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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Tue, 27 Dec 2016 11:23:19 -0800
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Subject: Re: [PATCH 2/2] mm: add PageWaiters indicating tasks are waiting for
 a page bit
To: Nicholas Piggin <npiggin@gmail.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
        Bob Peterson <rpeterso@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Steven Whitehouse <swhiteho@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Andreas Gruenbacher <agruenba@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, linux-mm <linux-mm@kvack.org>,
        Mel Gorman <mgorman@techsingularity.net>
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On Tue, Dec 27, 2016 at 10:58 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The other alternative is to keep the lock bit as bit #0, and just make
> the contention bit be the high bit. Then, on x86, you can do
>
>     lock andb $0xfe,flags
>     js contention
>
> which might be even better. Again, it would be a very special
> operation just for unlock. Something like
>
>    bit_clear_and_branch_if_negative_byte(mem, label);
>
> and again, it would be trivial to do on most architectures.
>
> Let me try to write a patch or two for testing.

Ok, that was easy.

Of course, none of this is *tested*, but it looks superficially
correct, and allows other architectures to do the same optimization if
they want.

On x86, the unlock_page() code now generates

        lock; andb $1,(%rdi)    #, MEM[(volatile long int *)_7]
        js      .L114   #,
        popq    %rbp    #
        ret

for the actual unlock itself.

Now to actually compile the whole thing and see if it boots..

                 Linus