Re: [ppc-linux-nat]: set access flag for h/w watchpoint even if it is only read or write

[Eli Zaretskii <eliz at gnu dot org>]

> Date: Thu, 6 Jul 2006 09:20:20 -0400
> From: Daniel Jacobowitz <drow@false.org>
> Cc: gdb-patches@sourceware.org
> 
> On Fri, Jun 09, 2006 at 11:40:26PM +0800, Wu Zhou wrote:
> > Hello all,
> > 
> > I found a bug in the current ppc-linux h/w watchpoint implementation:  
> > when we set read watchpoint to some expression, if there are any write 
> > operation to it before a read operation is hit, watchpoint_check will see 
> > that its value is changed. So user won't see the watchpoint is hit.
> > 
> > I make one change to the SET_DEBUGREG operation: even if it is only 
> > read or write watchpoint, we still set access flag.  Then, no matter 
> > what operation is on the watched address, a SIGTRAP will be triggered. 
> > The gdb code itself can determine if it is a write operation or read 
> > operation.  If it is write, watchpoint_check routine can update the 
> > bs->value to the latest.  
> 
> Eli, you're the most familiar with watchpoint support; do you have any
> comment on this?

I'm sorry, I missed it somehow.

Yes, this problem is known on x86 and elsewhere.  The problem is
extremely rare, as reading and writing to the same address in the same
instruction is a hard-to-accomplish treat.  Wu, could you show a
real-life example of where this matters?

The solution you suggest, in my experience, is worse than the problem:
it will cause a significant slow-down of the most frequent case, as
Daniel points out.

> Many targets can tell us what sort of watchpoint was hit; we could
> do something like extend target_stopped_data_address to also return
> what sort of watchpoint was triggered, if it knows.

Yes, that would be a welcome addition, I agree.