linux-kernel - Re: [PATCH 21/26] tty: Convert tty

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Message-ID: <20140903175606.GA9740@kroah.com>
Date:	Wed, 3 Sep 2014 10:56:06 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Jiri Slaby <jslaby@...e.cz>, linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 21/26] tty: Convert tty_struct bitfield to bools

On Wed, Sep 03, 2014 at 11:10:45AM -0400, Peter Hurley wrote:
> On 09/03/2014 08:19 AM, One Thousand Gnomes wrote:
> >> Ahh. Thanks for the insight, Alan.
> >>
> >> But set_bit() et. al. will generate an incredible amount of churn;
> >> what if I split the fields up to prevent false-sharing?
> > 
> > Do you feel lucky ;-)
> 
> Hahaha  :)
> 
> > I'd rather set_bit and friends were used. They exist largely for this
> > kind of reason and they also have atomic test/set methods which may in
> > the longer term be very useful.
> > 
> > Yes it is churn can't argue with that.
> 
> Yuck. There should be a better way. IXANY mode is suddenly going to
> have a ton of unnecessary bus locks on x86.

True, but at least it will be correct, which I'm guessing today it isn't
:(

> Note the ctrl_status field is a byte as well, which can't be RMW'ed by
> the bit-locked primitives, and definitely should not be aggregated with
> any adjacent field.

Never trust what an ia64 compiler can, and will, do...

greg k-h
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