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Subject[RFC PATCH 14/17] perf: Add ioctl(REATTACH) for detached events
Date
This adds an ioctl command to demote a detached event to a 'normal' one
that gets destroyed when its file descriptor is closed. It can still be
used to mmap the buffers, but not very useful otherwise.

Signed-off-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
---
include/uapi/linux/perf_event.h | 1 +
kernel/events/core.c | 32 +++++++++++++++++++++++++++++++-
2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 4cdd4fab9d..ae54bd496d 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -435,6 +435,7 @@ struct perf_event_attr {
#define PERF_EVENT_IOC_ID _IOR('$', 7, __u64 *)
#define PERF_EVENT_IOC_SET_BPF _IOW('$', 8, __u32)
#define PERF_EVENT_IOC_PAUSE_OUTPUT _IOW('$', 9, __u32)
+#define PERF_EVENT_IOC_REATTACH _IO ('$', 10)

enum perf_event_ioc_flags {
PERF_IOC_FLAG_GROUP = 1U << 0,
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f0b77b33b4..fbee221d19 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4532,7 +4532,19 @@ EXPORT_SYMBOL_GPL(perf_event_release_kernel);
*/
static int perf_release(struct inode *inode, struct file *file)
{
- perf_event_release_kernel(file->private_data);
+ struct perf_event *event = file->private_data;
+
+ /*
+ * For a DETACHED event, perf_release() can't have the last reference,
+ * because we grabbed one extra in the sys_perf_event_open, IOW it is
+ * always put_event(). In order for it to be the last reference, we'd
+ * first need to ioctl(REATTACH) on this event, which would drop the
+ * PERF_ATTACH_DETACHED attach state.
+ */
+ if (event->attach_state & PERF_ATTACH_DETACHED)
+ put_event(event);
+ else
+ perf_event_release_kernel(file->private_data);
return 0;
}

@@ -4885,6 +4897,11 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
void (*func)(struct perf_event *);
u32 flags = arg;

+ if (event->attach_state & PERF_ATTACH_DETACHED &&
+ cmd != PERF_EVENT_IOC_REATTACH &&
+ cmd != PERF_EVENT_IOC_ID)
+ return -EINVAL;
+
switch (cmd) {
case PERF_EVENT_IOC_ENABLE:
func = _perf_event_enable;
@@ -4948,6 +4965,19 @@ static long _perf_ioctl(struct perf_event *event, unsigned int cmd, unsigned lon
rcu_read_unlock();
return 0;
}
+ case PERF_EVENT_IOC_REATTACH:
+ /*
+ * DETACHED state is serialized on ctx::mutex
+ */
+ if (!is_detached_event(event))
+ return -EINVAL;
+
+ event->attach_state &= ~PERF_ATTACH_DETACHED;
+ tracefs_remove(event->dent);
+ event->dent = NULL;
+ put_event(event); /* can't be last */
+
+ return 0;
default:
return -ENOTTY;
}
--
2.14.1
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