Date: Tue, 25 Jun 2013 17:28:14 +0400 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Bruce Evans <brde@optusnet.com.au> Cc: Konstantin Belousov <kostikbel@gmail.com>, svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org Subject: Re: svn commit: r252032 - head/sys/amd64/include Message-ID: <20130625132814.GL1214@FreeBSD.org> In-Reply-To: <20130624211428.O2235@besplex.bde.org> References: <20130621090207.F1318@besplex.bde.org> <20130621064901.GS1214@FreeBSD.org> <20130621184140.G848@besplex.bde.org> <20130621135427.GA1214@FreeBSD.org> <20130622110352.J2033@besplex.bde.org> <20130622124832.S2347@besplex.bde.org> <20130622174921.I3112@besplex.bde.org> <20130623073343.GY91021@kib.kiev.ua> <20130624081056.GD1214@FreeBSD.org> <20130624211428.O2235@besplex.bde.org>
next in thread | previous in thread | raw e-mail | index | archive | help
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.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20130625132814.GL1214>