Re: [RFA]: Modified Watchthreads Patch

[Jeff Johnston <jjohnstn at redhat dot com>]

Daniel Jacobowitz wrote:
> On Fri, Dec 10, 2004 at 03:02:41PM -0500, Jeff Johnston wrote:
>
> > > On the technical side, two questions:
> > >
> > > 1) I can see that it will be a bit of work to rearrange i386-linux to
> > > use this, but it should be doable.  Do you know offhand of any
> > > i386-specific problems other than inserting watchpoints for all
> > > threads?
> >
> > Actually, with i386/x86-64 I discovered that the debug registers are global 
> > in scope for the setting of watchpoints (i.e. I didn't have to use the 
> > observer). The status register, however, is thread-specific for reporting 
> > them.  I have gotten the watchthreads.exp testcase working for both 
> > platforms.  Your lwp fix helps a lot with this.  We call TIDGET()/PIDGET() 
> > in the low-level code which used to get called in the wrong ptid mode so we 
> > kept checking the main-thread for the watchpoint.
>
>
> Er... do you know why the debug registers are global, and what kernel
> is this with?  They look thread-specific to me (kernel 2.6.10-rc1). 
> They are accessible using PEEKUSR/POKEUSR for each thread, and
> __switch_to updates them at context switches.

I am simply speaking from experience with the RHEL3 kernel.  I got it working 
without touching the insert/remove logic.  I am currently retrofitting new 
changes into the mainline gdb that are much "cleaner" than my previous fixes.  I 
haven't tried x86 on the latest kernel, but I am in the midst of putting 
together an uber-patch with the stuff here plus some other things needed for 
each platform.  IA64 is already finished and running watchthreads.exp on a 
next-release kernel.  I am about to start x86 so I will keep in mind your 
comment.  I'll let you know either way what I had to do to get it working.

> > > 2) What should to_stopped_by_watchpoint do in the presence of multiple
> > > threads?  It looks like it relies on inferior_ptid being the thread
> > > which stopped at a watchpoint; I'm worried that that may not be
> > > consistently true in a heavily threaded application.  Maybe it should
> > > iterate over all threads.
> >
> > It works fine for the watchthreads.exp test once all the mechanisms are in 
> > place (I have a few more patches to go).  We don't want to iterate over all 
> > threads unless we know the platform has a problem.  Otherwise, we won't be 
> > able to pin down a specific watchpoint triggered with the thread/source 
> > line that triggered it.  Is there a valid scenario where inferior_ptid 
> > should not be the thread for the signal chosen by the low-level linux-nat 
> > code?  If not, I would prefer to treat that as a bug that requires pinning 
> > down.
>
>
> We can delay this issue, then.  I am concerned about losing watchpoints
> when other events are active, e.g. a thread event breakpoint or dlopen
> breakpoint and a read watchpoint.  I'm sure GDB gets this wrong
> already.
>
> Please fix the whitespace at the end of s390-nat.c.  Otherwise, this is
> approved if Ulrich is OK with the S390 bits; let's give him a chance to
> comment.

Great.  Will make the white-space change and wait for Ulrich.

-- Jeff J.