fork parent and fork child. remove_new_fork_children checks the
> thread list to see if there is a fork parent, doesn't find one, checks
> the stop reply queue for a pending fork event, doesn't find one, and
> allows the fork child thread to be reported to GDB before the fork
> event has been handled. remote_update_thread_list calls
> remote_notice_new_thread and overwrites the current (main) thread in
> inferior_appeared. GDB has now lost all knowledge of the main thread,
> and an internal error results.
>
> The fix was to make sure that when the stop reply was removed from the
> stop reply queuei, all of the necessary fork event information was stored
> in the parent thread structure. In process_stop_reply we call a new
> function, update_thread_if_fork_parent, to store the pending_follow
> information from the fork stop reply in the fork parent thread.
>
> Tested on x86_64 and Nios II Linux. No regressions, but more failures,
> which are addressed in subsequent patches in this patchset.
>
Many thanks for the detective work!
> +static void
> +update_thread_if_fork_parent (struct stop_reply *stop_reply)
> +{
> + ptid_t ptid;
> +
> + ptid = stop_reply->ptid;
> + if (stop_reply->ws.kind == TARGET_WAITKIND_FORKED
> + || stop_reply->ws.kind == TARGET_WAITKIND_VFORKED)
> + {
> + struct thread_info *tp = find_thread_ptid (ptid);
> +
> + tp->pending_follow = stop_reply->ws;
> + }
> +}
So the fork event has been reported out of target_wait but it was
left pending on the infrun side (infrun.c:save_waitstatus)
IOW, the fork event hasn't been processed by handle_inferior_event
yet, so it hasn't made it to tp->pending_follow yet.
The information is not lost, we're just looking for it in the
wrong place. I think this would be the right fix:
>From 211e4553a500f7a81d11860f9707db97b0a53c45 Mon Sep 17 00:00:00 2001
From: Pedro Alves <palves@redhat.com>
Date: Mon, 1 Feb 2016 20:25:00 +0000
Subject: [PATCH] Fix PR remote/19496, internal error
---
gdb/remote.c | 7 ++++++-
gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp | 6 ------
2 files changed, 6 insertions(+), 7 deletions(-)
diff --git a/gdb/remote.c b/gdb/remote.c
index 8831b50..5dffb98 100644
--- a/gdb/remote.c
+++ b/gdb/remote.c
@@ -6180,7 +6180,12 @@ remove_new_fork_children (struct threads_listing_context *context)
fork child threads from the CONTEXT list. */
ALL_NON_EXITED_THREADS (thread)
{
- struct target_waitstatus *ws = &thread->pending_follow;
+ struct target_waitstatus *ws;
+
+ if (thread->suspend.waitstatus_pending_p)
+ ws = &thread->suspend.waitstatus;
+ else
+ ws = &thread->pending_follow;
if (is_pending_fork_parent (ws, pid, thread->ptid))
{
diff --git a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
index 6c72061..ff3ca9a 100644
--- a/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
+++ b/gdb/testsuite/gdb.threads/forking-threads-plus-breakpoint.exp
@@ -100,12 +100,6 @@ proc do_test { cond_bp_target detach_on_fork displaced } {
set fork_count 0
set ok 0
- if {$displaced == "off"
- && [target_info exists gdb_protocol]
- && ([target_info gdb_protocol] == "remote"
- || [target_info gdb_protocol] == "extended-remote")} {
- setup_kfail "remote/19496" *-*-*
- }
set test "inferior 1 exited"
gdb_test_multiple "" $test {
-re "Inferior 1 \(\[^\r\n\]+\) exited normally" {
--
1.9.3