mm: fix race in kunmap_atomic()

Christoph reported a nice splat which illustrated a race in the new stack
based kmap_atomic implementation.

The problem is that we pop our stack slot before we're completely done
resetting its state -- in particular clearing the PTE (sometimes that's
CONFIG_DEBUG_HIGHMEM).  If an interrupt happens before we actually clear
the PTE used for the last slot, that interrupt can reuse the slot in a
dirty state, which triggers a BUG in kmap_atomic().

Fix this by introducing kmap_atomic_idx() which reports the current slot
index without actually releasing it and use that to find the PTE and delay
the _pop() until after we're completely done.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Reported-by: Christoph Hellwig <hch@infradead.org>
Acked-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/arch/sparc/mm/highmem.c b/arch/sparc/mm/highmem.c
index 5e50c09b7dce..4730eac0747b 100644
--- a/arch/sparc/mm/highmem.c
+++ b/arch/sparc/mm/highmem.c
@@ -75,7 +75,7 @@ void __kunmap_atomic(void *kvaddr)
                 return;
         }
 
-        type = kmap_atomic_idx_pop();
+        type = kmap_atomic_idx();
 
 #ifdef CONFIG_DEBUG_HIGHMEM
         {
@@ -104,6 +104,8 @@ void __kunmap_atomic(void *kvaddr)
 #endif
         }
 #endif
+
+        kmap_atomic_idx_pop();
         pagefault_enable();
 }
 EXPORT_SYMBOL(__kunmap_atomic);