Re: [PATCH 04/16] perf, mmap: Factor out perf_get_fd()

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Apr 07, 2014 at 05:04:26PM +0200, Jean Pihet wrote:
> From: Robert Richter <robert.richter@linaro.org>
> 
> This new function creates a new fd for an event. We need this later to
> get a fd from a persistent event.
> 
> Signed-off-by: Robert Richter <robert.richter@linaro.org>
> Signed-off-by: Robert Richter <rric@kernel.org>
> Signed-off-by: Jean Pihet <jean.pihet@linaro.org>
> ---
>  kernel/events/core.c     | 14 ++++++++------
>  kernel/events/internal.h |  1 +
>  2 files changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 22ec8f0..9857475 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4206,6 +4206,11 @@ static const struct file_operations perf_fops = {
>  	.fasync			= perf_fasync,
>  };
>  
> +int perf_get_fd(struct perf_event *event, int f_flags)
> +{
> +	return anon_inode_getfd("[perf_event]", &perf_fops, event, f_flags);
> +}
> +
>  /*
>   * Perf event wakeup
>   *
> @@ -7028,7 +7033,6 @@ SYSCALL_DEFINE5(perf_event_open,
>  	struct perf_event *event, *sibling;
>  	struct perf_event_attr attr;
>  	struct perf_event_context *ctx;
> -	struct file *event_file = NULL;
>  	struct fd group = {NULL, 0};
>  	struct task_struct *task = NULL;
>  	struct pmu *pmu;
> @@ -7189,10 +7193,9 @@ SYSCALL_DEFINE5(perf_event_open,
>  			goto err_context;
>  	}
>  
> -	event_file = anon_inode_getfile("[perf_event]", &perf_fops, event,
> -					f_flags);
> -	if (IS_ERR(event_file)) {
> -		err = PTR_ERR(event_file);
> +	event_fd = perf_get_fd(event, f_flags);
> +	if (event_fd < 0) {
> +		err = event_fd;
>  		goto err_context;
>  	}
>  
> @@ -7257,7 +7260,6 @@ SYSCALL_DEFINE5(perf_event_open,
>  	 * perf_group_detach().
>  	 */
>  	fdput(group);
> -	fd_install(event_fd, event_file);
>  	return event_fd;
>  
>  err_context:

ISTR there being a good reason we only install the FD at the very last
moment. Of course I cannot quite recall it :/

I suspect it had something to do with not being able to fudge the state
while its being set-up or somesuch.