Subject: Re: Query on lock protection in random number driver
From: Nikanth Karthikesan <knikanth@suse.de>
To: Andi Kleen <andi@firstfloor.org>
Cc: linux-kernel@vger.kernel.org, mpm@selenic.com
In-Reply-To: <47879DA2.BANGALORE.BLR.100.174746A.1.EEA6.1@1:7.BANGALORE.BLR.100.0.1.0.1@16>
References: <1200049691.3844.44.camel@nikanth-laptop.blr.novell.com>
	 <47879DA2.BANGALORE.BLR.100.174746A.1.EEA6.1@1:7.BANGALORE.BLR.100.0.1.0.1@16>
Content-Type: text/plain
Date: Fri, 11 Jan 2008 16:58:28 +0530
Message-Id: <1200050908.3844.56.camel@nikanth-laptop.blr.novell.com>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1089
Lines: 28

On Fri, 2008-01-11 at 12:12 +0100, Andi Kleen wrote:
> Nikanth Karthikesan <knikanth@suse.de> writes:
> >
> > Also the globals random_read_wakeup_thresh and
> > random_write_wakeup_thresh are not at all protected by any locks! Why
> > locks are not needed for these?
> 
> Reading variables sizeof <= native word size (32bit or 64bit depending
> on architecture) is atomic by itself. This is not necessarily
> guaranteed in ISO-C or POSIX threads, but Linux can assume that.

Yes, I found that by checking the implementation of atomic_read.

But I didnt check the implementation of atomic_set before sending the
mail and assumed assigning to a variable may not be atomic on all arch,
and because of that, we may be reading a half-written, variable! But
assigning to an int is also atomic on all arch.

Thanks a lot.

Thanks
Nikanth Karthikesan

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/