On Mon, Jun 24, 2013 at 11:16:33PM +1000, Bruce Evans wrote: B> > K> This is quite interesting idea, but I still did not decided if it B> > K> acceptable. The issue is that we could add the carry to the other B> > K> processor counter, if the preemption kicks in at right time between B> > K> two instructions. I did not found any argument why would it be B> > K> wrong, the races for fetch operation seems to be the same with either B> > K> local update method. B> > B> > This would be wrong since update isn't locked. Thus, if we are put on B> > other CPU between two instructions, and in second instruction updating B> > another CPU counter simultaneously with the original CPU we were on, B> > then we are losing an update. B> B> Hmm, this is subtle. The update is never lost, but is applied to B> different counter, non-atomically at a later time. Non-atomicity B> only matters when there is a carry since the counter only goes B> transiently backards in this case. For example: initial state: B> B> CPU1 counter: 00000000 ffffffff B> CPU2 counter: 00000000 fffffffe B> B> Start adding 1 to the first counter, doing it non-atomically by B> incrementing the low word first. B> B> CPU1 counter: 00000000 00000000 (carry in CPU1 eflags) B> CPU2 counter: 00000000 fffffffe B> B> Get preempted at this point: B> B> CPU1 counter: 00000000 00000000 B> CPU2 counter: 00000000 fffffffe (carry in CPU2 eflags)
Nope, the non-atomicity isn't specific to the case when we carry bit from least significant part to most one. The non-atomicity is much simplier: two CPUs write to same memory address w/o using lock prefix. Thus, any update mechanism that first calculates absolute per-CPU address, then writes to it w/o lock, requires critical section. Because preemption between calculation and write, and later execution on other CPU leads to two CPUs writing to same address. -- Totus tuus, Glebius. _______________________________________________ svn-src-all@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/svn-src-all To unsubscribe, send any mail to "svn-src-all-unsubscr...@freebsd.org"
