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Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT

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On Tue, 12 Jan 2016, Peter Zijlstra wrote:

>On Mon, Jan 11, 2016 at 10:22:43AM -0800, Davidlohr Bueso wrote:
>> With d72da4a4d97 (rbtree: Make lockless searches non-fatal) our rbtrees
>> provide weak guarantees that allows us to do lockless (and very speculative)
>> reads of the tree. Such readers cannot see partial stores on nodes, ie
>> left/right as well as root. As such, similar to the WRITE_ONCE semantics when
>> doing rotations, use READ_ONCE when checking the root node in RB_EMPTY_ROOT.
>
>No objection, but is this actually used anywhere?

I found this because I wanted to use the waiter check in rtmutexes in a lockless
fashion (ie rt_mutex_has_waiters).

>
>Or is this a just-in-case completeness thing?

This too.

Thanks,
Davidlohr

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[PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT Davidlohr Bueso <dave@stgolabs.net> - 2016-01-11 19:30 +0100
  Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT Peter Zijlstra <peterz@infradead.org> - 2016-01-12 10:10 +0100
    Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT Davidlohr Bueso <dave@stgolabs.net> - 2016-01-12 16:20 +0100
      Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT Peter Zijlstra <peterz@infradead.org> - 2016-01-12 17:00 +0100

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