linux-kernel - Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit

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Date:	Fri, 25 Mar 2011 09:29:34 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Jack Steiner <steiner@....com>,
	Borislav Petkov <bp@...64.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Nick Piggin <npiggin@...nel.dk>,
	"x86@...nel.org" <x86@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Ingo Molnar <mingo@...hat.com>, tee@....com,
	Nikanth Karthikesan <knikanth@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH RFC] x86: avoid atomic operation in test_and_set_bit_lock
 if possible

On Fri, Mar 25, 2011 at 3:06 AM, Jan Beulich <JBeulich@...ell.com> wrote:
>
> The problem was observed with __lock_page() (in a variant not
> upstream for reasons not known to me), and prefixing e.g.
> trylock_page() with an extra PageLocked() check yielded the
> below quoted improvements.

Ok. __lock_page() _definitely_ should do the test_bit() thing first,
because it's normally called from lock_page() that has already tested
the bit.

But it already seems to do that, so I'm wondering what your variant is.

I'm also a bit surprised that lock_page() is that hot (unless your
_lock_page() variant is simply too broken and ends up spinning?).
Maybe we have some path that takes the page lock unnecessarily? What's
the load?

                Linus
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