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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Thu, 25 Mar 2021 15:18:20 +0100
From:   Ingo Molnar <mingo@kernel.org>
To:     Dmitry Vyukov <dvyukov@google.com>
Cc:     Marco Elver <elver@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@redhat.com>,
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        Namhyung Kim <namhyung@kernel.org>,
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Subject: Re: [PATCH v3 07/11] perf: Add breakpoint information to siginfo on
 SIGTRAP
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* Dmitry Vyukov <dvyukov@google.com> wrote:

> On Wed, Mar 24, 2021 at 3:05 PM Marco Elver <elver@google.com> wrote:
> >
> > On Wed, 24 Mar 2021 at 15:01, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > One last try, I'll leave it alone now, I promise :-)
> >
> > This looks like it does what you suggested, thanks! :-)
> >
> > I'll still need to think about it, because of the potential problem
> > with modify-signal-races and what the user's synchronization story
> > would look like then.
> 
> I agree that this looks inherently racy. The attr can't be allocated
> on stack, user synchronization may be tricky and expensive. The API
> may provoke bugs and some users may not even realize the race problem.

Yeah, so why cannot we allocate enough space from the signal handler 
user-space stack and put the attr there, and point to it from 
sig_info?

The idea would be to create a stable, per-signal snapshot of whatever 
the perf_attr state is at the moment the event happens and the signal 
is generated - which is roughly what user-space wants, right?

Thanks,

	Ingo