Re: [PATCH] fix gettimeofday vs. update_gtod race

Benjamin Herrenschmidt wrote:

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

quoted

+	/* 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

No?  I didn't find anything about mftb having synchronizing
behavior.  How should we ensure that temp_varp is assigned before
reading the timebase?

Surely at least a compiler barrier is needed?

and the race not completely closed imho... 

How so?  I could've missed something, but I've hammered the patch
pretty hard, fwiw.

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.

I did try something like that but found that a loop (i.e. multiple
get_tb's to "catch up") was necessary.