litmus/ctrldev: Fix stale readouts on ARMv7

For ARMv7 __S011 only declares the access bits, not the caching
policy. As a result the userspace mapping would bypass the cache,
causing the kernel to occasionally read stale entries from the cache.
Switch to using PAGE_SHARED instead, which assumes correct caching
defaults.

[With kind assistance by Alexander Zuepke <alexander.zuepke@hs-rm.de>,
who pointed out the correct direction to look for the caching bug.]

Signed-off-by: Roy Spliet <rspliet@mpi-sws.org>

1 files changed, 4 insertions, 4 deletions

diff --git a/litmus/ctrldev.c b/litmus/ctrldev.c
index 3e2ac2b3fbe8..877f2786b4c8 100644
--- a/litmus/ctrldev.c
+++ b/litmus/ctrldev.c
@@ -97,14 +97,14 @@ static int litmus_ctrl_mmap(struct file* filp, struct vm_area_struct* vma)
         vma->vm_ops = &litmus_ctrl_vm_ops;
         /* This mapping should not be kept across forks,
          * cannot be expanded, and is not a "normal" page. */
-        vma->vm_flags |= VM_DONTCOPY | VM_DONTEXPAND | VM_IO;
+        vma->vm_flags |= VM_DONTCOPY | VM_DONTEXPAND | VM_READ | VM_WRITE;
 
         /* We don't want the first write access to trigger a "minor" page fault
          * to mark the page as dirty.  This is transient, private memory, we
-         * don't care if it was touched or not. __S011 means RW access, but not
-         * execute, and avoids copy-on-write behavior.
+         * don't care if it was touched or not. PAGE_SHARED means RW access, but
+         * not execute, and avoids copy-on-write behavior.
          * See protection_map in mmap.c.  */
-        vma->vm_page_prot = __S011;
+        vma->vm_page_prot = PAGE_SHARED;
 
         err = alloc_ctrl_page(current);
         if (!err)