On Wed, 2015-10-21 at 12:36 -0700, Paul E. McKenney wrote: > On Wed, Oct 21, 2015 at 10:18:33AM +0200, Peter Zijlstra wrote: > > On Tue, Oct 20, 2015 at 02:28:35PM -0700, Paul E. McKenney wrote: > > > I am not seeing a sync there, but I really have to defer to the > > > maintainers on this one. I could easily have missed one. > > > > So x86 implies a full barrier for everything that changes the CPL; and > > some form of implied ordering seems a must if you change the privilege > > level unless you tag every single load/store with the priv level at that > > time, which seems the more expensive option. > > And it is entirely possible that there is some similar operation > somewhere in the powerpc entry/exit code. I would not trust myself > to recognize it, though. > > So I suspect the typical implementation will flush all load/stores, > > change the effective priv level and continue. > > > > This can of course be implemented at a pure per CPU ordering (RCpc), > > which would be in line with the rest of Power, in which case you do > > indeed need an explicit sync to make it visible to other CPUs. > > > > But yes, if Michael or Ben could clarify this it would be good. > > :-) ;-) ;-) Sorry guys, these threads are so long I tend not to read them very actively :} Looking at the system call path, the straight line path does not include any barriers. I can't see any hidden in macros either. We also have an explicit sync in the switch_to() path, which suggests that we know system call is not a full barrier. Also looking at the architecture, section 1.5 which talks about the synchronisation that occurs on system calls, defines nothing in terms of memory ordering, and includes a programming note which says "Unlike the Synchronize instruction, a context synchronizing operation does not affect the order in which storage accesses are performed.". Whether that's actually how it's implemented I don't know, I'll see if I can find out. cheers -- To unsubscribe from this list: send the line "unsubscribe stable" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html