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Re: RFA: lin-lwp bug with software-single-step or schedlock
- From: Mark Kettenis <kettenis at chello dot nl>
- To: drow at mvista dot com
- Cc: gdb-patches at sources dot redhat dot com, msnyder at redhat dot com
- Date: Wed, 23 Oct 2002 23:13:22 +0200 (CEST)
- Subject: Re: RFA: lin-lwp bug with software-single-step or schedlock
- References: <20021023042615.GA6358@nevyn.them.org>
Date: Wed, 23 Oct 2002 00:26:15 -0400
From: Daniel Jacobowitz <drow@mvista.com>
This bug was noticed on MIPS, because MIPS GNU/Linux is
SOFTWARE_SINGLE_STEP_P. There's a comment in lin_lwp_resume:
/* Apparently the interpretation of PID is dependent on STEP: If
STEP is non-zero, a specific PID means `step only this process
id'. But if STEP is zero, then PID means `continue *all*
processes, but give the signal only to this one'. */
resume_all = (PIDGET (ptid) == -1) || !step;
I'm fairly certain it's not without reason that I wrote this comment
as it is.
Now, I did some digging, and I believe this comment is completely
incorrect. Saying "signal SIGWINCH" causes PIDGET (ptid) == -1,
and it is assumed the signal will be delivered to inferior_ptid.
There's some other problem there - I think I've discovered that we
will neglect to single-step over a breakpoint if we are told to
continue with a signal, which is a bit dubious of a decision - but
by and large it works as expected.
I don't see directly why, but I wouldn't be surprised by it.
So if STEP is 0, we always resume all processes. STEP at this point _only_
refers to whether we want a PTRACE_SINGLESTEP or equivalent;
SOFTWARE_SINGLE_STEP has already been handled. We can't make policy
decisions based on STEP any more.
Indeed, there's something wrong here.
I tried removing the || !step. It's pretty hard to tell, since there are
still a few non-deterministic failures on my test systems (which is what I
was actually hunting when I found this!) but I believe testsuite results are
improved on i386.
There is one thing that might be affected. Suppose you have a signal
such as SIGUSR1 that stops the inferior but is also passed on to the
inferior. If a multi-threaded program gets this signal, GDB will
stop. If you now change the current thread to some other thread and
try to single-step. Will the signal be delivered to the origional
thread?
If your patch doesn't affect this, I think your patch is OK to check
in. Otherwise we'll have to think about this a bit more.
Mark