linux-kernel - Re: [RFC PATCH 04/16] x86/split_lock: Use non locked bit set instruction in set_cpu

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Message-ID: <20180621195540.GC13636@worktop.programming.kicks-ass.net>
Date:   Thu, 21 Jun 2018 21:55:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 04/16] x86/split_lock: Use non locked bit set
 instruction in set_cpu_cap

On Sun, May 27, 2018 at 08:45:53AM -0700, Fenghua Yu wrote:
> set_bit() called by set_cpu_cap() is a locked bit set instruction for
> atomic operation.
> 
> Since the c->x86_capability can span two cache lines depending on kernel
> configuration and building evnironment, the locked bit set instruction may
> cause #AC exception when #AC exception for split lock is enabled.

That doesn't make sense. Sure the bitmap may be longer, but depending on
if the argument is an immediate or not we either use a byte instruction
(which can never cross a cacheline boundary) or a 'word' aligned BTS.
And the bitmap really _should_ be 'unsigned long' aligned.

If it is not aligned, fix that too.

/me looks at cpuinfo_x86 and finds x86_capability is in fact a __u32
array.. see that's broken and needs fixing first.