Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT
Date	2016-01-12 17:00 +0100
Message-ID	<qQ9wu-2Zn-9@gated-at.bofh.it> (permalink)
References	<qPPo6-67U-19@gated-at.bofh.it> <qQ37I-7fJ-11@gated-at.bofh.it> <qQ8TM-2LK-21@gated-at.bofh.it>
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On Tue, Jan 12, 2016 at 07:09:46AM -0800, Davidlohr Bueso wrote:
> 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).

OK, fair enough.

Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

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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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