linux-kernel - Re: [RFC PATCH] tracepoint: Provide tracepoint_kernel_find_by

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Message-ID: <20180326204834.npkr26bdf5aprwnv@ast-mbp.dhcp.thefacebook.com>
Date:   Mon, 26 Mar 2018 13:48:36 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC PATCH] tracepoint: Provide tracepoint_kernel_find_by_name

On Mon, Mar 26, 2018 at 03:10:31PM -0400, Mathieu Desnoyers wrote:
> Provide an API allowing eBPF to lookup core kernel tracepoints by name.
> 
> Given that a lookup by name explicitly requires tracepoint definitions
> to be unique for a given name (no duplicate keys), include a
> WARN_ON_ONCE() check that only a single match is encountered at runtime.
> This should always be the case, given that a DEFINE_TRACE emits a
> __tracepoint_##name symbol, which would cause a link-time error if more
> than one instance is found. Nevertheless, check this at runtime with
> WARN_ON_ONCE() to stay on the safe side.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Alexei Starovoitov <ast@...nel.org>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Ingo Molnar <mingo@...nel.org>

Mathieu, that's not enough.
Commit log is also wrong.
It needs to state that something like this is needed only because
changing for_each_tracepoint_range() semantics will break lttng.
I'll post a follow up patch shortly.