linux-kernel - Re: [PATCH 2/2] mm: add PageWaiters indicating tasks are waiting for a page bit

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Message-ID: <CA+55aFyXXKdjbidzVC=waiaAaUJpwqZQZv-kKoZfaiWtYy3z=A@mail.gmail.com>
Date:   Tue, 27 Dec 2016 11:40:38 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Bob Peterson <rpeterso@...hat.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Steven Whitehouse <swhiteho@...hat.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-mm <linux-mm@...ck.org>,
        Mel Gorman <mgorman@...hsingularity.net>
Subject: Re: [PATCH 2/2] mm: add PageWaiters indicating tasks are waiting for
 a page bit

On Tue, Dec 27, 2016 at 11:24 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> Oops. I should include the actual patch I was talking about too, shouldn't I?

And that patch was completely buggy. The mask for the "and" was computed as

+               : "Ir" (1 << nr) : "memory");

but that clears every bit *except* for the one we actually want to
clear. I even posted the code it generates:

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

which is obviously crap.

The mask needs to be inverted, of course, and the constraint should be
"ir" (not "Ir" - the "I" is for shift constants) so it should be

+               : "ir" ((char) ~(1 << nr)) : "memory");

new patch attached (but still entirely untested, so caveat emptor).

This patch at least might have a chance in hell of working. Let's see..

              Linus

View attachment "patch.diff" of type "text/plain" (2830 bytes)