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RE: [draft patch] <unavailable> unwinder for btrace [Re: [rfc 3/5] record: make it build again]
- From: "Metzger, Markus T" <markus dot t dot metzger at intel dot com>
- To: Jan Kratochvil <jan dot kratochvil at redhat dot com>
- Cc: Markus Metzger <markus dot t dot metzger at googlemail dot com>, "gdb-patches at sourceware dot org" <gdb-patches at sourceware dot org>
- Date: Thu, 28 Mar 2013 12:37:53 +0000
- Subject: RE: [draft patch] <unavailable> unwinder for btrace [Re: [rfc 3/5] record: make it build again]
- References: <1360337423-27095-1-git-send-email-markus dot t dot metzger at intel dot com> <1360337423-27095-4-git-send-email-markus dot t dot metzger at intel dot com> <20130210221059 dot GC4819 at host2 dot jankratochvil dot net> <A78C989F6D9628469189715575E55B2307B78A04 at IRSMSX102 dot ger dot corp dot intel dot com> <20130211141451 dot GA8962 at host2 dot jankratochvil dot net> <20130211171319 dot GA17524 at host2 dot jankratochvil dot net> <A78C989F6D9628469189715575E55B2307BA9030 at IRSMSX102 dot ger dot corp dot intel dot com> <9B969C1D-95E8-4AD5-BEF0-E269FF8771DF at gmail dot com> <20130328062747 dot GA27157 at host2 dot jankratochvil dot net> <A78C989F6D9628469189715575E55B2307BA984B at IRSMSX102 dot ger dot corp dot intel dot com> <20130328091204 dot GA15793 at host2 dot jankratochvil dot net>
> -----Original Message-----
> From: gdb-patches-owner@sourceware.org [mailto:gdb-patches-owner@sourceware.org] On Behalf Of Jan Kratochvil
> Sent: Thursday, March 28, 2013 10:12 AM
> > The data structure should only change when there is new trace, which requires
> > the target to continue. "info record" should, like any other record-btrace command,
> > fetch the new trace once and then operate on the cached trace data.
>
> In non-stop mode I belive there will be new btrace info on each "info record"
> command, won't be? I have not tried it but it seems so to me.
Record_btrace_info, like any other record btrace command, calls btrace_fetch.
This queries the target for new branch trace. If there is no new trace, it returns.
Only if there is new trace, it will call btrace_clear and then reconstruct the branch
trace.
All this is per-thread, so the branch trace for each thread should only be reconstructed
when it has changed - in non-stop and stop-all mode.
> > Is there a guarantee that frame_info and frame_id objects are destroyed
> > when the target resumes? Or could I trigger their destruction from within
> > btrace_clear?
>
> "trigger frame_info and frame_id destruction" == reinit_frame_cache().
>
> Accessing frame_info after reinit_frame_cache() is always a crash.
>
> Accessing frame_id after reinit_frame_cache() is safe but one needs to be
> prepared frame_find_by_id may return NULL if it is no longer available.
>
> When you introduce new reinit_frame_cache() call one just needs to be careful
> no caller holds that time a frame_info * pointer in a local variable.
> It would be a bug in such caller to call some non-trivial caller while holding
> frame_info * but there were many such bugs in GDB.
>
> I would not rely on any reinit_frame_cache() calls, calling
> reinit_frame_cache() more times is zero-cost, I think you should call
> reinit_frame_cache() from btrace_clear as you ask above.
Thanks. I'm doing just that. So far, I have not seen any issues.
Regards,
Markus.
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