3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Dima Zavin <redacted>

commit 435a7ef52db7d86e67a009b36cac1457f8972391 upstream.

We can't be holding the mmap_sem while calling flush_cache_user_range
because the flush can fault. If we fault on a user address, the
page fault handler will try to take mmap_sem again. Since both places
acquire the read lock, most of the time it succeeds. However, if another
thread tries to acquire the write lock on the mmap_sem (e.g. mmap) in
between the call to flush_cache_user_range and the fault, the down_read
in do_page_fault will deadlock.

[will: removed drop of vma parameter as already queued by rmk (7365/1)]

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Dima Zavin <redacted>
Signed-off-by: John Stultz <redacted>
Signed-off-by: Will Deacon <redacted>
Signed-off-by: Russell King <redacted>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 arch/arm/kernel/traps.c |    2 ++
 1 file changed, 2 insertions(+)

--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c

@@ -496,7 +496,9 @@ do_cache_op(unsigned long start, unsigne
 		if (end > vma->vm_end)
 			end = vma->vm_end;
 
+		up_read(&mm->mmap_sem);
 		flush_cache_user_range(start, end);
+		return;
 	}
 	up_read(&mm->mmap_sem);
 }