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Date:   Tue, 17 Nov 2020 18:08:19 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     rostedt <rostedt@goodmis.org>
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        Matt Mullins <mmullins@mmlx.us>,
        Ingo Molnar <mingo@redhat.com>,
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        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>,
        netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
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Subject: Re: [PATCH] tracepoint: Do not fail unregistering a probe due to
 memory allocation
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Thread-Topic: tracepoint: Do not fail unregistering a probe due to memory allocation
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----- On Nov 17, 2020, at 5:16 PM, rostedt rostedt@goodmis.org wrote:

> On Tue, 17 Nov 2020 16:22:23 -0500 (EST)
> Mathieu Desnoyers <mathieu.desnoyers@efficios.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