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Date: Wed, 3 Aug 2016 14:27:17 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
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        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [RFC PATCH v7 1/7] Restartable sequences system call
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On Tue, Jul 26, 2016 at 03:02:19AM +0000, Mathieu Desnoyers wrote:
> We really care about preemption here. Every migration implies a
> preemption from a user-space perspective. If we would only care
> about keeping the CPU id up-to-date, hooking into migration would be
> enough. But since we want atomicity guarantees for restartable
> sequences, we need to hook into preemption.

> It allows user-space to perform update operations on per-cpu data without
> requiring heavy-weight atomic operations.

Well, a CMPXCHG without LOCK prefix isn't all that expensive on x86.

It is however on PPC and possibly other architectures, so in name of
simplicity supporting only the one variant makes sense.