linux-kernel - Re: [PATCH] tracing/user_events: Run BPF program if attached

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Message-ID: <20230509164244.3163e421@rorschach.local.home>
Date:   Tue, 9 May 2023 16:42:44 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Beau Belgrave <beaub@...ux.microsoft.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-trace-kernel@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf <bpf@...r.kernel.org>,
        David Vernet <void@...ifault.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] tracing/user_events: Run BPF program if attached

On Tue, 9 May 2023 16:30:50 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> >From the user space side, which does:  
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/samples/user_events/example.c#n60
> 
> 	/* Check if anyone is listening */
> 	if (enabled) {

Hmm, looking at this deeper, we should update it to prevent the
compiler from optimizing it, and keeping "enabled" in a register. Which
would not work. Should probably add:

	if (*(const volatile int *)&enabled) {

-- Steve


> 		/* Yep, trace out our data */
> 		writev(data_fd, (const struct iovec *)io, 2);
> 
> 		/* Increase the count */
> 		count++;
> 
> 		printf("Something was attached, wrote data\n");
> 	}