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mm: fix race in kunmap_atomic()

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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 <[email protected]>
Reported-by: Christoph Hellwig <[email protected]>
Acked-by: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Peter Zijlstra 2010-10-27 15:32:58 -07:00 • committed by Linus Torvalds
commit 20273941f2
10 changed files with 26 additions and 9 deletions
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3
arch/tile/mm/highmem.c
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@ -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);