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Date: Wed, 19 Aug 2009 18:19:36 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Ingo Molnar <mingo@elte.hu>, Paul Mackerras <paulus@samba.org>,
       Arnaldo Carvalho de Melo <acme@redhat.com>,
       Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org,
       stephane eranian <eranian@googlemail.com>
Subject: Re: [PATCH 4/4][RFC] perf_counter: Allow sharing of output channels
Message-ID: <20090819161934.GD4972@nowhere>
References: <20090819091823.916851355@chello.nl> <20090819092023.980284148@chello.nl>
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On Wed, Aug 19, 2009 at 11:18:27AM +0200, Peter Zijlstra wrote:
> Provide the ability to configure a counter to send its output to
> another (already existing) counter's output stream.
> 
> [ compile tested only ]
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: stephane eranian <eranian@googlemail.com>
> +int perf_counter_set_output(struct perf_counter *counter, int output_fd)
> +{
> +	struct perf_counter *output_counter = NULL;
> +	struct file *output_file = NULL;
> +	struct perf_counter *old_output;
> +	int fput_needed = 0;
> +	int ret = -EINVAL;
> +
> +	if (!output_fd)
> +		goto set;
> +
> +	output_file = fget_light(output_fd, &fput_needed);
> +	if (!output_file)
> +		return -EBADF;
> +
> +	if (output_file->f_op != &perf_fops)
> +		goto out;
> +
> +	output_counter = output_file->private_data;
> +
> +	/* Don't chain output fds */
> +	if (output_counter->output)
> +		goto out;



If you don't chain them, how do you propagate more than one
path of output redirected?