On Fri, Feb 22, 2013 at 03:05:39PM +0100, Peter Zijlstra wrote: > On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote: > > > Which is a problem how? > > > > So here is a possible scenario, CPU 0 reads a kcpustat value, and CPU > > 1 writes > > it at the same time: > > > > //Initial value of "cpustat" is 0xffffffff > > == CPU 0 == == CPU 1 == > > > > //load low part > > mov %eax, [cpustat] > > inc [cpustat] > > //Update the high part if necessary > > jnc 1f > > inc [cpustat + 4] > > 1: > > //load high part > > mov %edx, [cpustat + 4] > > > > > > Afterward, CPU 0 will think the value is 0x1ffffffff while it's > > actually > > 0x100000000. > > > > atomic64_read() and atomic64_set() are supposed to take care of that, > > without > > even the need for _inc() or _add() parts that use LOCK. > > > Sure I get that, but again, why is that a problem,.. who relies on > these statistics that makes it a problem?
I guess we want to provide at least some minimal reliability in /proc/stat I mean we don't mind if the read is slightly off, reading stats from userspace is inherently racy anyway, but if it suddenly shows a wrong increase of 4 billions which disappear soon after, it looks like a bug to me. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
