netdev - Re: [PATCH v2] net: tracepoint: fix print wrong sysctl

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Date:   Tue, 08 Sep 2020 19:49:52 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     dust.li@...ux.alibaba.com
Cc:     kuba@...nel.org, edumazet@...gle.com, satoru.moriya@....com,
        netdev@...r.kernel.org
Subject: Re: [PATCH v2] net: tracepoint: fix print wrong sysctl_mem value

From: Dust Li <dust.li@...ux.alibaba.com>
Date: Tue,  8 Sep 2020 10:09:39 +0800

> @@ -98,7 +98,7 @@ TRACE_EVENT(sock_exceed_buf_limit,
>  
>  	TP_STRUCT__entry(
>  		__array(char, name, 32)
> -		__field(long *, sysctl_mem)
> +		__array(long, sysctl_mem, 3)
>  		__field(long, allocated)
>  		__field(int, sysctl_rmem)
>  		__field(int, rmem_alloc)
> @@ -110,7 +110,9 @@ TRACE_EVENT(sock_exceed_buf_limit,
>  
>  	TP_fast_assign(
>  		strncpy(__entry->name, prot->name, 32);
> -		__entry->sysctl_mem = prot->sysctl_mem;
> +		__entry->sysctl_mem[0] = prot->sysctl_mem[0];
> +		__entry->sysctl_mem[1] = prot->sysctl_mem[1];
> +		__entry->sysctl_mem[2] = prot->sysctl_mem[2];

I can't understand at all why the current code doesn't work.

We assign a pointer to entry->sysctl_mem and then print out the
three words pointed to by that.

It's so wasteful to copy this over every tracepoint entry so
the pointer approach is very desirable.