linux-kernel - Re: [PATCH 04/16] perf, mmap: Factor out perf_get

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Date:	Fri, 25 Apr 2014 15:54:13 +0200
From:	Robert Richter <rric@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jean Pihet <jean.pihet@...aro.org>, Borislav Petkov <bp@...en8.de>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org,
	Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH 04/16] perf, mmap: Factor out perf_get_fd()

On 22.04.14 16:27:59, Peter Zijlstra wrote:
> On Mon, Apr 07, 2014 at 05:04:26PM +0200, Jean Pihet wrote:
> > From: Robert Richter <robert.richter@...aro.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@...aro.org>
> > Signed-off-by: Robert Richter <rric@...nel.org>
> > Signed-off-by: Jean Pihet <jean.pihet@...aro.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.

I guess its this patch from Al you referring to:

 ea635c6 Fix racy use of anon_inode_getfd() in perf_event.c

I think we do not introduce it back due to:

 a6fa941 perf_event: Switch to internal refcount, fix race with close()

which removes the user of event_file.

-Robert
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