Messages in this thread Patch in this message | | | Date | Tue, 16 Jun 2015 13:16:20 +0200 | From | Peter Zijlstra <> | Subject | Re: ring_buffer_attach && cond_synchronize_rcu (Was: percpu-rwsem: Optimize readers and reduce global impact) |
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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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