On Sat, 2010-03-06 at 19:36 +0000, Russell King - ARM Linux wrote: > On Sat, Mar 06, 2010 at 04:17:23PM +0530, James Bottomley wrote: > > On a fault in of exec data, we first try to get the page out of the page > > cache. If it's not present, we put the faulting process to sleep and > > fetch it in from storage. When we do the read, on the PIO path, the > > kernel alias for the page becomes dirty. Some time later, we place the > > page into the user space (updating the pte entry that caused a fault). > > At this point, we'll call both flush_icache_page() and > > update_mmu_cache() ... this is where the I/D resolution should be done. > > No - this is where things get extremely icky. OK, but the point I'm trying to make is that the page cache code, including the I/O layer, only manages kernel D alias state (either by flushing or marking it dirty). The user space I/D handling is done in the mm code (I'm not claiming it's done correctly there, just claiming it's done there). > The problem at this point occurs on SMP architectures. As soon as you > update the PTE entry, it is visible to other threads of the application. > If you do I-cache handling after updating the PTE, then there is a window > where another CPU can execute the page: > > CPU0 CPU1 > speculatively prefetches from page N via kernel > mapping, loads garbage into I-cache > attempts to execute P > page fault > page N allocated > set_pte_at > executes P > *splat* > flush I-cache OK, so I can believe this. We see extremely rare segfaults on parisc which look to be the result of some I flush race like this. However, I think for a discussion of problems with the arch and mm interfaces, we should probably move off the usb list and onto linux-arch. Our specific problem on parisc is that being VIPT we can't do an I (or D) user flush without a mapping. We have two schemes for fixing this: One is to use a PAGE_FLUSH flag for the mapping ... it allows the flushes to work but refuses any type of RWX access (can do this because we have a software TLB). The other is to use a flush area within the kernel where we flush a page congruent to the userspace address ... I haven't got this working yet, and it's a bit wasteful of kernel address space because our congruence modulus is 4MB. James -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
