[RFC 0/3] Use reinsert breakpoint for vCont;s

Yao Qi
2016-05-06T10:32:33Z

Nowadays, reinsert breakpoint is used in GDBserver to step over a
breakpoint. I want to use it to handle vCont;s. The motivation
of this work is to exercise software single step in GDBserver side.
In the past two weeks, I am fixing various test fails, but still
can't fix all of them. I want to post something here, and hope
people can help me on this area.

Suppose GDB is able to send vCont;s to GDBserver using software
single step (done by patch 3), what should GDBserver do? It call
function single_step if lwp->resume->kind is resume_step. See
patch 2.

With this change, reinsert breakpoint is used for two purposes,
1) step over GDBserver breakpoint, 2) handle vCont;s. Here are some
facts or assumptions in my mind,

- reinsert breakpoints can be inserted for both step over and
vCont;s together. GDBserver should finish all step-overs
before resuming the threads, see scenario b) below,
- GDB doesn't send more than one vCont s actions in one vCont
packet, although RSP doc doesn't say this.

It is straightforward to insert reinsert breakpoints for vCont;s,
but I am not sure when to delete them. Here are some scenarios,

a) vCont;s thread A, and vCont;c thread B. Thread A hits the reinsert
breakpoints, and GDBserver can remove them. What is the proper
place to remove them?

b) vCont;s thread A, and vCont;c thread B. Thread B hits breakpoints
(not reinsert), do we remove reinsert breakpoints? My answer is
no. In the following step-over, reinsert breakpoints for step-over
are deleted, but reinsert breakpoints for vCont;s (thread A) are still
there.

c) vCont;s thread A, and vCont;c thread B. Thread B hits the reinsert
breakpoints (for thread A vCont;s), do we remove reinsert breakpoints?
I think no, we can just step over it for thread B.

d) vCont;s thread A, and vCont;c thread B. A signal arrives, do we remove
reinsert breakpoints? Yes, I think so.

IMO, b) requires reinsert breakpoint thread specific, so that we can delete
reinsert breakpoints for step-over of thread B, but keep reinsert breakpoints
for vCont;s of thread A. That is what patch 1 does.

I tried different ways to remove reinsert breakpoints in GDBserver, but still
can't fix fails in gdb.threads/schedlock.exp, that the program gets SIGILL or
SIGSEGV. These fails can't happen in every run, and they are disappeared
when I turn on debugging output in GDBserver. I suspect they are about the
improper management to reinsert breakpoints.

(gdb) PASS: gdb.threads/schedlock.exp: schedlock=off: cmd=next: call_function=1: next to increment (5)
next^M
78 while (*myp > 0)^M
(gdb) next^M
^M
Thread 1 "schedlock" received signal SIGILL, Illegal instruction.^M
[Switching to Thread 3797.3797]^M
0x000087f8 in thread_function (arg=0x0) at /home/yao/SourceCode/gnu/gdb/git/gdb/testsuite/gdb.threads/schedlock.c:78^M
78 while (*myp > 0)^M
(gdb) FAIL: gdb.threads/schedlock.exp: schedlock=off: cmd=next: call_function=1: next to increment (6)

Any ideas on the overall design, how to handle vCont;s in GDBserver using
software single step? or is it a completely wrong thing to handle vCont;s
using software single step?

*** BLURB HERE ***

Yao Qi (3):
make reinsert breakpoint thread specific
use reinsert breakpoint for vCont;s
[GDBserver] Support vCont s and S actions with software single step

gdb/gdbserver/linux-low.c | 37 ++++++++++++++++++++++++++++++++-----
gdb/gdbserver/mem-break.c | 29 ++++++++++++++++++++++++++---
gdb/gdbserver/mem-break.h | 13 +++++++++----
gdb/gdbserver/server.c | 13 ++++++++-----
4 files changed, 75 insertions(+), 17 deletions(-)

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1.9.1