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Date: Tue, 12 Jan 2016 16:59:31 +0100 From: Peter Zijlstra <peterz@...radead.org> To: Davidlohr Bueso <dave@...olabs.net> Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de> Subject: Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT 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@...radead.org>
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