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, 2 insertions, 1 deletions

diff --git a/arch/tile/mm/highmem.c b/arch/tile/mm/highmem.c
index 8ef6595e162c..abb57331cf6e 100644
--- a/arch/tile/mm/highmem.c
+++ b/arch/tile/mm/highmem.c
@@ -241,7 +241,7 @@ void __kunmap_atomic(void *kvaddr)
                 pte_t pteval = *pte;
                 int idx, type;
 
-                type = kmap_atomic_idx_pop();
+                type = kmap_atomic_idx();
                 idx = type + KM_TYPE_NR*smp_processor_id();
 
                 /*
@@ -252,6 +252,7 @@ void __kunmap_atomic(void *kvaddr)
                 BUG_ON(!pte_present(pteval) && !pte_migrating(pteval));
                 kmap_atomic_unregister(pte_page(pteval), vaddr);
                 kpte_clear_flush(pte, vaddr);
+                kmap_atomic_idx_pop();
         } else {
                 /* Must be a lowmem page */
                 BUG_ON(vaddr < PAGE_OFFSET);