linux-kernel - Re: [PATCH] tracepoint: Do not fail unregistering a probe due to memory allocation

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Message-ID: <1227896553.48834.1605654499161.JavaMail.zimbra@efficios.com>
Date:   Tue, 17 Nov 2020 18:08:19 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     rostedt <rostedt@...dmis.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Matt Mullins <mmullins@...x.us>,
        Ingo Molnar <mingo@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] tracepoint: Do not fail unregistering a probe due to
 memory allocation

----- On Nov 17, 2020, at 5:16 PM, rostedt rostedt@...dmis.org wrote:

> On Tue, 17 Nov 2020 16:22:23 -0500 (EST)
> Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> 
>> If we don't call the stub, then there is no point in having the stub at
>> all, and we should just compare to a constant value, e.g. 0x1UL. As far
>> as I can recall, comparing with a small immediate constant is more efficient
>> than comparing with a loaded value on many architectures.
> 
> Why 0x1UL, and not just set it to NULL.
> 
>		do {							\
>			it_func = (it_func_ptr)->func;			\
>			__data = (it_func_ptr)->data;			\
>			if (likely(it_func))				\
>				((void(*)(void *, proto))(it_func))(__data, args); \
>		} while ((++it_func_ptr)->func);

Because of this end-of-loop condition ^
which is also testing for a NULL func. So if we reach a stub, we end up stopping
iteration and not firing the following tracepoint probes.

Thanks,

Mathieu

> 
> 
> -- Steve

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com