On Mon, Jan 21, 2013 at 03:24:40PM +0000, Russell King - ARM Linux wrote: > On Fri, Jan 18, 2013 at 11:37:25PM -0500, Konrad Rzeszutek Wilk wrote: > > On Fri, Jan 18, 2013 at 01:45:27PM -0800, Greg Kroah-Hartman wrote: > > > On Fri, Jan 18, 2013 at 09:08:59PM +0000, Russell King - ARM Linux wrote: > > > > On Fri, Jan 18, 2013 at 02:24:15PM -0600, Matt Sealey wrote: > > > > > Hello all, > > > > > > > > > > I wonder if anyone can shed some light on this linking problem I have > > > > > right now. If I configure my kernel without SMP support (it is a very > > > > > lean config for i.MX51 with device tree support only) I hit this error > > > > > on linking: > > > > > > > > Yes, I looked at this, and I've decided that I will _not_ fix this export, > > > > neither will I accept a patch to add an export. > > > > > > > > As far as I can see, this code is buggy in a SMP environment. There's > > > > apparantly no guarantee that: > > > > > > > > 1. the mapping will be created on a particular CPU. > > > > 2. the mapping will then be used only on this specific CPU. > > > > 3. no guarantee that another CPU won't speculatively prefetch from this > > > > region. > > > > I thought the code had per_cpu for it - so that you wouldn't do that unless > > you really went out the way to do it. > > Actually, yes, you're right - that negates point (4) and possibly (2), > but (3) is still a concern. (3) shouldn't be that much of an issue > _provided_ that the virtual addresses aren't explicitly made use of by > other CPUs. Is that guaranteed by the zsmalloc code? (IOW, does it > own the virtual region it places these mappings in?) It does own them but it does also hand them off. So the users of it might be put on a different CPU. I think, I need to trace the call-chain. > > What is the performance difference between having and not having this > optimization? Can you provide some measurements please? Oh boy, there were somewhere. > > Lastly, as you hold per_cpu stuff across this, that means preemption > is disabled - and any kind of scheduling is also a bug. Is there > any reason the kmap stuff can't be used? Has this been tried? How > does it compare numerically with the existing solutions? It was really dependent on the architecture. On x86 the copying was superior, but on ARM it was sllow. _______________________________________________ devel mailing list devel@xxxxxxxxxxxxxxxxxxxxxx http://driverdev.linuxdriverproject.org/mailman/listinfo/devel
