Commit c0fe5dcb authored Aug 27, 2014 by Linus Torvalds

Merge tag 'trace-fixes-v3.17-rc1-2' of...

Merge tag 'trace-fixes-v3.17-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace

Pull trace buffer epoll hang fix from Steven Rostedt:
"Josef Bacik found a bug in the ring_buffer_poll_wait() where the
condition variable (waiters_pending) was set before being added to the
poll queue via poll_wait(). This allowed for a small race window to
happen where an event could come in, check the condition variable see
it set to true, clear it, and then wake all the waiters. But because
the waiter set the variable before adding itself to the queue, the
waker could have cleared the variable after it was set and then miss
waking it up as it wasn't added to the queue yet.

Discussing this bug, we realized that a memory barrier needed to be
added too, for the rare case that something polls for a single trace
event to happen (and just one, no more to come in), and miss the
wakeup due to memory ordering. Ideally, a memory barrier needs to be
added on the writer side too, but as that will kill tracing
performance and this is for a situation that tracing wasn't even
designed for (who traces one instance of an event, use a printk
instead!), this isn't worth adding the barrier. But we can in the
future add the barrier for when the buffer goes from empty to the
first event, as that would cover this case"

* tag 'trace-fixes-v3.17-rc1-2' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace:
trace: Fix epoll hang when we race with new entries

parents 68e37028 4ce97dbf

kernel/trace/ring_buffer.c

+15 −1

Original line number	Diff line number	Diff line
		@@ -626,8 +626,22 @@ int ring_buffer_poll_wait(struct ring_buffer *buffer, int cpu,
		work = &cpu_buffer->irq_work;
		}

		work->waiters_pending = true;
		poll_wait(filp, &work->waiters, poll_table);
		work->waiters_pending = true;
		/*
		* There's a tight race between setting the waiters_pending and
		* checking if the ring buffer is empty. Once the waiters_pending bit
		* is set, the next event will wake the task up, but we can get stuck
		* if there's only a single event in.
		*
		* FIXME: Ideally, we need a memory barrier on the writer side as well,
		* but adding a memory barrier to all events will cause too much of a
		* performance hit in the fast path. We only need a memory barrier when
		* the buffer goes from empty to having content. But as this race is
		* extremely small, and it's not a problem if another event comes in, we
		* will fix it later.
		*/
		smp_mb();

		if ((cpu == RING_BUFFER_ALL_CPUS && !ring_buffer_empty(buffer)) \|\|
		(cpu != RING_BUFFER_ALL_CPUS && !ring_buffer_empty_cpu(buffer, cpu)))

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