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SubjectRe: ring_buffer_attach && cond_synchronize_rcu (Was: percpu-rwsem: Optimize readers and reduce global impact)

I made that the below.

Are you OK with the Changelog edits and the added SoB ?

---
Subject: perf: Fix ring_buffer_attach() RCU sync, again.
From: Oleg Nesterov <oleg@redhat.com>
Date: Sat, 30 May 2015 22:04:25 +0200

While looking for other users of get_state/cond_sync. I Found
ring_buffer_attach() and it looks obviously buggy?

Don't we need to ensure that we have "synchronize" _between_
list_del() and list_add() ?

IOW. Suppose that ring_buffer_attach() preempts right_after
get_state_synchronize_rcu() and gp completes before spin_lock().

In this case cond_synchronize_rcu() does nothing and we reuse
->rb_entry without waiting for gp in between?

It also moves the ->rcu_pending check under "if (rb)", to make it
more readable imo.

Cc: tj@kernel.org
Cc: mingo@redhat.com
Cc: der.herr@hofr.at
Cc: dave@stgolabs.net
Cc: torvalds@linux-foundation.org
Cc: josh@joshtriplett.org
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Fixes: b69cf53640da ("perf: Fix a race between ring_buffer_detach() and ring_buffer_attach()")
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20150530200425.GA15748@redhat.com
---
kernel/events/core.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4307,20 +4307,20 @@ static void ring_buffer_attach(struct pe
WARN_ON_ONCE(event->rcu_pending);

old_rb = event->rb;
- event->rcu_batches = get_state_synchronize_rcu();
- event->rcu_pending = 1;
-
spin_lock_irqsave(&old_rb->event_lock, flags);
list_del_rcu(&event->rb_entry);
spin_unlock_irqrestore(&old_rb->event_lock, flags);
- }

- if (event->rcu_pending && rb) {
- cond_synchronize_rcu(event->rcu_batches);
- event->rcu_pending = 0;
+ event->rcu_batches = get_state_synchronize_rcu();
+ event->rcu_pending = 1;
}

if (rb) {
+ if (event->rcu_pending) {
+ cond_synchronize_rcu(event->rcu_batches);
+ event->rcu_pending = 0;
+ }
+
spin_lock_irqsave(&rb->event_lock, flags);
list_add_rcu(&event->rb_entry, &rb->event_list);
spin_unlock_irqrestore(&rb->event_lock, flags);

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