[PATCH] fix gettimeofday vs. update_gtod race

[Benjamin Herrenschmidt]

On Fri, 2006-08-11 at 15:41 -0500, Nathan Lynch wrote:

> +	/* Sampling the time base must be done after loading
> +	 * do_gtod.varp in order to avoid racing with update_gtod.
> +	 */
> +	rmb();
> +	tb_ticks = get_tb() - temp_varp->tb_orig_stamp;

The barrier isn't necessary and the race not completely closed imho... I
need to think about it a bit more closely but what about instead just
check if tb_ticks goes negative, and if yes, just do get_tb() again ?
That might be faster than having a sync in there and should still be
correct.

>  	temp_tb_to_xs = temp_varp->tb_to_xs;
>  	temp_stamp_xsec = temp_varp->stamp_xsec;
>  	xsec = temp_stamp_xsec + mulhdu(tb_ticks, temp_tb_to_xs);
> @@ -464,7 +469,7 @@ void do_gettimeofday(struct timeval *tv)
>  		tv->tv_usec = usec;
>  		return;
>  	}
> -	__do_gettimeofday(tv, get_tb());
> +	__do_gettimeofday(tv);
>  }
>  
>  EXPORT_SYMBOL(do_gettimeofday);
> 
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