linux-kernel - Re: Query on lock protection in random number driver

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Message-Id: <1200050908.3844.56.camel@nikanth-laptop.blr.novell.com>
Date:	Fri, 11 Jan 2008 16:58:28 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	linux-kernel@...r.kernel.org, mpm@...enic.com
Subject: Re: Query on lock protection in random number driver

On Fri, 2008-01-11 at 12:12 +0100, Andi Kleen wrote:
> Nikanth Karthikesan <knikanth@...e.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

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