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

Robert Richter
2014-04-25T13:54:20Z

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@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.

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