Date: Thu, 31 Oct 2013 19:16:54 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>, Namhyung Kim <namhyung.kim@lge.com>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Hyeoncheol Lee <cheol.lee@lge.com>,
        Hemant Kumar <hkshaw@linux.vnet.ibm.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        "zhangwei(Jovi)" <jovi.zhangwei@huawei.com>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Subject: Re: [PATCH 10/13] tracing/uprobes: Fetch args before reserving a
	ring buffer
Message-ID: <20131031181654.GA11208@redhat.com>
References: <1383029621-7384-1-git-send-email-namhyung@kernel.org> <1383029621-7384-11-git-send-email-namhyung@kernel.org>
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On 10/29, Namhyung Kim wrote:
>
> @@ -630,6 +653,19 @@ probe_event_enable(struct trace_uprobe *tu, int flag, filter_func_t filter)
>  	if (trace_probe_is_enabled(&tu->p))
>  		return -EINTR;
>  
> +	if (atomic_inc_return(&uprobe_buffer_ref) == 1) {
> +		int cpu;
> +
> +		uprobe_cpu_buffer = __alloc_percpu(PAGE_SIZE, PAGE_SIZE);
> +		if (uprobe_cpu_buffer == NULL) {
> +			atomic_dec(&uprobe_buffer_ref);
> +			return -ENOMEM;
> +		}
> +
> +		for_each_possible_cpu(cpu)
> +			mutex_init(&per_cpu(uprobe_cpu_mutex, cpu));
> +	}
> +
>  	WARN_ON(!uprobe_filter_is_empty(&tu->filter));
>  
>  	tu->p.flags |= flag;
> @@ -646,6 +682,11 @@ static void probe_event_disable(struct trace_uprobe *tu, int flag)
>  	if (!trace_probe_is_enabled(&tu->p))
>  		return;
>  
> +	if (atomic_dec_and_test(&uprobe_buffer_ref)) {
> +		free_percpu(uprobe_cpu_buffer);
> +		uprobe_cpu_buffer = NULL;
> +	}
> +
>  	WARN_ON(!uprobe_filter_is_empty(&tu->filter));

Do we really need atomic_t? probe_event_enable/disable is called under
event_mutex and we rely on this fact anyway.

Otherwise this logic looks racy even with atomic_t, another thread could
use the uninitialized uprobe_cpu_buffer/mutex if it registers another probe
and the handler runs before we complete the initialization, no?

Oleg.

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