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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 5 Jun 2017 15:51:52 -0700
Message-ID: <CA+55aFwo5UXidsA23FcbD6+VCSbWC9ZHt65it9q9f0L_hycPLw@mail.gmail.com>
Subject: Re: [RFC 01/11] x86/ldt: Simplify LDT switching logic
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To: Andy Lutomirski <luto@kernel.org>
Cc: X86 ML <x86@kernel.org>, Borislav Petkov <bpetkov@suse.de>, Andrew Morton <akpm@linux-foundation.org>, Mel Gorman <mgorman@suse.de>, "linux-mm@kvack.org" <linux-mm@kvack.org>, Nadav Amit <nadav.amit@gmail.com>, Rik van Riel <riel@redhat.com>

On Mon, Jun 5, 2017 at 3:40 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> I think the "LDT didn't match" was really just a simpler and more
> efficient way to say "they weren't both NULL".

In fact, looking back in the history, it used to instead add the sizes
of the context (and then similar logic: "if the sum is non-zero, one
or the other was non-zero").

Commit 0bbed3beb4 ("[PATCH] Thread-Local Storage (TLS) support") in
the historical tree then did this:

-               if (next->context.size+prev->context.size)
+               if (unlikely(prev->context.ldt != next->context.ldt))

I'm ok with your change, but I reacted to the commit log about how
this was "overcomplicated". It was actually an optimization exactly to
avoid two compares..

               Linus

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