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Date: Wed, 5 Oct 2016 09:32:03 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: kernel-hardening@...ts.openwall.com
Cc: keescook@...omium.org, Elena Reshetova <elena.reshetova@...el.com>,
Hans Liljestrand <ishkamiel@...il.com>, David Windsor <dwindsor@...il.com>
Subject: Re: [RFC PATCH 12/13] x86: x86 implementation for
HARDENED_ATOMIC
On 10/05/2016 09:18 AM, Jann Horn wrote:
> 1. Pipeline flushes because of branch prediction failures caused by
> more-or-less random cmpxchg retries? Pipeline flushes are pretty
> expensive, right?
> 2. Repeated back-and-forth bouncing of the cacheline because an increment
> via cmpxchg needs at least two accesses instead of one, and the
> cacheline could be "stolen" by the other thread between the READ_ONCE
> and the cmpxchg.
> 3. Simply the cost of retrying if the value has changed in the meantime.
> 4. Maybe if two CPUs try increments at the same time, with exactly the
> same timing, they get stuck in a tiny livelock where every cmpxchg
> fails because the value was just updated by the other core? And then
> something slightly disturbs the timing (interrupt / clock speed
> change / ...), allowing one task to win the race?
I can speculate about it, but I don't know for sure. The topdown tool
from pmu-tools is usually a great way to figure out what's causing these
kinds of bottlenecks in the CPU:
https://github.com/andikleen/pmu-tools
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