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Date: Tue, 26 Jan 2016 18:24:25 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        vince@deater.net, eranian@google.com,
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        Jiri Olsa <jolsa@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>,
        Wang Nan <wangnan0@huawei.com>
Subject: Re: [PATCH v2] perf: Synchronously cleanup child events
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On Tue, Jan 26, 2016 at 05:16:37PM +0100, Peter Zijlstra wrote:
> > +struct file *perf_event_get(unsigned int fd)
> >  {
> > +	struct file *file;
> >  
> > +	file = fget_raw(fd);
> 
> fget_raw() to guarantee the return value isn't NULL? afaict the O_PATH
> stuff does not apply to perf events, so you'd put any fd for which the
> distinction matters anyway.
> 
> > +	if (file->f_op != &perf_fops) {
> > +		fput(file);
> > +		return ERR_PTR(-EBADF);
> > +	}
> >  
> > +	return file;
> >  }

It is not possible for one thread to concurrently call close() while
this thread tries to fget() ? In which case, we must check the return
value anyway?