linux-kernel - Re: [PATCH 0/3] nohz: Convert tick dependency mask to atomic

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Message-ID: <CA+55aFxzR_gu9b0U6RG+8eGvjtuxjjOf4jfwjOKxhV1UShk6yg@mail.gmail.com>
Date:	Tue, 29 Mar 2016 07:23:31 -0500
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/3] nohz: Convert tick dependency mask to atomic_t

On Tue, Mar 29, 2016 at 4:44 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> Harmonizing thread_info::flags does not look easy, given how much assembly code
> accesses this field.

It might not be too bad.

For 32-bit architectures (which is still most of them), it's just a

   unsigned int/long -> atomic_t

and for 64-bit architectures you end up with three choices:

 - it's already 32-bit (alpha, ia64, x86):

        unsigned int -> atomic_t

 - little-endian long:

        atomic_t flags
        unsigned int padding;

 - big-endian long (only powerpc? Maybe there's a big-endian MIPS still?)

        unsigned int padding;
        atomic_t flags;

so you could do that fairly mindlessly. You *could* even use a nasty
macro from hell to do it automatically, with the above rules in some .

Then each architecture could clean itself up and get rid of the
padding field if they want to.

              Linus