[RFA]: Modified Watchthreads Patch

Jeff Johnston jjohnstn@redhat.com
Fri Dec 10 22:18:00 GMT 2004

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
>>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 
> 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.

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