[tip:perf/urgent] perf: Fix ring_buffer_attach() RCU sync, again

From: tip-bot for Oleg Nesterov
Date: Fri Jun 19 2015 - 13:59:16 EST

Commit-ID: 2f993cf093643b98477c421fa2b9a98dcc940323
Gitweb: http://git.kernel.org/tip/2f993cf093643b98477c421fa2b9a98dcc940323
Author: Oleg Nesterov <oleg@xxxxxxxxxx>
AuthorDate: Sat, 30 May 2015 22:04:25 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Fri, 19 Jun 2015 09:38:45 +0200

perf: Fix ring_buffer_attach() RCU sync, again

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: dave@xxxxxxxxxxxx
Cc: der.herr@xxxxxxx
Cc: josh@xxxxxxxxxxxxxxxx
Cc: tj@xxxxxxxxxx
Fixes: b69cf53640da ("perf: Fix a race between ring_buffer_detach() and ring_buffer_attach()")
Link: http://lkml.kernel.org/r/20150530200425.GA15748@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
kernel/events/core.c | 14 +++++++-------
1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index eddf1ed..0ceb386 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4331,20 +4331,20 @@ static void ring_buffer_attach(struct perf_event *event,

old_rb = event->rb;
- event->rcu_batches = get_state_synchronize_rcu();
- event->rcu_pending = 1;
spin_lock_irqsave(&old_rb->event_lock, flags);
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);
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
Please read the FAQ at http://www.tux.org/lkml/