linux-kernel - Re: [PATCH v5] tracing: Guard __DECLARE_TRACE() use of __DO_TRACE

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Message-ID: <CAADnVQKMR6kkqC5JQceg6A8F6qSd_cYQE--1ToxwDUxTytPbGA@mail.gmail.com>
Date: Tue, 13 Jan 2026 15:44:23 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Steven Rostedt <rostedt@...dmis.org>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	LKML <linux-kernel@...r.kernel.org>, 
	Linux trace kernel <linux-trace-kernel@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v5] tracing: Guard __DECLARE_TRACE() use of
 __DO_TRACE_CALL() with SRCU-fast

On Tue, Jan 13, 2026 at 6:23 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> On 2026-01-12 09:19:58 [-0800], Alexei Starovoitov wrote:
> > > Now if you are saying that BPF will handle migrate_disable() on its own
> > > and not require the tracepoint infrastructure to do it for it, then
> > > this is perfect. And I can then simplify this code, and just use
> > > srcu_fast for both RT and !RT.
> >
> > Agree. Just add migrate_disable to __bpf_trace_run,
> > or, better yet, use rcu_read_lock_dont_migrate() in there.
>
> Wonderful, thank you.
>
> Is this "must remain on the same CPU and can be re-entrant" because BPF
> core code such memory allocator/ data structures use per-CPU data
> structures and must use the same through the whole invocation?

It's per-cpu maps.
htab_percpu_map_lookup_elem() returns a pointer to per-cpu value
which needs to be valid for the duration of the program.
Not much else.