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Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Mon, 21 Apr 2014 18:47:52 -0700
To: Andrew Lutomirski <amluto@gmail.com>
CC: "H. Peter Anvin" <hpa@linux.intel.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>,
        Alexander van Heukelum <heukelum@fastmail.fm>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Borislav Petkov <bp@alien8.de>,
        Arjan van de Ven <arjan.van.de.ven@intel.com>,
        Brian Gerst <brgerst@gmail.com>,
        Alexandre Julliard <julliard@winehq.com>,
        Andi Kleen <andi@firstfloor.org>, Thomas Gleixner <tglx@linutronix.de>
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Race condition (although with x86 being globally ordered, it probably can't actually happen.) The bitmask is probably the way to go.

On April 21, 2014 6:28:12 PM PDT, Andrew Lutomirski <amluto@gmail.com> wrote:
>On Mon, Apr 21, 2014 at 6:14 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> I wanted to avoid the "another cpu made this allocation, now I have
>to free" crap, but I also didn't want to grab the lock if there was no
>work needed.
>
>I guess you also want to avoid bouncing all these cachelines around on
>boot on bit multicore machines.
>
>I'd advocate using the bitmap approach or simplifying the existing
>code.  For example:
>
>+       for (n = 0; n < ESPFIX_PUD_CLONES; n++) {
>+               pud = ACCESS_ONCE(pud_p[n]);
>+               if (!pud_present(pud))
>+                       return false;
>+       }
>
>I don't see why that needs to be a loop.  How can one clone exist but
>not the others?
>
>--Andy

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