From mboxrd@z Thu Jan  1 00:00:00 1970
From: der.herr@hofr.at (Nicholas Mc Guire)
Date: Sat, 31 Jan 2015 13:04:20 +0100
Subject: spinlock variable protection
In-Reply-To: <CAJs94EZK6PAT6RbkuAvJgdNce_6WananEJ06NAC6vJirro1hMw@mail.gmail.com>
References: <mafu9s$mhn$1@ger.gmane.org>
	<A827E844-3767-4BE6-A45F-B3E1CC615E36@gmail.com>
	<CAJs94EZK6PAT6RbkuAvJgdNce_6WananEJ06NAC6vJirro1hMw@mail.gmail.com>
Message-ID: <20150131120420.GA21819@opentech.at>
To: kernelnewbies@lists.kernelnewbies.org
List-Id: kernelnewbies.lists.kernelnewbies.org

On Fri, 30 Jan 2015, Matwey V. Kornilov wrote:

> 2015-01-30 16:52 GMT+03:00 buyitian <buyitian@gmail.com>:
> >>
> > Please check the assembly code to double confirm the GCC behavior.
> > Why will GCC change the order as what you mentioned? Only assembly code can tell you.
> 
> It does not change at the moment. I think it can change it.
> 
> Because from line
>   ret = hdl->count;
> until line
>   return ret;
> there is no access to either ret or hdl->count. So it is reasonable to
> optimizer to think that their values are the same and eliminate
> unneeded variable.
>
If your worry is that it will optimize it out then pack it into
an ACCESS_ONCE and that should prevent GCC from doing so.
for your case I think ret = ACCESS_ONCE(hdl->count);
would be sufficient.

thx!
hofrat