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From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, Davidlohr Bueso <dbueso@suse.de>
Subject: Re: [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT
Date: Tue, 12 Jan 2016 07:09:46 -0800	[thread overview]
Message-ID: <20160112150946.GA1927@linux-uzut.site> (raw)
In-Reply-To: <20160112090629.GO6344@twins.programming.kicks-ass.net>

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

  reply	other threads:[~2016-01-12 15:09 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-01-11 18:22 [PATCH] rbtree: use READ_ONCE in RB_EMPTY_ROOT Davidlohr Bueso
2016-01-12  9:06 ` Peter Zijlstra
2016-01-12 15:09   ` Davidlohr Bueso [this message]
2016-01-12 15:59     ` Peter Zijlstra

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