[PATCH] [ARM] Do not call flush_cache_user_range with mmap_sem held

[Dima Zavin]

This patch is probably not as efficient as it could be since the flush
will end up faulting in pages the user never touched. I think we
should print a warning regarding that and skip over the unmapped
pages, I just wasn't sure how to do that correctly.

--Dima

2010/4/28 Dima Zavin <dima at android.com>:
> 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.
>
> Also, since we really can't be holding the mmap_sem while calling
> flush_cache_user_range AND vma is actually unused by the flush itself,
> get rid of vma as an argument.
>
> Signed-off-by: Dima Zavin <dima at android.com>
> Cc: Russell King <rmk+kernel at arm.linux.org.uk>
> Cc: Arve Hjønnevåg <arve at android.com>
> ---
>  arch/arm/include/asm/cacheflush.h |    2 +-
>  arch/arm/kernel/traps.c           |    4 +++-
>  2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm/include/asm/cacheflush.h b/arch/arm/include/asm/cacheflush.h
> index 0d08d41..b68a2b4 100644
> --- a/arch/arm/include/asm/cacheflush.h
> +++ b/arch/arm/include/asm/cacheflush.h
> @@ -336,7 +336,7 @@ extern void flush_cache_page(struct vm_area_struct *vma, unsigned long user_addr
>  * Harvard caches are synchronised for the user space address range.
>  * This is used for the ARM private sys_cacheflush system call.
>  */
> -#define flush_cache_user_range(vma,start,end) \
> +#define flush_cache_user_range(start,end) \
>        __cpuc_coherent_user_range((start) & PAGE_MASK, PAGE_ALIGN(end))
>
>  /*
> diff --git a/arch/arm/kernel/traps.c b/arch/arm/kernel/traps.c
> index 1621e53..2455fd3 100644
> --- a/arch/arm/kernel/traps.c
> +++ b/arch/arm/kernel/traps.c
> @@ -453,7 +453,9 @@ do_cache_op(unsigned long start, unsigned long end, int flags)
>                if (end > vma->vm_end)
>                        end = vma->vm_end;
>
> -               flush_cache_user_range(vma, start, end);
> +               up_read(&mm->mmap_sem);
> +               flush_cache_user_range(start, end);
> +               return;
>        }
>        up_read(&mm->mmap_sem);
>  }
> --
> 1.6.6
>
>