Re: [GIT PULL] tracing: Updates for v6.13

[Linus Torvalds]

On Fri, 22 Nov 2024 at 14:12, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> Hmm, if we make a __DO_TRACE_SYSCALL(), I don't think it needs to even have
> that condition parameter.

That was my point. The whole conditional - and the parameter - seems
to be completely pointless as far as I can tell.

That said, I think you can actually simplify things even further: if
you move the TO_CONDITION() checking into the caller, you could move
the locking there too.

IOW, instead of this pattern:

                if (static_branch_unlikely(&__tracepoint_##name.key))   \
                        __DO_TRACE(name,                                \
                                TP_ARGS(args),                          \
                                TP_CONDITION(cond), 0);                 \

you could make it be something like this instead:

                if (static_branch_unlikely(&__tracepoint_##name.key)) \
                        if (TP_CONDITION(cond)) \
                                scoped_guard(preempt_notrace) \
                                        __DO_TRACE(name, TP_ARGS(args)); \

where __DO_TRACE() would get neither the "cond" argument _nor_ that
locking argument, because both are just done by the two users (the
other one would use "scoped_guard(rcu_read_trace)" of course.

And look, this is another reason why unconditional locking is a good
thing: now you can use the "guard()" model for the lock, and don't
need an explicit unlock, simplifying the code more.

Of course, you want "guard(rcu_read_trace)" (for system call events)
and "guard(preempt_notrace)" (for the regular trace event case), and
we don't have the "notrace" versions of those guard classes yet.

But adding those would literally be trivial, ie something like

  DEFINE_LOCK_GUARD_0(rcu_notrace,
        rcu_read_lock_notrace(), rcu_read_unlock_notrace())

wouldn't that make it all look really nice?

NOTE NOTE NOTE! I didn't actually try any of the above in real life,
so I might be missing some important detail. I'm just pointing out
that making this all unconditional and not based on random flags has
the potential for even more cleanups.

And I might have gotten the different lock names confused too.

              Linus