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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 <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>
This commit is contained in:
Peter Zijlstra
2010-10-27 15:32:58 -07:00
committed by Linus Torvalds
parent a8e23a2918
commit 20273941f2
10 changed files with 26 additions and 9 deletions
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5
include/linux/highmem.h
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@@ -88,6 +88,11 @@ static inline int kmap_atomic_idx_push(void)
return idx;
}
static inline int kmap_atomic_idx(void)
{
return __get_cpu_var(__kmap_atomic_idx) - 1;
}
static inline int kmap_atomic_idx_pop(void)
{
int idx = --__get_cpu_var(__kmap_atomic_idx);