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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/mips/mm/highmem.c
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@ -74,7 +74,7 @@ void __kunmap_atomic(void *kvaddr)
return;
}
type = kmap_atomic_idx_pop();
type = kmap_atomic_idx();
#ifdef CONFIG_DEBUG_HIGHMEM
{
int idx = type + KM_TYPE_NR * smp_processor_id();
@ -89,6 +89,7 @@ void __kunmap_atomic(void *kvaddr)
local_flush_tlb_one(vaddr);
}
#endif
kmap_atomic_idx_pop();
pagefault_enable();
}
EXPORT_SYMBOL(__kunmap_atomic);