tree 0e7cf0df48f172f99b65bb152b0cbfff3c5636a1
parent 68e370289c29e3beac99d59c6d840d470af9dfcf
parent 4ce97dbf50245227add17c83d87dc838e7ca79d0
author Linus Torvalds <torvalds@linux-foundation.org> 1409155956 -0700
committer Linus Torvalds <torvalds@linux-foundation.org> 1409155956 -0700
mergetag object 4ce97dbf50245227add17c83d87dc838e7ca79d0
 type commit
 tag trace-fixes-v3.17-rc1-2
 tagger Steven Rostedt <rostedt@rostedt.homelinux.com> 1409058413 -0400
 
 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.
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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