Re: [patch, gdbserver] Uninsert bpkt when regular and fast tracepoint are set at the same address

[Stan Shebs <stanshebs@earthlink.net>]

On 10/26/11 7:56 PM, Yao Qi wrote:
> Hi,
> I find a program will receive segv fault when I set a regular tracepoint
> and a fast tracepoint at the same address, start tracing and resume program.
>
> gdbserver has taken care of this situation in many places of the code,
> when uninserting breakpoint or fast tracepoint, write_inferior_memory is
> called to take care of layering breakpoints on top of fast tracepoints.
>   However, it is not right to me.  Here is an example to illustrate this
> problem.
>
> Supposing I set a regular tracepoint and a fast tracepoint on 0x080484fc,
>
>      0x080484fc<+3>:     e8 f3 ff ff ff  call   0x80484f4<func>
>
> During insertion, trap insn (for regular tracepoint) and jmp insn (for
> fast tracepoint) are inserted, and gdbserver takes care of them to make
> sure trap insn is *always* inserted on top of jmp insn.

I'm looking at this and wondering, why are we inserting the fast 
tracepoint jump insn at all?
Shouldn't be it sufficient to let the trap handler do the work of both 
slow and fast tracepoints at that location?  Since hitting the trap has 
already put us on the slow path, there's not going to any noticeable 
additional penalty for not going to the IPA and interpreting conditional 
bytecodes instead of compiled ones, etc.  We may not even need to sync 
trace buffers (I'm not sure about that, code is tricky).

Stan
stan@codesourcery.com