On Thursday, April 14, 2011, Alan Stern wrote: > On Wed, 13 Apr 2011, Mike Frysinger wrote: > > > > On Wed, Apr 13, 2011 at 18:49, Rafael J. Wysocki wrote: > > > > In my opinion is an architecture problem, not the freezer code problem. > > > > OK, we have a patch pending locally which populates all barriers with > > this logic, but based on my understanding of things, that didnt seem > > correct. i guess i'm reading too much into the names ... i'd expect > > the opposite behavior where "rmb" is only for UP needs while "smp_rmb" > > is a rmb which additionally covers SMP. > > You are misinterpreting the names and the concepts, both. > > First, you need to understand that memory barriers are needed only for > purposes of synchronizing between two different entities capable of > accessing memory (obviously it's not necessary to synchronize an entity > with itself). One of those entities is always a CPU, of course; the > other entity could be a DMA-capable device or it could be another CPU. > > A device driver might need to use memory barriers even on a UP > platform, because it might need to synchronize the CPU with the device > it is driving. > > But core kernel code is concerned only with CPUs. Therefore on UP > systems, core kernel code (such as the freezer) never needs to use > memory barriers. > > That's the difference between rmb() and smp_rmb(). rmb() _always_ > generates a memory barrier, so it should be used only in device > drivers. smp_rmb() generates a memory barrier only if CONFIG_SMP is > enabled; otherwise it merely generates a compiler barrier. > > In the freezer, there is no reason to use rmb() and wmb(). It should > use smp_rmb() and smp_wmb(). OK, I think you're right, but that's because rmb() and wmb() cause too much overhead to happen. Thanks, Rafael _______________________________________________ linux-pm mailing list linux-pm@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/linux-pm
