Re: [RFA]: Modified Watchthreads Patch

[Daniel Jacobowitz <drow at false dot org>]

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.

> >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.

-- 
Daniel Jacobowitz