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: <alpine.DEB.2.21.1806280945120.1778@nanos.tec.linutronix.de>
Date:   Thu, 28 Jun 2018 09:53:17 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     Peter Zijlstra <peterz@...radead.org>, 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 Wed, 27 Jun 2018, Fenghua Yu wrote:
> On Thu, Jun 21, 2018 at 09:55:40PM +0200, Peter Zijlstra wrote:
> > 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.
> 
> Do you mean x86_capability's type should be changed from __u32 to unsigned
> long?
> 
> Changing x86_capability's type won't directly fix the split lock in
> set_cpu_cap(), right? BTS still may access x86_capability across cache
> line no matter x86_capability's type.

Errm. No.  BTS & al are accessing a single 64bit location which is

   base_address + (bit_offset % 64) * 8

So if the base address is properly aligned then BTS & al will _NEVER_ have
to lock more than a single cache line. And it does not matter wheter we fix
the type or enforce 64bit alignement of the array by other means.

If that's not true then BTS & al are terminally broken and you can stop
working on #AC right away.

Thanks,

	tglx