linux-kernel - Re: [PATCH] nohz: Change tick_dep_mask from 'unsigned long' to 'unsigned int'

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Date:	Tue, 15 Mar 2016 09:30:49 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] nohz: Change tick_dep_mask from 'unsigned long' to
 'unsigned int'

On Tue, Mar 15, 2016 at 5:15 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> Hm, so the problem I did not consider is the following:
>
>  triton:~/tip> git grep tick_dep_mask kernel | grep _bit
>  kernel/time/tick-sched.c:       clear_bit(bit, &ts->tick_dep_mask);
>  kernel/time/tick-sched.c:       clear_bit(bit, &tsk->tick_dep_mask);
>  kernel/time/tick-sched.c:       clear_bit(bit, &sig->tick_dep_mask);

Ahh. I missed that too.

Ok, leave the 64-bit field for now, while we think about this. But one
option is to just use "atomic_andnot()" instead of clear_bit().

That would imply using an "atomic_t", which would be fairly natural
(and would be 32-bit).

And wouldn't it be so nice if "thread_info->flags" would just be
atomic_t too. Right now we use a mixture of bit-ops and ACCESS_ONCE()
(and many codepaths then doing neither, and just accessing it
directly, ignoring any races.

Oh well.

                Linus