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

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Message-ID: <20160315172803.GB5058@lerouge>
Date:	Tue, 15 Mar 2016 18:28:04 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	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 09:30:49AM -0700, Linus Torvalds wrote:
> 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);

Ah! Now I remember why I used unsigned long :-s

> 
> 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).

I can try that. And then we would end up with atomic_fetch_or() and maybe
the scheduler could keep its ad-hoc version until thread_info types get unified
(if that's possible).

> 
> 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.

I can try that too while at it :-)