perf: Fix ring_buffer_attach() RCU sync, again

commit 2f993cf093643b98477c421fa2b9a98dcc940323 upstream.

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.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dave@stgolabs.net
Cc: der.herr@hofr.at
Cc: josh@joshtriplett.org
Cc: tj@kernel.org
Fixes: b69cf53640da ("perf: Fix a race between ring_buffer_detach() and ring_buffer_attach()")
Link: http://lkml.kernel.org/r/20150530200425.GA15748@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 7 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index eddf1ed4155e..0ceb386777ae 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4331,20 +4331,20 @@ static void ring_buffer_attach(struct perf_event *event,
                 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);