ARM: 7409/1: Do not call flush_cache_user_range with mmap_sem held

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 <dima@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
index 55b2f3d..63d402f 100644 (file)
--- a/arch/arm/kernel/traps.c
+++ b/arch/arm/kernel/traps.c
@@ -496,7 +496,9 @@ do_cache_op(unsigned long start, unsigned long end, int flags)
                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);
 }